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Hiwatt custom 20 tube hum problem

Started by billyjoe24, November 07, 2009, 01:00:20 PM

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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.


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?


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.)



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. ???



Would this be right for the old V1?
Or am I out to lunch?
Don't know how to do pin6.


J M Fahey

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.
You can still use most of your hardware.


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.


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.

J M Fahey

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.


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?

J M Fahey

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.


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?


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?