Hiwatt custom 20 tube amp hum problem

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The transformer on another hiwatt custom 200 has the numbers
GT MOT C310 on the label.
The one I have has the numbers GT MOT 0310.
What the difference is I don't know.
Would the wrong tranformer cause hum and buzz?
 
frank57 said:
The transformer on another hiwatt custom 200 has the numbers
GT MOT C310 on the label.
The one I have has the numbers GT MOT 0310.
What the difference is I don't know.
Would the wrong tranformer cause hum and buzz?
Click to expand...

Very unlikely, it sounds like the guy looking at it doesn't have the slighest clue?.
 
So what could it be?
There is too much voltage on tube 4. 350 instead of 285.
Could this cause the buzz.
There is no noise from the board and all the parts check out.
 
frank57 said:
So what could it be?
Click to expand...

Impossible to say without seeing it, but it's a crude simple circuit, and should be trivial to isolate the fault - unfortunately most repair techs are too young, and don't have a clue about valve gear.

There is too much voltage on tube 4. 350 instead of 285.
Click to expand...

In which case I would suspect it's not drawing enough current, or none at all.

Could this cause the buzz.
There is no noise from the board and all the parts check out.
Click to expand...

Did you ever locate a circuit diagram for the unit?, first move is to remove specific valves in order to locate where the hum is coming from - valve amps are easy to repair partly because of this.
 
Well, he has 30 years experience.
The problem seems to be coming from the power supply.
Another tech agreed with that as well.
There was actually a Marshall amp with similar symptoms and it was a case of the wrong transformer installed at the factory.
Would too much voltage on tube 4 cause the buzz hum problem?
At one point someone installed low pass filters on the input and output of the preamp circuit, I think to get rid of it.

There are no bad parts on the board and the tubes are all good.
The solder joins are all good. There were 20 cold solder joins.
I think he was looking at putting in resistors for the hum.
The output transformer on this amp is bigger than the power transformer.
There is no schematic as far as I know, only the layout I drew.
Would the next step be to test the transformer?
Maybe the part number doesn't mean much.
Can a new transformer be easily tested?
 
Like I said, it's impossible to trouble shoot without it in front of me, but you don't fault find by testing all the components - and I doubt they have been tested in any way helpful towards this fault. As for the valves, replacement is really the only way of testing them - but you can eliminate most of them in seconds, no testing required.

I just wish you were over here, it shouldn't take more than half an hour to find the problem.
 
Well all I can tell you is that the tubes test good.
There are no leaky caps or anything like that.
It might be like the hughes and kettner custom 20 tube , but I cn't find the schematic.
I'll talk to my tech tomorrow and see what the story is on this amp.
Any ideas off hand?
 
frank57 said:
Well all I can tell you is that the tubes test good.
Click to expand...

You mean on a valve/tube tester?, it's unlikely to pick up things like hummimng - they are only pretty basic tests.

There are no leaky caps or anything like that.
It might be like the hughes and kettner custom 20 tube , but I cn't find the schematic.
I'll talk to my tech tomorrow and see what the story is on this amp.
Any ideas off hand?
Click to expand...

Only to isolate where the hum is entering - for a start does it still hum if all valves apart from the outputs are removed?.
 
If I remember correctly, it did not hum with only the power tubes.
He's looking with the scope and without a schematic it's tough to follow the signal.One thing about the OT transformer, this one has a yellow and white wire instead of two yellow wires.Could c mean class c?
Does that mean anything?
I think the hum goes away if you pull out the first 2 tubes.
Before the board was restored it was the reverse.
But everyone sees a problem with the power supply.
The ct is grounded and I believe the voltages from the pins are all good.
He doesn't think it's the transformer.Could those crap connectors be part of the problem?
There were 2 low pass filters on the preamp circuit on the input and output before it was restored. If they were there to eliminate the buzz then it
might be an idea to put them back. What exactly is a low pass filter?
 

No, class C is completely useless for audio.

Should be no problem following it without a circuit, it's a valve amp so EXTREMELY simple.

Does that mean anything?
I think the hum goes away if you pull out the first 2 tubes.
Click to expand...

In which case everything after that is almost certainly fine.


The power supply is even simpler than the rest, the number of components is only in single figures, and a scope will show any problem in seconds.

There were 2 low pass filters on the preamp circuit on the input and output before it was restored. If they were there to eliminate the buzz then it
might be an idea to put them back. What exactly is a low pass filter?
Click to expand...

Low-pass filters eliminiate high frequencies - so wouldn't stop mains hum.
 
Then what's going on that these experienced techs can't find the problem with the scope? Is it a case of simply not wanting to spend the time on it?
 
frank57 said:
Then what's going on that these experienced techs can't find the problem with the scope? Is it a case of simply not wanting to spend the time on it?
Click to expand...

I can only imagine it's down to them now knowing the slightest thing about valve amplifers?, certainly in the UK it's common that musical equipment repairers don't know anything about them.

Valve amplifiers are crude and simple, dead easy to repair.

Here's a fairly representative 20W power amplifier, if a little more complicated than many.

https://ampage.org/schematics/blkpwr20.gif

Really the only things on that that could cause hum are the electrolytics or the valves.
 
Well I think they do, but maybe when it comes to something without a schematic ...How much time should it take to track the problem down?
How tough would it be to do a schematic from the layout I drew?
The thing is I 've been told the caps are good by 2 guys in a row.
Maybe it needs better filtering?
One guy mentioned the diodes. He said there was a glitch on the board. The diodes are good but shouldn't be.
Could it be that the caps are actually not good ?
 
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frank57 said:
Well I think they do, but maybe when it comes to something without a schematic ...How much time should it take to track the problem down?
Click to expand...

I notice you've posted this across various forums, and other people have mentioned the same - if the engineer was competent, he wouldn't be struggling.

How tough would it be to do a schematic from the layout I drew?
Click to expand...

Pretty tough, because of the completely bizarre method of connecting to the valves - but from the actual unit it wouldn't take long at all.


Only someone with it in their hands could say.
 
Could those connectors be eliminated?
My tech hasn't thrown in the towel yet.
I think next week I'll know.
If it's bad news should I just get rid of this amp?
In spite of it all, it does sound quite good.
But is the layout I drew of no help to a tech?

What cap would be the most likely culprit?
 
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Does removing ONE particular valve stop the hum? - you mentioned removing two did, but removing just one should. If so, which one?.
 
I don't think so.
I think right now it's both 12ax7 preamp tubes.
I can't remember. Before it was the phase splitter, but without the phase splitter in I didn't get any sound.
Hopefully, my tech can figure it out.
If not when I get it back I'll check the tubes again.
 
I've had a bit of spare time today, so I've been drawing the preamp circuit, the first valve only uses one half, the other half isn't used, and is a preamp before the gain control.

The next valve is two preamp stages, driving the tone stack which feeds the effect send.

The last valve shown is a buffer from the effect return, and feeds the volume control. The second part of this valve is used as a voltage 'regulator' to bias the phase splitter.

BTW, ignore the value of C7, it should be 470pF.
 

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can you get an electronic tech to come out of retirement to check it out? Young techs don't understand tubes.
I had the last semester of tube training at the tech center in 1976.
 
Wow Nigel! You're a life saver!
This is tremendous work!
I can't thank you enough!

Is V3 actually V2A?
If you can finish it you'd be a real Christmas Angel!
This is exactly what these techs need.
By the way Merry Christmas!
 
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