Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Tubes and Hybrids => Topic started by: billyjoe24 on November 07, 2009, 01:00:20 PM

Title: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 07, 2009, 01:00:20 PM
Took this amp in for repair and it has 60 hz hum.
I've been told the rectifier diodes might be the source of the problem,
but without a schematic the cost is through the roof.
Any ideas what the exact cause is?
Title: Re: Hiwatt custom 20 tube hum problem
Post by: J M Fahey on November 07, 2009, 03:15:44 PM
Hi, tell us some things first.
Does it practically dissapear if putting the volume or any other control (or all) in "0" or unplugging the guitar/turning its volume into "0"?
Does it change if you move the guitar or if you touch the chassis with your bare hands?
Is it constant or rises with time?
Is the amplifier grounded?
Good luck.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 07, 2009, 04:25:36 PM
Had some trouble figuring out how to post!
Anyway, the problem is that turning up the master volume even with the gain low will cause a dramatic increase in noise and hum. If we attempt to put the gain high and master low, we can't go much higher with the master because again we get more noise and hum.
This is with no guitar plugged in.
The amp was restored to the original values on the board(it's pcb) and 2 traces were reconnected. Supposedly the amp is grounded. It wasn't before.
Someone tried to get around the issue by killing the gain, but the problem was there.
Is this amp fixable without a schematic?
They're saying it'll be very expensive.
So if we turn everything off there is no hum.
There is also a buzzy static as well.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: J M Fahey on November 07, 2009, 05:06:18 PM
Just a hunch. On your 20VAC scale, check the voltage from pin 9 to 4/5 (they're usually joined) on any 12AX7 socket and then from any of them to ground.
Post what you measure.
It sounds as if the center tap of the filament winding is ungrounded.
If not, we'll go on.
Any experienced tech can service it without schematic, because most (99%) tube amps use the same basic "building blocks" and you can check one-by-one.
That's not so on SS gear.
The problem is that we need both to have access to the same schematic, to be able to ask you to check certain things on "remote control".
EMail Hiwatt, adding the model and  serial number and explaining the problem, they might send you a copy.
Take some sharp, well illuminated pictures if possible, specially around the "repaired tracks".
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 07, 2009, 05:37:29 PM
Hiwatt doesn't have a schematic, so that's out.
I'll ask them to do what you said but I think they're telling me it's too expensive to fix.
Not sure if they want to do any more on it.
Would it be a good idea to just take the amp back at this point on Monday and try some other tech? I do have a picture of the components on the board.
One other tech told me this was unfixable. It was badly designed.
These guys think it's the power supply.
Would the fact that the transformers are not 90 degrees to each other be a factor?
I have apicture of the components don't know how to post it.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: J M Fahey on November 08, 2009, 10:12:47 AM
Your post amazes me, let's see it part by part:
Quote
Hiwatt doesn't have a schematic
Maybe not, if it's old.
Quote
they're telling me it's too expensive to fix
That means "I have no idea, take it away"
Quote
Not sure if they want to do any more on it
Agree.
Quote
just take the amp back and try some other tech?

Try to find a good one, ¿Where do you live?
Quote
One other tech told me this was unfixable. It was badly designed.
That means "I have no idea, take it away"[2]
Quote
Would the fact that the transformers are not 90 degrees to each other be a factor?
At worst it would have a very slight 60Hz hum, not affected by any control.
Quote
I have apicture of the components don't know how to post it.
Post it in some images server (see what the other guys use) and post the links here. Avoid "Rapid"share and any, such as Flickr, that forces you to register.
On Monday take some other sharp pictures.
Do you have some technician/nerdy friend who can solder, measure and read a schematic properly?
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 08, 2009, 11:50:45 AM
I'm amazed too by my situation but not in a good way.
Hiwatt or Fernandes Guitars the manufacturer of this amp told me they have no schematic for this amp and that's for sure.This in itself I can't believe.It's from 2004 this amp,
but that's the situation.
I have sent the tech people some emails with suggestions from here,but tomorrow I thinkI'll phone them and pick it up.
 The amp was restored to the stock configuration,
which has increased the gain. I was told if it's a design flaw then forget it.
I just can't understand why this amp seems unfixable!
I'm in Lasalle Canada near Montreal. As for soldering, I certainly can do that.
But the voltages in this amp I don't think I want to mess with.
The tubes don't appear to be the source of the problem although one tube was bad.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 08, 2009, 01:11:31 PM
Link to photo of components before repair.
Also They seem to think the problem is in the power supply section.
Without a schematic it will take 4 or 5 hours.
 (http://img176.imagevenue.com/loc410/th_03737_HiwattsPcb_122_410lo.jpg) (http://img176.imagevenue.com/img.php?image=03737_HiwattsPcb_122_410lo.jpg)
Title: Re: Hiwatt custom 20 tube hum problem
Post by: J M Fahey on November 08, 2009, 11:58:55 PM
Now I know there are Three (maybe more) Custom Hiwatt 20's.
There never was a "real" Hiwatt 20, for the very good reason that they always made Professional, stage rated amplifiers, so they went from 50W ones to 400W beasts, the 100W, 2x4x12" being the most popular.
The first and the third were made by "something" brothers in UK, hand made, PTP wired, and *very* expensive.
The one you have is Korean made, which not only uses a PCB (nothing wrong with that, if well made) but connectors for everything, specially the tubes (what were they thinking?)
Even if you get a schematic, probably it will be from the British one.
I distinctly remember having seen a Custom 20 innards and they were absolutely different, just two long thin strips of turret connectors, chassis mounted sockets , *VERY* clean wiring and a simpler circuit overall.
Maybe it had 3 12AX7's or even 2; definitely not four, and was single-channel, like its big brothers.
What irks me is that I can repair it in an hour or two, no schematic needed, but we are half a world apart, and don't know how to explain you how to do it.
Oh well.
As a last chance, when you get it back check those filament voltages I told you to.
I've seen you asked everywhere with not much luck.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 09, 2009, 09:24:32 AM
Yes, that's me everywhere trying to get help.
Maybe you should talk to my repair place, but I think I'm just going to get it back
and just get rid of it.I'm going to phone them today.

This is a one channel amp as well.
It drives me nuts that you can fix it while all I get here is:
it's a piece of crap and can't be fixed!

Would more photos help?
You can always try to explain what to do anyway.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: J M Fahey on November 09, 2009, 10:41:10 AM
Hi Billyjoe.
Fact is, in USA
Quote
and just get rid of it
is normal, here it is usually not an option; getting schematics or factory parts neither, so we have to sharpen our skills.
As they say, "the fat cat doesn't catch the mice".
As a last chance, tell them to measure the filament voltage, from pins 4+5 to ground , from pin 9 to ground, in any 12AX7 and from pin 9 to 4+5.
They should have 6.3VAC in the third case and 3.15VAC in the first and second.
If not, your filament supply is not grounded, that causes terrible hum.
Some other guy suggested something similar to you.
Another possibility, is that the PCB grounding is missing or very poor, I'd need some extra pictures to tell you what to measure.
In the worst case, don't junk it, you can build a PTP wired "real Hiwatt" or a "Marshall 18W" or a "Tiny Terror/AC15" or an "Atomic16" clone using the original transformers and some of the filter capacitors (you'll need "real" chassis mounted sockets though) *or* if you don't want to work with high voltages you can house there some preamp plus a 15W TDA2050 amp or even an LM3886 50 W power amp.
Good luck.
Juan Manuel Fahey.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 09, 2009, 11:10:54 AM
Just spoke to my guy.
He's worried that if we go ahead and repair it because there may be a design flaw and it will still hum. He can't guarantee it will work.He wants to know if it's a known issue with the amp.
He told me if there was a problem with the filament wires the tubes wouldn't light up.
If I want to check the caps and diodes possibly the filament wires they will charge another 60.00. He doesn't seem to think it's a component problem.
Could the ceriatone board fit in here? Their layout is free.
The thing is I would try to keep it at 20 watts.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: J M Fahey on November 09, 2009, 09:12:29 PM
Hi billyjoe.
Just get your amp back.
As a side comment, an ungrounded filament winding will light them beautifully, but will hum like crazy too.
Anyway, stop wasting money.
I've found the British Hiwatt20.
Have a look:
http://mhuss.com/Hiwatt/ipage.php?n=8 (http://mhuss.com/Hiwatt/ipage.php?n=8)
(http://mhuss.com/Hiwatt/images/C20Guts.jpg)
(http://mhuss.com/Hiwatt/Schematics/AB_SA210.jpg)
try to get some greybeard technician.
Good luck.
Juan Manuel Fahey.(http://)
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 10, 2009, 08:57:46 AM
That's a beautiful amp.
Looking at it I can only I think I can wire a copy!
Famous last words.
But yes I will get the amp back,and figure out what to do.
I will post more pics.
If it's not the filaments or perhaps a dried out cap, what could it be?
Title: Re: Hiwatt custom 20 tube hum problem
Post by: phatt on November 11, 2009, 12:06:42 AM
Hi billyjoe,
              As *JMF* has already noted it's hard to work out as many things might be happening altogether.

My thoughts on this;
ANY Tube amp with that many preamp triodes is bound to hum, some worse than others. From the pics I can't tell if the preamp heaters are DC or AC powered?

If they are AC heaters then with so much gain you *WILL have Substantial HUMMMMMMMMMMMMM* :'(

I recently worked on a Laney 30 watt unit and it's 2nd channel Hum was so bad the owner wanted to sell it for similar reasons you mention here.

The gain on the laney was insane so I just turned it all down to an exceptable level and Now it gets played at all his gigs. 8|

So I can understand the chaps reluctance to fix it knowing that without some modification it will always hum.
Other comment is the yellow wire crossing the top of the 3rd Electro from left is Cut rather badly I'd get that fixed.

You could ask a good teck to pull one of the triodes out of circuit and rewire it to a more civilised circuit,, you will still have more gain than you will ever use,,, well if you know valve amps you know it can be done.

I get looks of confusion when I try to explain to the young players that Brian May used the *Normal* Ch on his Vox amps,,, not the *Bright Ch* as most would assume.
Now if you follow the signal in a Vox Amp the Normal Ch passes through one Triode (half an AX7) and then after passing through a Volume control goes Directly to the PI and power Amp.

With a good guitar and a simple treble booster you can have masses of distortion,,,,,,,and very little hum even though it's still using AC heaters.
Anyone who builds an all Valve Amp using more than 3 triode stages
(1 1/2 AX7's) does not understand valve teck.
So even if you get the potential ground issue fixed it may still hum quite a bit.
Sorry I can't be of more help. Phil.

Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 11, 2009, 09:32:32 AM
Hi,
It's not a high gain amp though.
At low levels everything is fine.
There seems to be a buzz coming from the gain control.
The thing is it was diagnosed as 60 hz hum and static. That's not normal.
The amp sounds quite good in actual fact. Very much a Hiwatt vibe.
Great clean sound and nice distortion, not high gain.
I think the mod was to get rid of the buzz.
The real Custom 20 has the exact same valve configuration.
I spotted that cut wire too and they didn't fix it despite my pointing it out.
I'm not opposed to modification, but we have to rule out some of the ideas mentioned.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: phatt on November 11, 2009, 10:01:13 AM
Hi,
It's not a high gain amp though.
At low levels everything is fine.
There seems to be a buzz coming from the gain control.
The thing is it was diagnosed as 60 hz hum and static. That's not normal.
The amp sounds quite good in actual fact. Very much a Hiwatt vibe.
Great clean sound and nice distortion, not high gain.
I think the mod was to get rid of the buzz.
The real Custom 20 has the exact same valve configuration.


Note bold text, Sorry billy but with that many stages there will be hum.
I agree static noise might mean some other problem, like dirty valve pin or cold solder somewhere.

Second;
           The schematic of the DR210 custom 20 shows 3 AX7's and your board has 4 AX7's so something is wrong there for a start.

Yes at low level it might be clean but once you want to open up the master all hell will break loose. This is a serious hi gain amp and I doubt it has DC heaters so even worse with 2 more triode stages.
If it does not distort like crazy when you wind it right up something is very wrong.
Phil.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 11, 2009, 11:18:38 AM
Take a look at some other hiwatt amps. The 4 tube preamp design is classic hiwatt.
This is not regular hum! Hiwatt amps do not distort like crazy.
It's not some kind of super high gain sounding amp.
None of the hiwatts aside from the hiwatt hi-gain series are.
The problem is not with opening up the master all the way. You have to understand that.
There seems to be extra hum from somewhere. The possibility mentioned was the power supply section. Sure it could be like you say. It has to be checked out. The buzz is definitely a problem.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: phatt on November 12, 2009, 04:01:10 AM
Take a look at some other hiwatt amps. The 4 tube preamp design is classic hiwatt.This is not regular hum! Hiwatt amps do not distort like crazy.
It's not some kind of super high gain sounding amp.

That is a big asumption on your part,, When the truth is you have no idea (and niether do I) that this is a direct copy of a Hiwatt.
It looks as though it is a cheap copy,
And the way the heater wiring is laid out it will have some hum.
JM Fahey has already asked you to check the heater circuit ground issue
and you have not replied.
If you want help you will have to do some homework.
I doubt there will be a quick cheap fix for your amp.

Replace all valves,, replace all Electros and see what happens?
Hope it helps somewhat.
Phil.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 12, 2009, 08:40:53 AM
Hi,
Didn't mean to get into an argument!
I meant that Hiwatt used 4 tubes in the preamp in a lot of their amps.
Of course I need help. A lot of it.

I am just repeating what was told to me by these repair people.
It's Class a and a quality amp so they said.
It's debatable. Does sound good though. Actually quite good indeed.
The tubes were replaced by them but one I think has quickly started to squeal when I turn it up.
It's uncertain that the caps and so on were checked .
I couldn't get a straight answer. Yes one day no the next.
They saw a problem, what can I say?
The static is a problem for sure. No doubt about that.
Could bad solder joints cause that?
I'm uncertain how to check
 the pins on the socket. Which ones are 4 and 5 and so on.
Can you circle them?
The amp has been off for about 24 hours now.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: phatt on November 12, 2009, 08:54:46 AM
Hi billy ,,no harm done ;)

Here are just some differences for you to ponder, don't hold me to it cause I can't see the valve pin connections. I'm using the DR210 schematic JMF posted as refference. Then looking at your board.

C1 on your board is 4u7, DR= 220n (so different Value)
R5 is scary as it looks like 2x1k giving 500 ohms (not a common value)
The print on board says 1k5,, again diff from DR
**I would get that changed back to 1k at least as it could take out the triode **:(
C5 or 3 is 2u2,, DR says 220n
Past that point I can't tell but at the other end of the circuit is a *Presense knob usually only found on amps with *Feedback*  Again the DR has *No Feedback*

So the only thing the same is the name :(

Now something I just noticed that may help;
You may have a Ground loop,, Follow the *Green* wire that comes from the board and bolted via an eyelet to the chassis. Then Note the wide track that it's soldered to on the PCB. "That is the Ground plane/track on the board".
Now Follow that ground track all the way across until you come to the *Blue* wire.

(It's the only blue wire there in the middle)

That Blue wire is *Also grounded* at the *main Earth bolt*. *If* that is true and they also join on the board then it's almost certain that you have an ground loop issue.
This would not help by any means.
The blue wire looks as though it has been tampered with at some stage.
PS,
The 2 yellow twisted wires mounted on the back side of *V4* should be the heaters.
Phil.

Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 12, 2009, 09:22:07 AM
The blue wire is for the standby switch.
The standby switch is not grounded to the bolt.
The green wire is from the transformer.

Would more photos help?
They told me the values on the board were returned to stock.
Would changing values to match more the real Hiwatt help?
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 12, 2009, 11:44:00 AM
Bad resistors ?

The piggy back resistors are ok. They are 2  x 3000 or whatever the right value is.
Should be replaced with one 1.5 1/2 watt.

Here's what i found so far:
 board values are first my readings next:
R6 470k  298
R7 330k  245
R8 470k  298
R20 220k 185
R22 Wrong value! 220k on board, 100k

I would say there is a strong probability that the caps are bad too.

I will list all the values later .
I am getting odd readings on the preamp pots.
With the controls full blast, for mid I get 46 instead of 100.

Gain is 455 for 500k so ok.
Mater Volume is 211 for 250 so ok.
Are those pots bad or does it have to do with the circuit?

I tried to check the sockets but I must be doing that wrong, because from pin 9 to the ground on the chassis I get zippo.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: J M Fahey on November 12, 2009, 10:04:48 PM
Hi BillyJoe.
If you measure resistors in circuit (still soldered to the board) you´ll get "wrong" values, in fact always lower, because through the tracks they are in parallel with something else, with a few exceptions, so that, per se, is not bad.
Please try to record and post some sample of your hum, explaining "this is with everything on "0", now I open only the master, to 5, now to ten ... " or something like that.
Also turn your amp on, BEWARE, THERE'S 300 TO 450V EVERYWHERE , with everything on "0", put your multimeter on the scale: 20VAC, press firmly the black tip on the chassis and the red tip on pin 9 of any 12AX7 , write down what you measure. Repeat 2 more times, touching pins 4 and then pin 5 with the red tip, the black one still touching the chassis. Write that too. Then repeat and write with the black tip on pin 9 and red tip on pin 4 or 5. You are looking at the "socket" pcb from below, the tracks/solder side, count pins clockwise, the Nº1 is the first after the "empty space".
The socket "should" have 10 pins, but they use 9 so the tube goes in only one way.
Be very careful when measuring, it's easy to slip a tip and touch something ; the chassis should be stable too, use a couple books or similar to hold it steady.
Good luck.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 13, 2009, 09:44:16 AM
Yes it's a rookie mistake :)
I figured it out after the first one!
But I removed the pigtail monster that was on R5
with a single 1.5 k resistor. They left quite a bit of flux I think on the board there.
Also R22 was the wrong value: 100k instead of 220k,
so that was fixed.


I sense a slight improvement in that I can use the gain at any setting with the Master very very low. Before there seemed to be a buzz even then.
Is this a gain controlled preamp?
I think it has more in common with the DR504 this amp.

I have found a very experienced and very good tech and we are going to see what we can do to improve things.It's a free estimate so I'll let you know what happens.
I'm too chicken to mess with the high voltages.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: J M Fahey on November 13, 2009, 12:51:30 PM
Quote
I'm too chicken to mess with the high voltages.
Don't worry, me too, and i guess we are in good company here.
I have *two* very experienced, very professional friends, very dead , because of the most stupid accidents, which are the ones that get you.
One of them, an Opera Theater lightning master, who studied and worked for years at the Scala of Milan, no less.
The other was installing an extra 3600V (or 4600, for a human it's the same) power transformer at a Continental Foods "frozen chicken in a bag" plant.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 13, 2009, 02:03:37 PM
You never know what's going to happen in life,
even to people who are very experienced. Tragic things happen,
so you never know.
I'll let you know what happens with amp,
I have a good feeling about it.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 22, 2009, 11:32:28 AM
I drew out the board!
This is the view from the component side.
I would say this is 98% accurate.
Just one or two numbers I couldn't see clearly.
Would this help my tech at all?

There is a piece or gap at the top of the board is that by design?

Let me know if anybody spots a problem with the design.
I revised the image with the correct jumper wire position at the input.
I revised R22 which read r27 on rev.1 next to tube 4
(http://img148.imagevenue.com/loc591/th_52736_SchematicRev.2_copy_122_591lo.jpg) (http://img148.imagevenue.com/img.php?image=52736_SchematicRev.2_copy_122_591lo.jpg)
Title: Re: Hiwatt custom 20 tube hum problem
Post by: J M Fahey on November 22, 2009, 03:06:21 PM
Hi Billyjoe, congratulations!! Impressive work, it really shows you are comitted to solve it.
I'll print and trace it, trying to reverse-engineer the schematic.
You do the same, "4 eyes see more than 2".
What I don't like are those connectors for the tubes, but that's nothing a soldering iron can't solve ;)
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 22, 2009, 03:49:49 PM
It took a bit of time, but with good photos all is possible.
Anything I wasn't sure of I put a question mark.
I'm not sure how to do a proper schematic from the board though.

I sent it to my tech as well so maybe he can figure something out as well.

I really would like to solve the problem, because this amp does sound quite good!

This amp might be an improved version of a Hiwatt made by Sterling Import in California,
in the eighties.It had the exact same name and controls and a 10 inch speaker  and a similar more Marshall type sound rather than classic Hiwatt. Of course it had a hum and buzz problem too but not on all of them. I can't find any pictures of that amp though.
At least on the Korean version they separated the tubes from the main board.
The american one had plenty of tube problems because the tubes were on the board.

How are the tubes usually connected to the board?
Title: Re: Hiwatt custom 20 tube hum problem
Post by: J M Fahey on November 22, 2009, 10:45:22 PM
Hi.
Good cables soldered at both ends, the board and the tube socket.
This last one can't be avoided, but thanks to "old technology", each female terminal is a tinned sheet metal tube (it *should* be tinned phosphor bronze) , slit along one side to give it some elasticity, and surrounding 90% of the tube pin.
The typical ribbon connector uses a square post (not bad) but the thin  metal strip that mekes contact with it touches only 1 (one) side, incredible as it may seem.
*Some* high end connectors have an "Y" shaped terminal that touches two sides.
I personally *don't* use the ubiquitous .1" spacing connectors and only use the much fatter and stronger .156" ones on power boards that might have to be exchanged by the musician himself on a faraway province (state).
Google some old Marshall and Fender boards or layouts and you'll see the "old way".
Problem with modern Bugeras and the like is over abuse of connectors everywhere to simplify automated manufacturing.
Even Peavey Classics or Bravos, or Fender HRD's and similar ones have problems that are solved by pulling the connectors and soldering the wires straight into the now free PCB holes.
They *do* work reasonably on computers, because they are usually goldplated, carry very low voltage and current, but mainly because computers lay resting on desks or tables.
If you took your CPU every weekend to a smoky Club, soon you'd start to have malfunctions.

Title: Re: Hiwatt custom 20 tube hum problem
Post by: phatt on November 23, 2009, 12:13:03 AM
Two things that need to be looked at before you go to far;

1/ AC heater wire up and layout. If AC and no CT (centre tap) then you are looking for two small value R's (50 to 100 ohms) across the heater circuit going to ground.

Without a ground reference then there is *going to be excessive hum*.
These R's (If present) maybe on the Valve PCB. (remember the Valve pcb is double sided) look on the Valve side of the board.

My mate's Fender Pro Junior has these hidden on the valve PCB and you have to pull it right out to see them.

The only other way is to lift the CT of heaters to a reference voltage but I doubt this is done on this amp.

 2/  Transformer leakage. Check how close the power transformer is to the Preamp valves. If the valves have no shielding cans and PTr's are close then it quite possible to have induced hum.

You can ground a flat plate of steel via an aligator clip and slide it between the Tr and most sensitive preamp tubes to see if it reduces the hum.
Some tube geeks build the whole circuit and then move the Tr's around untill they find the best mounting point.

old Tr's came with shielding bell ends but these days they don't seem to bother.
Some TR's have a *copper strap* wrapping around the core to help reduce EMF but the old bell ends where the best way to go.

Yes JMF is on the ball all those connectors are painszzzz.
"If you want it to be bullet proof then Solder everything"
That comment came from an old chap who spent his life in AM radio.

If I get time then I'll have a crack at a schematic as well but the 2nd pcb would help,,,
just a pic of the other side with valves removed would help out.

Good work on the picture :tu:  Phil.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: phatt on November 23, 2009, 12:58:14 AM
One thing I just noticed,
The wire jumper next to the notch does not line up with your previous picture.
The little yellow wire crosses to ground track on pic.

Which raises an interesting thought,, hum.

The ground of that input socket would be better connected straight to the Main Green Ground wire.
The way it's pictured the small signal high imp path will certainly pick up unwanted noise as it passes all the *high current* components on the long way back through
the *Earth Track* to ground.

This is bad practice if you wish to reduce noise/hum.

The efx loop is a dozzy as it takes straight off the tone stack,, ouch don't think that's a good idea either.  but no matter,,, hum problem first.
phil.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: phatt on November 23, 2009, 09:38:35 AM
Hi some schematic's,,
There may well be errors but others will soon pickup any oversight.

If your Heater circuit *is* Center taped then consider the ground plane situation I mentioned before.
I've found two ways for that if you are interested.
Hope it helps,, Phil.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 25, 2009, 11:24:52 AM
One thing I notice now after looking at a few Hiwatt schematics,
is that there does seem to be an attempt to follow the older Hiwatt scheme of things,
at least here and there.
I think Tube 1 is a variation of the normal channel on the Hiwatt sa412.
 On the old Hiwatt the other half of the tube  would be the brilliant channel.
We see similar values, the big 1m resistor at the beginning and so on.
Not exactly the same thing, but there is some resemblance.
The new Fernandes amps are a little different.

I think it's a bit of a mish mash of different Hiwatt amps, not an exact duplicate of anyone in particular.
Perhaps the Sterling Import one, but there are a lot of hiwatt amps without much info on them.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: phatt on November 27, 2009, 12:00:11 AM
I've never seen an *SA412* but with your amp the ONLY thing that is some semblace of HiWatt is the PI topography ,,,datz it.

AFAIK,, No Hiwatt used the extra front end. It's dumb noise prone stupidity.
Marshall hot rodders found out the fatal flaws of wiring up the extra triode like that. they just osscilate and any extra gain is rendered useless.

My advice for you is just loose the first AX7 and wire the preamp back up to standard hiwatt specs,, you will thank me one day.

If you wish to underatnad how hard it is to get that many stages to work without issue then download the *Carvin Legacy* schematic and have a squizz at the Hot channel and count the triodes.  ALL heaters are DC,,, only way you can make it happen.

Cheers phil.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 27, 2009, 09:45:10 AM
Here's a schematic of the sa412.
(http://img209.imagevenue.com/loc118/th_32628_hiwatt_sa412_copy_122_118lo.jpg) (http://img209.imagevenue.com/img.php?image=32628_hiwatt_sa412_copy_122_118lo.jpg)

Could you explain more about the front end?
It's a bit over my head.
I just thought in a general way a bit of something here and there.
Was the person who cut the traces attempting to do what you say?
Is the gain all coming from tube 2 on this amp?
I think that this amp is a reissue of the american sterling import amp.
But how much that resembles a hiwatt I don't know.
I was thinking that if I get bad news about the repair, how tough would it be to maybe stick the trinity amps triwatt in there or the ceriatone board?
Can dc power even be done on this amp and if so what would the cost be?
Title: Re: Hiwatt custom 20 tube hum problem
Post by: phatt on November 28, 2009, 09:31:36 PM
Hi billyjoe,
I'll assume your use of the word *Gain* means some kind of OD/distortion?
That big fat rich OD is coming from the power stage not preamp triodes.
Triodes are only doing voltage amplification/ Voltage gain.

Generally they only do halfwave clip which aids the powerstage fatness a bit but without additional circuitry,,, No big fat distortion comes from them.
The whole point of tube amps is power stage compression without that you will be eternally frustrated.

You can build a no frills EL84 amp with 2x AX7 and 2x EL84 with all the gain you will ever
need,, just by refining the *POWER STAGE*.
Maximising the compression effect *Inside the powerstage* is the secret to success.

In the words of King TUT; "If you want MAX tubyness then simple cathode biased amps are for you."  All the bigger stuff is much harder (READ $$$$) to get it to happen.

I personally went round in a big complex multi triode preamp circle and now I use one of the first tube amps I ever built.:)
a very simple cathode biased amp. (2 knobs,, volume and tone.) 
ALL the rest is done via external mostly SS little trick circuits I've built.
When I do need extra volume I just take a speaker line out to a second 120watt SS poweramp and a big quad box.
This negates all the needless complexity and expense of big tube amps.

I play at a local muso club and I'm competing with Cyber twins, Carvin legacy's and the
manditory Fender devilles. I have no problems pulling all the needed tones of a good guitar rig. If truth where known I think it pains them moreso because they know I'm also using a $150 stock chinese strat copy.  :o

The master volume era was created so one could overdrive the input and adjust the master to lower level. Although it may seem like a logical step it's conterproductive as it kills off the magic of the power stage. That's why you don't split valve preamps from valve power stages with things like efx loops or SS reverb loops as you break the magic bond between the preamp and poweramp. SS amps are completely different so fine do what ever you want but tube amps work best as one complete unit.


My learning is thus; **Little amps always sound sweeter**,
because by design the amp spends more time in *Powertube Compression* mode.
in a nut shell;
The bigger and stronger the power stage the harder it gets to make magic as the bigger amps have to run a lot louder before they compress and the power supply sags.
You fight an endless circle constantly adjusting between preamp dist and master volume.
some big fenders run so clean you may as well be using SS.

Re heaters;
Don't worry bout that for now as you are obviously considering gutting this amp and if you start out with a simple EL84 powerd Ceriatone kit you won't ever have to concern yourself with needless complexity of DC heaters.

Re gutting the amp.
The trinity hiwatt uses 6v6 or kt66 so not even close, different tubes give different
sound/tone/feel/dynamics. but hey if it's well thought out it might be just the ticket.

Get this one nailed on your list of things worth knowing It may help unravell some
confusion when discussing power valves.

TC (transconductance) in simple terms *Gain*
(the most over used word in the amplifier world)

TC of common guitar power tubes,
(Tetrodes verses Pentodes)
T 6V6 TC= 3,800
T 6L6 TC= 8,000

P EL84 TC= 11,000 (apprx)
P EL34 TC= 11,200

Armed only with that info even a novice can now see why the term,
"British sound verses American sound" comes from.
The Type of output tube makes a big difference to the end result.

An EL84 only needs a 30 volt swing on it's grid to go full output whereas a 6V6 needs about 70 volts swing. (I think a KT88 needs 90 odd volts on the grid)

Now that does not mean that a 6V6 can't produce good distortion but you have to know how to build an output stage and know the tubes you are using.

The Tone stack in the SA412 it the best you'll get and you can build it as a stand alone unit if you wish and take it with you to any amp.
Read my blab under Schematics and layouts
http://www.ssguitar.com/index.php?topic=1136.0 (http://www.ssguitar.com/index.php?topic=1136.0)

Keep in mind a big part of Hiwatt sound is due to the use of *FANE* Speakers

Some homework;
Try Valve wizard pages,, Tone Lizard site (go to the tone lounge for some good grounding in Valve gear.

Have fun Phil.

Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on November 29, 2009, 10:25:44 AM
Thanks for the info it is much appreciated :)

I tried to put in a speaker that was as close to the Fane spirit as I could find.
I also put in an original Mullard phase splitter 12At7.
I put in an eminence ramrod 10 inch which actually I think is more like a celestion 12 inch.
The frequency response is like that of a 12 inch. If you use the Mullard el 84 reissue tubes from New Sensor, you get a tremendous bass sound, very much like a 12 inch speaker.

I looked at the celestion 10 inch speakers but the price was a little steep and the reviews not so incredible. Right now there are jj power tubes in there, but I think the tone is actually better with the Mullards. You lose some rock edge though. Of the new preamp tubes I tried,New Sensor Tung Sol seemed to sound best but didn't last past 4 months without microphonics at high volume. One thing I found was if you put in a high gain tube after the phase splitter like a Ruby tube 12ax7, you actually clean up the distortion and get a smoother singing sound. Right now there are Mesa Boogie chinese 12ax7's and a mesa boogie Sovtek after the phase splitter. The tubes tested bad so they were replaced.

But I think this week we'll know exactly what's going on with this amp and if there's a solution or whether I might have to throw in the towel on it. Maybe the Egnater Rebel 30 is a good alternative or Mesa Boogie 5:25?
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on December 17, 2009, 02:21:35 PM
Amp is with my tech right now.
There's absolutely nothing wrong with any parts on the board or the tubes.
Tube 4 has 350volts instead of 285 however. Can this cause buzz?
The hum seems to be coming from the power supply. Any ideas?
We thought it might be a too small output transformer or they installed the wrong one but the numbers match the tranny's on someone else's Hiwatt amp and he has no hum.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on December 19, 2009, 12:45:39 PM
Okay. I think it's the wrong output transformer.
According to information I have from someone who has a hum free Hiwatt it's supposed to be an output transformer labelled MOT 21 GT C310.
Mine is labelled MOT 21 GT 0310 . Don't know what the difference is.
Would this cause these problems I've mentioned?
The Power transformer labels match.
Any ideas?
Title: Re: Hiwatt custom 20 tube hum problem
Post by: phatt on December 20, 2009, 10:29:57 AM
Okay. I think it's the wrong output transformer.
According to information I have from someone who has a hum free Hiwatt it's supposed to be an output transformer labelled MOT 21 GT C310.
Mine is labelled MOT 21 GT 0310 . Don't know what the difference is.
Would this cause these problems I've mentioned?
The Power transformer labels match.
Any ideas?

NO!!  that's about as reliviant as the lunar cycle :duh

*Heater Ground reference First*,,, Until that is known I doubt anyone can help.
JMF has explained all that part already?

Then establish all Com points are in fact at zero,,
Then check diodes are all ok then check All PSU Electros.
(one might have gone open causing excessive hum)
Phil.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on December 20, 2009, 11:24:19 AM
From what I was told the diodes, tubes caps are all good.
He suspected the transformer because this was the case with a Marshall amp. They installed the wrong part in that case.

 But two techs agree it's coming from the power supply.
But what exactly do they mean?I believe he checked all the pins and so on and it was good.
One said that there was a glitch on the board the diodes tested good but they shouldn't be.
Could the transformer be faulty somehow? I think he's going to test that next.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on December 20, 2009, 01:39:42 PM
The center tap is grounded and the voltages check out from the tubes.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: rednefceleb on December 20, 2009, 06:24:35 PM
I have been following posts about 20 watt Korean made Hiwatt 1x10" combos that have  a buzz/hum problem. I believe it's hopeless.  If anyone figures it out, spread the word,    P.S. Don't buy Korean amplifiers!    American or British made only   !!!!!   Happy Sunday.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: phatt on December 22, 2009, 06:29:27 AM
Hi, rednef,,Thanks for the timely input.

I believe I know what ails this amp that being *Deleting* the utterly useless V1 and jumping a few wires to bring back to a more civilised setup.

I'm just going off *billyjoe's* layout so do double check for possible errors.

There maybe even better results by lowering the gain pot to 250k as this amp will likely still have heaps of gain that will never be used.

Cheer's Phil.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on December 22, 2009, 01:59:07 PM
This is getting interesting!
I'm kind of chicken to cut traces though.
Next week I'll find out what's up on this amp.


What I seem to be getting is that the lack of schematic is
making it a bit tougher than usual to follow the signal with the scope.
It's coming from the power supply though.

It's interesting that you're cutting the same two traces as before,
but I can tell you the hum was still there almost as bad.
There was also very little gain.Low gain in my opinion for lead.

There was no buzz with the distortion however.
There were two low pass filters on the preamp on the input and output, were they there
to eliminate the buzz? There is high static on this amp.
If so they worked.

Could it be that on rednefceleb's amp the board is slightly different?
Would too high voltage on tube 4 cause buzz?
How  tough would it be to get rid of all those connectors?
Title: Re: Hiwatt custom 20 tube hum problem
Post by: phatt on December 23, 2009, 08:04:19 AM
Quote from billyjoe;
"It's interesting that you're cutting the same two traces as before,
but I can tell you the hum was still there almost as bad.
There was also very little gain.Low gain in my opinion for lead."

Yes but look at the EFX loop *THAT* is where a lot is lost.
Replace *R15(100k)* with a 100 Ohm to 1k Resistor and Delete R16 (56k)* and you will have all the gain you will ever be able to use.

((This assumes you loose V1 and ALSO disconnect the whole EFX loop setup *permanantly*.))

With V1 left *as is* it will then probably just squeal it's head off.

V1 and the efx loop setup just kills this thing IMO.
A 56k resistor across a tone stack is just about as dumb as you can get.

Oh but the market demands you must have an efx loop otherwise it's unkool.
EFX loops kinda work with SS but on valve amps it's a no brainer.
This amps efx loop is about the worst setup you could possibly design in a valve amp.

I've just done some simulations for you and with the mods suggested the Amp will start to distort at half gain volume. With the master at max this would be a very responsive amp.


The way it was setup;
The original circuit I simulated with a 100mV input and with the gain pot on *1* the signal swings to the supply rails after the tone stack triode. That's around 200 volt swing ??? OUCH! (up the gain and you are probably getting into blocking distortion,, whatever it would sound horrible)

So I'm sorry but with the amp *as is* there's not much chance of any dynamic compression or touch sensitivity.
Not a nice user friendly amp.

If you are still having issues with hum after all these mods then consider the way the heaters are connected.
In an amp like this the tracks tend to run Parallel and often around the valve sockets,, not good.

my only other thought is Tr placement might be a problem,, ie remount. and also the tr's may not be well shielded and if so will induce some 50/60 hZ hum.

An outside possibility; *IF* any of the valves are very close to the speaker magnet that can play merry havoc on the sound.
This subject came up a while back on a valve forum,, a Peavy amp if I remember.
Rule of thumb is about 75mm away, the further the better really.

You mentioned loosing the jumper wires ,,Yes but makes for a lot of work.
You would be far better off gutting the amp and hard wire it all using tag or eyelet boards.
run all your heaters as twisted pairs (read up on How to do it the right way not just flimsy loose hand twisting, needs to be tight twisting).
And learn what is known by the guru's, which is run all the signal wiring at *right angles* to heaters and other power wires.
(just about impossible with single sided PCB valve amps, not all things PCB are good)
You will at least learn a heck of a lot about valve amps in the process and that can't be all that bad :)
Here is a schematic of what I suggest, only up to the efx loop point.
You should not need to change the rest.
Just to be sure you understand; When AC heaters are used with old designs (Lower valve count)
Then Hum is not normally a problem but start adding extra and the hum will quickly climb.

A friend just purchased a brand new Pro Junior amp which is about as simple as you can get,,
2 by AX7's and a pair of EL84's,  2 knobs vol and tone.
He rang me after a couple of weeks and asked if it was supposed to hum when you turn it up high?,, is there something wrong? Should I take it back?
I laughed and said welcome to the wonderful world of valve amps and good old AC heater hum.
(Understandable as it was his first valve Amp.)

Phil.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on December 23, 2009, 11:18:39 AM
It's very cool stuff! 8)
I really appreciate your help :)
A bit over my head but I am learning a lot.
At this point I don't know what to do with this amp if worse comes to worse. :-\
Is it a piece of junk design wise? :(
I was thinking of putting in a ceriatone board, but I don't think it will fit.
The chassis on their design is too big.
What about making a new pcb? Perhaps based on your design?Dumb idea?

It could be like you said before that it needs some kind of dc voltage or reference on the filaments to kill the hum. That would seem to be the only solution.
My tech will tell me next week what the story is on this amp.
I'm sure he can fix this thing.

If he fixes it, do you see ways of improving it by changing values?
That's something I could do.
If you could finish off the schematic you'd be doing me a huge favour! 0:)
I simply don't have a clue how to do it.

The funny thing is that with the reissue mullard power tubes it actually was very touch responsive. One thing I did before was put in a High gain ruby tube after the phase splitter, which seemed to reduce the gain down a little and smooth it out.
What about a 12ay7 in v1?
Yes the transformers are tight. But I wonder why the fellow who has the same amp doesn't have these problems? That's what I don't get. ???
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on December 23, 2009, 01:32:26 PM
Is V2A the old V2B?
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on December 23, 2009, 03:48:31 PM
Would this be right for the old V1?
Or am I out to lunch?
Don't know how to do pin6.

  (http://img195.imagevenue.com/loc654/th_01188_Untitled-1_copy_122_654lo.jpg) (http://img195.imagevenue.com/img.php?image=01188_Untitled-1_copy_122_654lo.jpg)
     
Title: Re: Hiwatt custom 20 tube hum problem
Post by: J M Fahey on December 24, 2009, 11:39:21 AM
I guess this is getting long on the tooth (for non-native English speakers: "real old and hopeless")
*Maybe* you should ditch the PCB and build a PTP board.
They are easier to home make than a PCB, you only need some 1/8" nickel plated eyelets , a drill and a suitable piece of insulating board.
Fiber glass would be king, phenolic the standard, I have even used Formica !!!
The Hoffman guys have *very* simple boards and layouts.
I have not found any Hiwatt there, but the boosted Marshall 18W they call the Stout is a fire breathing dragon.
http://www.el34world.com/schematics.htm#Hoffman%20Board%20kit%20information.
You can still use most of your hardware.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on December 24, 2009, 01:44:39 PM
What about these guys?
http://guitaramplifierpcbs.com/liteiib.aspx
Title: Re: Hiwatt custom 20 tube hum problem
Post by: phatt on December 25, 2009, 08:08:13 AM
Sorry for my oversight,, yes The *New* input would be on pin7 of V2A as V1 is no longer present.

Be careful comparing other amps of same name.
Has to be EXACTLY the same,, ie same *Model* nu written on the back, don't just assume by a front name plate as it often means little.

Be very aware that manufacturers are always changing things even though it may look identical it may well be a very different amp inside.
Without lifting the hood you can't be absolutly sure.

I believe I've already given you the schematic so this is just getting a little hard to follow. If you wish to continue this maybe PM me and I'll review this thing for you in private. 8|
Yes we do have to consider the other folks who read this.
Cheers, Phil.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on January 06, 2010, 04:38:32 PM
I believe the amp has been fixed!
I'm going to pick it up tomorrow and hopefully no surprises.
He put in a trim pot to adjust the bias and replaced one of the caps with a bigger one.
I can't remember the details, but with the standby on the voltage was -24 and with it off -14.
He adjusted it to 35 and the hum and buzz were reduced or eliminated.
Not sure. Something along those lines.
I'll know for sure tomorow.
So far so good I think.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: J M Fahey on January 07, 2010, 03:38:27 PM
I smell something fishy here.
Please try the amp *clean*, at low volume, play complex chords, let them die naturally.
Are they chopped off abruptly?
Your amp may be biased colder than Novosibirsk in December.
As a reference, the EL84 datasheets *stop* at -15V.
Of course hum, noise, your music, everything dissapears at -35V !!!!
You may even pull the output tubes, it will hum even less.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on January 07, 2010, 04:01:55 PM
I'm hoping I didn't get screwed here!
Just got the amp back so I will test it.
I heard it at the tech's place and it sounded okay,
but it wasn't a super thorough test.
I will shoot a picture as well.

He told me it was biased too low.
He changed a capacitor for the higher voltage. Does that sense ?
Perhaps he was talking about the current per tube?
If it was 14ma would it make sense then?
I will check it out further tomorrow, it's winter here so the amp is a little frozen from the 45 minute drive back.

He told me the input jack is not a shorting jack as well.
Can this be replaced? Any reason why it can't be changed?
Title: Re: Hiwatt custom 20 tube hum problem
Post by: J M Fahey on January 07, 2010, 07:11:33 PM
Yes, you *should* have a shorting jack input, like all guitar amps.
Very high gain ones have an extra switch that shorts the first or second stage, to *look* silent.
The best way to test is to put a shorted jack in the input, so whatever noise you hear, comes from the amp (or some interference it catches directly, not through the guitar).
The overbiased output tubes become noise gates; problem is, they chop low level music.
In fact, when I have to repair an amp with only a bare multimeter or worse (it happens all the time), a quick and acceptable way to setup bias is to rise gain until some hum/hiss is heard, bias it cold until noise dissapears, and lower bias voltage until hiss/hum come back, and then a little more.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on January 07, 2010, 08:46:39 PM
The amp sounds terrible! There is no clean sound now.
With the master at 9 am and the gain at 9 am I get distortion.
This was not the case before.
There is no dynamic response from the tubes either.
Chords seem to be cutting off. Especially at low volumes.

He told me the bias is set at -30. Is that too low or too high?
He told me it's the right bias for the circuit.
Maybe it's a speaker or connection etc.
I think I might have been had here.
What should the bias be for el 84?
It was -14, sounds right to me.I'm going there tomorrow,

A  pot was installed where the resistor was for the bias and a higher voltage cap was installed just before it.
There are absolutely no resistors that I can see under the tube pcb.
Would a 68k resistor on tube 1 get rid of hum?
Should I make him put the bias back where it was?



Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on January 08, 2010, 01:58:00 PM
What's going on here?
I was told this amp is not cathode biased according to the schematic and is class ab.
Pushpull. -14 was too low and now it's -30.
R38 was replaced with a pot and higher voltage cap for c23. Does this make sense to anyone?
I'm getting mighty confused and angry.
What exactly is the bias type originally and now?
Title: Re: Hiwatt custom 20 tube hum problem
Post by: tonyharker on January 08, 2010, 02:55:24 PM
Just looked at the characteristics for the Philips EL84.  For class AB Push pull grid bias should be -14.7 with B+ at 300v and -11.6 with B+ at 250v. 
-30v is sending your EL84s into cutoff which is why you are getting so much distortion.  The quiescent plate current with no signal should be 7.5mA in each tube.   For 17W output the plate current should be 46mA.  so your original bias of -14 was correct and you should get your tech guy to put it back to that value!  Alternatively get someone else to look at it who knows what they are doing!  ::)
Regards Tony
Title: Re: Hiwatt custom 20 tube hum problem
Post by: J M Fahey on January 08, 2010, 03:03:08 PM
Hi billyJoe, calm down.
I bet the technician is as angry, confused, pissed, out of his mind, whatever,  as you.
You two are killing yourselves over a terribly designed piece of cr*p.
Consider pulling the PCB and building something like Marshall 18 Watt, Soldano Atomic 16, a Tiny Terror, an AC15, or even a mini-JCM800 with 2xEL84.
You can later add as much gain as you wish, but start with a no nonsense working amp.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: phatt on January 09, 2010, 09:08:13 AM
Just looked at the characteristics for the Philips EL84.  For class AB Push pull grid bias should be -14.7 with B+ at 300v and -11.6 with B+ at 250v. 
-30v is sending your EL84s into cutoff which is why you are getting so much distortion.  The quiescent plate current with no signal should be 7.5mA in each tube.   For 17W output the plate current should be 46mA.  so your original bias of -14 was correct and you should get your tech guy to put it back to that value!  Alternatively get someone else to look at it who knows what they are doing!  ::)
Regards Tony

I agree with it ,,,cept the 48mA @300VHT.
Assuming the plate voltage is normally close to B+

Then 300 x 048 = 13.66 ? Ouch!
Last time I looked Max plate dissapation was 12 Watts for EL84?

A vast majority of these copycat import amps have overly high B+
and this amp has the screens running at close to B+ so even at -15volts Bias
The Valves have little safty margin.

My bet is that this amp will have a B+ in excess of 350 VDC so my first port of call on this amp is insert larger value screen drop.
This concept is now becoming almost a unversal understanding as more people realise the screen grid is the weakest link in the valve.

It's interesting to note that there is only a couple of watts difference in output power from 250 to 320VDC and then an inaudible difference from 300 to 350VDC.

The funny part is that a pair of EL84's sound so much sweeter running at 250VDC.
Bigger voltage = less tone,, but no one gets the joke. :duh

Cheers, Phil.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on January 09, 2010, 04:17:36 PM
Checked it again; still hums and buzzes.
With gain high and master volume pretty low it's okay. The guitar covers it and it's a ral rocking sound.
Should I leave the pot in or put back in the resistor?
The resistor would make  -14 again.
Did some tests.
Pulling v3 and v4 nothing connected to input,
no buzz hum or anything.
V2 pulled out, no buzz, some hum.
V1 out, buzz some hum.
Plug a guitar cable to  input but not to guitar buzz goes up.
Could the input jack be part of the problem here as well?
Title: Re: Hiwatt custom 20 tube hum problem
Post by: J M Fahey on January 10, 2010, 09:14:56 AM
Hi billyjoe.
What are you talking about?
We are talking -14 *VOLTS* bias *VOLTAGE* for  pair of 6BQ5.
You just referred to 25 to 27 *MILLIAMPERES* cathode *CURRENT* (which is quite low), yet set the bias VOLTAGE to that value. :loco
You are looking everywhere on the world to blame the hum and hiss, beyond the real suspect: poor design and layout, hidden behind a label they don't deserve. .
*If* the bias supply is poorly filtered (which I don't think is) , the hum will be inaudible, because it is balanced out; and in a *very* poor situation, it might be heard as 50Hz tremolo, generating "ghost notes", but anyway, it would be *constant*, not depending on gain or any control, which is not your case.
If you wish to do something with that amp (besides selling or junking it), pull the board and build a Tiny Terror, that has gobs of gain yet is simple enough to be built with reasonable possibilities of success, or even better, a Marshall 18 which is killer, although you'll have to drive it with some pedal of your choice for the high gain sounds.
Don't waste any more time and money on that nightmare.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: Brymus on January 10, 2010, 04:28:20 PM
What caps are you talking about ?
And seriously the wiring on that is a mess,and probably part of the reason for your hum.
I cant believe how they did that,It was obviously to save labor,using harnesses to connect the tube pcb to the main pcb.
But the lead and dress is suffering horribly from it.
Not to mention the tubes look way to close together.I would guess they are part of the problem as well.
You already have the most expensive parts needed to build a good amp.
The tubes ,chassis,and most costly the trannies.
You could salvage the pots and the "candy" caps (the brown ones) as well.
Maybe even the filter caps too.
Ditch that and build an 18 watt Marshall,or something similar.
Using an eyelet board and some proper lead dress.
Otherwise I doubt you will ever be happy with that.
Yeah they (techs) try to fix it for you.
But if they told you,a new amp was needed you would just think they were trying to scam you (probly why they havent said that here)
So rather than risk losing some work in todays economy - they do what you ask and try.
You can polish a turd all you want....but its still a turd.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: J M Fahey on January 11, 2010, 12:03:06 AM
Hi billyjoe.
Then your *only* chance is to borrow your friend's silent one, open both chassis, place them side by side on a table, get *good* lighting, and compare them component by component, track by track, wire by wire.
I understand his is an original (or correctly repaired), yours has been heavily abused, get both exactly alike and they *probably* will be and sound alike.
Not even that is a guarantee of success.
Gremlins do not exist (that coming from somebody having Irish ancestors is a hard thing to say), but they *should*, to explain many electronics things.
I understand how you feel, in fact all we do here in SSGuitar, you are aware of that, but only so much can be done on "remote control".
Good luck.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on January 11, 2010, 10:14:14 AM
I certainly appreciate your help and everyone else who's chipped in.
It's a frustrating amp this thing.
I think the amp had a somewhat better sound at -14.
Could the bias affect the loudness?
It seems louder than before.
But the output transformer is very, very close to tube 1.
I think the hum is tube 1 position, buzz from tube 2 position.

You know the problem with the guy who has the amp, he's way over in California.
If he could check the parts on the board and shoot some photos maybe you could see something but  I don't think he can.
I have one last fellow lined up, who is a retired tech,
so I think he's the best man for this troubleshooting job.
After that :'(

Title: Re: Hiwatt custom 20 tube hum problem
Post by: phatt on January 11, 2010, 09:47:23 PM

It's a good hi-gain sounding amp, on the noisy side.
I will probably try to sell it at a good affordable price for someone.
Like I said, turn it up and it roars, noise and all.
It will kick the crap out of a vox night train or Haze or tiny terror any day, believe me!

Get your facts sorted if you want good advice!

From the very start I made the comment that this was a high gain monster but you totally denigned that stating it was way to clean and nice.  So make up your mind :duh

I've done enough simulations to know that this preamp is capable of swinging in excess of 150 volts at the master volume knob at which point the amp has gone way past distortion and into crud tone,,, Far out!!! :o Ouch and plain stupid circuit.  :loco

An EL84 only needs 30Volts signal swing on its input grid to produce full power.

Put an audio signal (CD cable) into efx return and see if you can get **Clean music** coming out the speaker.  If you still have crossover distortion there then the bias is way off.
Untill you define the B+ voltage there is no way to establish a bias point anyway.
I and others here have made several intelligent suggestions,,,, most of which seems to get fobbed off and you then become fixated on another part of the circuit.
So you are really adding to your own confusion. :-*

I'll leave to you to go back and read the many comments that have been laid down,,otherwise I'll just be repeating myself  forever ever ever.
Have fun,,, Phil.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: J M Fahey on January 12, 2010, 05:10:25 AM
That makes two of us .....  >:(
Specially after fresh comments like:
Quote
I spoke with my tech a very knowledgeable and great guy.
Here we go again
 
Quote
   
There is room to move the transformers, but someone would either have to use, say every second socket on the existing pcb and add in a new one, to space them out,
or make a new pcb, punch in some holes and move the transformers back.
Sounds easy , try it.  :tu:
EDIT: I found this, a few posts behind:
Quote
Well they should be honest and say "i can't fix it" and that 's fine.
Don't know why, but I believe telling you that is impossible.
In a nutshell: after 6 pages of answers, over 80 replies, over 3800 reads, we are back on square one?
Count me out.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: phatt on January 12, 2010, 09:51:59 AM
When you get off the high and mighty and act in a rational manner you will get all the help I can give,,, till then bash a wall.

"You think it can be fixed? Then fix it!",,, IF you Give me the info I ask for then I can improve it somewhat.
But without modification this amp will always HUM.

The Amp DOES have major design flaws and if you keep insisting it must be a quite amp because your mate has one then I doubt I can help you.
AS is the Amp is a dud.

YES!!! the Tubes Are to close together.***
YES the power transformer has no shielding.
YES it has to many gain stages.
YES the EFX loop is killing it.
NO!!!! the board is not right. (Ground plane is wrong.)

NO Without knowing the B+ Voltage I have no way of knowing the correct bias.

NO I can't do a FRIGGIN thing to help you if you get stroppy.

BTW I think there may be a misunderstanding about bias voltage,
that being the bias of -35VDC or whatever,,, was being taken while the amp was in standby,,,,,You need to know the **PLATE Voltage** and the bias voltage while the amp is ON,, not on standby.

Don't get angry get even,,, Educate yourself and learn how to fix/build these things yourself,,,, but that requires a lot of patience and reading. another hint ;)

Do the ground work and I'll help,, go off half cocked and get mad again and I'll leave.
Cheers Phil.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on January 13, 2010, 11:01:02 AM
How about getting the ceriatone 504 head and using it with the amp?
I don't think i can fit the chassis in here.
It's too big. The board alone is not expensive.
But can it fit in the existing chassis?
Can the wattage be lowered on it from 50?
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on January 13, 2010, 12:25:38 PM
Is it possible to change the mounting bracket on these transformers?
I f the bracket had longer slots I could move the shielded transformer further from v1, probably a good 1/2 inch.There's plenty of room on that side.
What about flip flopping the transformers? Changing sides?

 Changing value of gain or master volume have any effect?
The other transfomer is a bit more of a problem, but I think I can get bell ends.
It's really  a last try, after that...

A 12ay7 in v1 lowers the hum somewhat, about same effect as cutting traces.
Zero effect on buzz.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: phatt on January 13, 2010, 09:45:45 PM
Delete V1 per my previous recommendation, Then take it from there.
Phil.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on January 14, 2010, 08:34:53 AM

If there is info you need I could ask someone to check that and just go with a list:
this is the info we need.
There are some guys who don't charge or charge very little for looking it over.
 Do the caps have to be removed from the board to be checked?




You're right there is a lot of gain here :)
(stop laughing) :) I admit defeat.
In my defense I haven't heard the thing in months.
For me it's not a super high gain amp, but you don't need a distortion pedal.
Perhaps it's high gain 7- 7.5 out of 10? with 10 being out of control hi gain monster?
Maybe I can post a power chord so you can hear it.Or the hum.
But it's the crap tubes they put in too.
Sovtek wb are not helping. Yuk.

The clean sound is (from 0 to 10)
Mv at 3 gain at 3.
Going up with MV control gets more hum. Gain up, some buzz.
Can the buzz be filtered out as before with filters,rather than hunt down the cause?

You could use the amp with mv very low (below 1 really) and use the gain at whatever setting. Not very loud but no buzz or hum.

I'm trying to get the transformers out of there but the one thing they did right was glue the bloody bolts on like there's no tomorrow! How do these bolts work?
If moving the transformers has no effect then :-\
But now I can't get them out.


What about a 470k resistor on tube 2 where the trace was cut?
I'm going to try this.The solder is there.
I see it on a lot of Hiwatt amps including the sa412.
This should cut down the gain somewhat?
I'm going to try it tomorrow just to see what happens.
Dumb idea maybe but what the hell.Why not?
What about a 68 k resistor on tube one? A grid stopper. What exactly is that?
Title: Re: Hiwatt custom 20 tube hum problem
Post by: phatt on January 15, 2010, 08:20:50 AM
Hello billyjoe, Sigh :'(

This is the very reason you go in circles:
I quote
"What about a 470k resistor on tube 2 where the trace was cut?
I'm going to try this.The solder is there.
I see it on a lot of Hiwatt amps including the sa412.
This should cut down the gain somewhat?
I'm going to try it tomorrow just to see what happens.
Dumb idea maybe but what the hell.Why not?
What about a 68 k resistor on tube one? A grid stopper. What exactly is that?"


Everything you just mentioned will be solved via ****DELETION OF V1****
So If you don't get the JOKE now you probably never will.

Here is a way to quickly delete V1 from the signal path with minimum work.
_________________
BUT ***READ THIS***First;

I am only going on a rough drawing you posted of this PCB layout so I have no way of knowing if it is *Accurate*

If you have no idea what I'm talking about don't do it!!!
If you don't know why the WARNING is there then YOU DOn'T HAVE THE Knowledge and therefore I will *ASSUME* you are inteligent enough not to touch it, RIGHT!!
So get someone to help you with it.
____________
If this works well and the amp has much less hum problem then you just might be
interested in persuing my first comment and suggestions.

If you persist in making it complicated then YOU  *Not I*  will have to endure the
frustration and pain of such pointless complexity.

Get V1 OUT and work from there.

Till that happens you are just shooting arrows with no target.
Have fun and don't do something you may not live to regret. Wink Phil.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on January 15, 2010, 10:40:46 AM
The board is accurate.Don't worry about that. I should repost it with the cap numbers finalized.
I can take out the cap and resistor, but I will get it done to avoid getting fried.
And that's no laughing matter.
But I am pretty sure the hum will still be there.
Can this amp be modified to be even more like the sa412? Dumb question?

Is it a waste of time messing with the transformers then?
I think you mentioned putting a metal piece there with an alligator clip to see if it does anything.
I am having a devil of a time getting two of the bolts loose on the PT.
I can move the OT way back using two of the existing holes, but the laminate will touch the baffle. Or I can drill 4 new holes.

Can this be of any use,like you said before:
Quote
The only other way is to lift the CT of heaters to a reference voltage but I doubt this is done on this amp.
Definitely no resistors under tube board. This is proving very difficult to unscrew though.
Of all the things they did wrong, this they had to do right!

One thing I would like to ask is , is there any info you need regarding plate voltages and so on? Let me know.
I could get it checked for a very small fee and get some written answers once and for all.
So you , and myself included,know exactly what is what with the bias and everything else.

The only odd things  that have come up was too much voltage on one of the tubes.Tube 3 I think.
I think tech  2 mentioned something about the power tubes.
One wasn't dissipating as much as the other?
Can't remember.I asked him twice the next day about it but I didn't get an an answer.
I made him do the pin to ground test on the sockets and there was something he mentioned that didn't make sense. He said the noise on the a tube was 1 point something instead of 3 point something.Can't remember.
And do caps have to be removed from the board to be checked?
Certainly, these were not.
One thing the last guy said was the tubes felt hot.
"Imagine if they were running at 220 like fender." ???
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on January 15, 2010, 04:07:31 PM
Just on a side note. I have been able to move the OT a little further sideways away
from v1.  Probably can go a little further too.If I could find a mounting bracket with longer slots I could get at least 1/2 inch of separation. Gonna have to look around.
Title: Re: Hiwatt custom 20 tube hum problem
Post by: billyjoe24 on January 19, 2010, 08:28:20 AM
There is a problem with the wiring on tube 2.
There is a grey wire which has a blackwire on one end and a black and white wire on the other. The black wire goes to the Ground on tube 2 and the white to 7.
The ground was on very flimsily. I managed to recrimp it, but I am not getting a
continuity beep when I check the black wires. When I check the black and white I get a beep. Is that normal?  I think someone mentioned an open electro as a possible source of hum.
Is that what this is? What kind of wire did they use here?
It looks like someone attempted to repair these by soldering but they have come loose.