Strange Tube Amp Supply Problem

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Strange Tube Amp Supply Problem

Postby rogerk8 » Sun Nov 11, 2012 11:38 pm


I have a strange tube amp supply problem.

I have manufactured 10 pcs of Williamson Tube Power Amps (took me over 20 years...).

These are of stereo configuration.

One out of these ten has this kind of strange problem:

With no tubes connected I get 205VAC (using a special step-down supply transformer) at each channel supply input. And some 284VDC at the rectifier output. Which is exactly the same channel-wise.

Loading the capacitors (with 10k) I do however get 247VDC(Vr<0,70VAC) at the left channel and 259VDC(Vr<0,82VAC) at the right channel.

I have checked that the ripple actually is 100Hz, i.e all diodes work.

Driving AC is in the loaded case exactly the same, i.e 197VAC.

But why do I get different VDC?

In a desperate attempt I have checked that the forward voltage drop of the rectifying bridge diodes is the same. And they are approximatelly the same (differs only some thousands of volt).

When I applied voltage without load I did approximatelly get the same discharge behaviour (when I removed the voltage). That is, I don't think there's anything wrong with the capacitors.

So I have come to the conclusion that:

1) There's nothing wrong with the supply transformer.
2) There seems to be nothing wrong with the electrolythic capacitors

So what is wrong then? The left channel delivers a too low voltage at the same time as the right channel delivers a too high voltage.

The only reason I can think of is that there is something wrong with the rectifying bridges. But I do actually not now.

Tomorrow I will test my desperate theory by changing the rectifying bridge(s) to anothe one.

Does anyone have a clue of this problem?

Thankful for tips!

Best regards, Roger
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Joined: Wed Apr 11, 2012 8:35 pm

Re: Strange Tube Amp Supply Problem

Postby Doc10 » Mon Nov 12, 2012 10:42 am

can you post the schematic?
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Joined: Wed Oct 03, 2012 3:44 pm
Location: Belgium

Re: Strange Tube Amp Supply Problem

Postby rogerk8 » Mon Nov 12, 2012 11:03 pm

Hi Doc10!

It looks something like:

But I do not use caps over the rectifying diodes. And I have verified that the NTC's do not contribute to the problem.

Today I actually tried switching the rectifier bridges. It didn't help.

I now suspect the capacitors (33+33uF) which have a tolerance of +/-20%.

But I am confused that the peak value of the rectifier input voltage (which is the same channel-wise by the way) do not present itself on the outputs. The only difference shoud be the two diode voltage drops minus some ripple which is negligeble in this case (<1 VAC).

But I might be wrong here. Maybe it is more to it than this.

And maybe there is some serious difference in capacitance which could explain why the voltages and ripples are so different (LCH: 244VDC/0,69VAC, RCH: 258VDC/0,83VAC)

Anyway, I lose 32V at the left channel and some 18V at the right channel relative to 195*SQRT(2)=276Vp. And i think this increses if I turn up the driving AC.

All the other 9 Amps showed none of this bad behaviour. They all yielded some 436VDC at full drive. And the overall difference was less than a volt!

I understand nothing!

Best regards, Roger
Posts: 24
Joined: Wed Apr 11, 2012 8:35 pm

Re: Strange Tube Amp Supply Problem

Postby rogerk8 » Thu Nov 15, 2012 11:09 pm


I have solved it!

The mains transformer is the problem.

I have no clue of how but the mains transformer actually delivers different kind of AC to the rectifying bridges (while loaded, only).

One channel's AC is cut and even wider than the other (which probably is why they measure exactly the same with my DVM).

The "cut" channel delivers a lower DC after the bridge than the other channel.

Which is nowadays understandable.

But why?

I have no idea but if I don't throw this unit into the garbage bin I will have to sell it at some discount.

Because who wants a maximum output power of 15W for the left channel and 20W for the right?

Best regards, Roger
Posts: 24
Joined: Wed Apr 11, 2012 8:35 pm

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