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Author Topic: Hiwatt custom 20 tube hum problem  (Read 45905 times)

billyjoe24

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Re: Hiwatt custom 20 tube hum problem
« Reply #15 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.
« Last Edit: November 11, 2009, 09:36:25 AM by billyjoe24 »

phatt

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Re: Hiwatt custom 20 tube hum problem
« Reply #16 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.

billyjoe24

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Re: Hiwatt custom 20 tube hum problem
« Reply #17 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.
« Last Edit: November 14, 2009, 08:25:41 AM by billyjoe24 »

phatt

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Re: Hiwatt custom 20 tube hum problem
« Reply #18 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.

billyjoe24

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Re: Hiwatt custom 20 tube hum problem
« Reply #19 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.

phatt

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Re: Hiwatt custom 20 tube hum problem
« Reply #20 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.

« Last Edit: November 12, 2009, 08:57:53 AM by phatt »

billyjoe24

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Re: Hiwatt custom 20 tube hum problem
« Reply #21 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?
« Last Edit: November 14, 2009, 08:21:13 AM by billyjoe24 »

billyjoe24

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Re: Hiwatt custom 20 tube hum problem
« Reply #22 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.
« Last Edit: November 14, 2009, 08:22:11 AM by billyjoe24 »

J M Fahey

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Re: Hiwatt custom 20 tube hum problem
« Reply #23 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.

billyjoe24

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Re: Hiwatt custom 20 tube hum problem
« Reply #24 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.
« Last Edit: November 14, 2009, 08:26:31 AM by billyjoe24 »

J M Fahey

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Re: Hiwatt custom 20 tube hum problem
« Reply #25 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.

billyjoe24

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Re: Hiwatt custom 20 tube hum problem
« Reply #26 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.

billyjoe24

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Re: Hiwatt custom 20 tube hum problem
« Reply #27 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
« Last Edit: November 26, 2009, 11:21:05 AM by billyjoe24 »

J M Fahey

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Re: Hiwatt custom 20 tube hum problem
« Reply #28 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 ;)

billyjoe24

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Re: Hiwatt custom 20 tube hum problem
« Reply #29 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?