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Old 13th May 2013, 12:44 AM   #1
rvmeush is offline rvmeush  United States
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Default Looking for expertise in adjusting Williamson amp

I have been rebuilding a Sound Valves M60 monoblock amp. The power supply needed quite a bit of work, and while I was at it I replaced many of the smaller electrolytics. In getting the amp working again I discovered an adjustment pot in the preamp section, but no instructions for how to set it properly. I'm attaching the schematic and the pot is P1(20K) on the 12AT7. In other amps like this there were identical value resistors used in the plate circuit and the cathode circuit. I could just adjust the pot so that the plate and cathode resistance values are identical - but it seems like they could have done this with fixed resistors in the design. So it seems like there is some other reason for this adjustment.

I have searched for info on the Sound Valves amps and how to adjust this pot, but can't locate anything.

I'd appreciate anyone who has experience with this particular amp or this type of amp to give me some insights on how to make this adjustment.

Thanks.
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Old 13th May 2013, 02:21 AM   #2
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Is that P1 on the panel, or not? This schemo looks a lot like that of a guitar amp, and P1 would be another twiddle knob to deliberately unbalance the front end to add H2 to the output.

If it isn't, then I'd set P1 until its resistance matches the plate load (100K given here, but measure to be sure). Cathodynes don't require external balance like some phase splitter topologies, and so that thingie looks superfluous.
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Old 13th May 2013, 03:02 AM   #3
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It looks like an AC balance control to get identical output amplitudes from both outputs of the concertina phase splitter. You might input a test signal, say 1kHz or so, and look for identical AC amplitudes at the plates of both driver stage plates, which would balance that stage as well as the splitter. Or you can look for a distortion minimum at the properly loaded speaker terminals if you have a distortion analyzer.
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Old 13th May 2013, 03:04 AM   #4
rvmeush is offline rvmeush  United States
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The Pot P1 is buried inside the case on the circuit board - so it's not readily accessible. There are adjustments (P2 and P3) that are user adjustable for setting the bias of the output tubes however.
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Old 13th May 2013, 03:07 AM   #5
rvmeush is offline rvmeush  United States
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Quote:
Originally Posted by egellings View Post
It looks like an AC balance control to get identical output amplitudes from both outputs of the concertina phase splitter. You might input a test signal, say 1kHz or so, and look for identical AC amplitudes at the plates of both driver stage plates, which would balance that stage as well as the splitter. Or you can look for a distortion minimum at the properly loaded speaker terminals if you have a distortion analyzer.
I did come across some info that the setting of P1 had to do with minimizing distortion. Is there some way to get an approximate setting without a distortion analyzer - I don't have one of those in my collection of test equipment.
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Old 13th May 2013, 03:21 AM   #6
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imho, best to leave them alone if you do not have a distortion meter....
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Old 13th May 2013, 05:58 AM   #7
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The desing is a bit strange. P1 is obviously for the AC-balance for whole amplifier, i.e. to compensate the gain difference of output tubes.
But the place of this adjustment is wrong, because the balanced voltage amplifier after phase splitter tends to compensate the phase difference at it's input.
If the total unbalance were for example 1 dB, the cathodyned should be adjusted to, say 3 dB unbalance. Better AC-balance adjustment would be to have one of the anode resistor of balanced amplifier adjustable.

Most people have distortion meter eventhough they do not know it.
Any laptop or PC with sound card together with one of those free or low priced audio programs form a good audio analyzer. Like this one: Latest News. Audio Rightmark
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Old 13th May 2013, 09:01 AM   #8
DF96 is offline DF96  England
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Yes, that is the wrong place for a phase-balance adjustment. And why would one be needed anyway - I thought guitar amps are supposed to have distortion?

Interestingly, there is a small capacitor (C6) which deliberately unbalances the PS at the cathode - unless it is compensating for wiring strays in the anode circuit.
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Old 13th May 2013, 01:59 PM   #9
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Do you have an oscillator or other source of test tone and a DVM?
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Old 13th May 2013, 02:37 PM   #10
rvmeush is offline rvmeush  United States
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Quote:
Originally Posted by simon7000 View Post
Do you have an oscillator or other source of test tone and a DVM?
I do have an oscillator that I could use for a test as well as a Fluke multimeter. How would you suggest using them to make this adjustment?

I also have a Rigol scope with an FFT function that I thought might serve as a crude distortion meter.

My first thought was that I should adjust P1 so that I had equal gain at the plates of the 12AU7, or at the plates of the 6550 tubes. I wasn't sure if I could make any meaningful measurements at the output transformer since I don't have a distortion analyzer.

I haven't found any other Williamson type amps with the same sort of Pot adjustment at the inverter. If anyone has a detailed service manual for the Sound Valves monoblock that might have some info on this adjustment.

Any other thoughts would be appreciated.
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