Hiwatt custom 20 tube hum problem

[J M Fahey]

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.

[phatt]

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   Phil.

[phatt]

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.

[phatt]

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.

[billyjoe24]

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.

[phatt]

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.

[billyjoe24]

Here's a schematic of the sa412.


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?

[phatt]

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. 

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

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.

[billyjoe24]

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?

[billyjoe24]

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.

[billyjoe24]

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?

[phatt]

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

*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.

[billyjoe24]

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.

[billyjoe24]

The center tap is grounded and the voltages check out from the tubes.

[rednefceleb]

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.