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Old 11th May 2012, 10:47 PM   #1
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Question Strange Stability Enhancement Trick

Hi !

in the article published in Audiocraft by David Hafler, "Modernize your Williamson Amplifier", author introduced strange trick - connecting 1 MOhm resistors parallel to coupling caps leading to output stage. As Mr Hafler wrote, this was done for phase correction purposes in order to enhance stability of the amplifier.

Someone could explain if this technique really working, and if yes, how?

These resistors raise positive DC on the grids of output tubes, and if second 6SN7 fails (e.g. worn out cathode), is very likely to cause overheating of output tubes because of lowering BIAS voltage.

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Old 11th May 2012, 11:05 PM   #2
DF96 is offline DF96  England
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This will be to improve LF stability, by turning a highpass filter into a shelf. This reduces LF phase shift.
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Old 12th May 2012, 12:10 PM   #3
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Hi, DF86,

Thanks for explanation!

I wonder is it possible to use this technique with LTP + PP, as shown on the schematic attached (circled red)? I'm not sure LTP with DC coupling to the output stage (albeit with 1 MOhm resistors) is a good thing.

There is another cap in question left from original schematic (circled green), which have no effect on oscillation.

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Old 12th May 2012, 02:45 PM   #4
DF96 is offline DF96  England
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Whenever you introduce DC coupling you necessarily cause the bias of the stages to interact. This is usually unhelpful, and usually unnecessary as AC coupling is fine for most audio. As the phase splitter is already direct coupled to the input stage for bias purposes you have scope for a cascade of problems if anything fails. Maintaining correct bias in the output stage will be difficult, so crossover distortion or overheating are likely.

I don't know what C5 does, but I guess it is something to do with HF stability.
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Old 12th May 2012, 05:12 PM   #5
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Hi, DF86,

You are certainly right about stability issues of 3 DC coupled stages in a row.

I'm thinking of decoupling pre-driver and LTP, transforming LTP into AC coupled. Due to DC coupling BIAS on LTP is suboptimal, resulting in high THD at high signal amplitude.
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