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Old 22nd August 2012, 02:03 AM   #1
Sherman is offline Sherman  United States
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Default Basic Bias Question for PP Amp

I'm putting together the pieces for yet another KT88 P-P amp with Mullard topology. I am designing the power supply and have a basic question about setting up the bias system. Virtually all the schematics I've examined show a single bias pot for each pair of output tubes, however I've heard several recommendations for using a separate pot for each tube. My question is basically is it worth it to set up the bias circuit with four pots or am I just overly complicating things?


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Old 22nd August 2012, 02:23 AM   #2
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I always use separate individual bias pots for each tube. I want the control they provide. With only one pot for a pair, you either need to closely match the tubes or provide a balance pot, which brings you back to two pots.
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Old 22nd August 2012, 05:59 AM   #3
cerrem is offline cerrem  United States
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There is always pros and cons with any methods...
With a pot on each tube.... you are effectively applying a delta in bias grid voltage.... For example -40 on one tube and -48 on the other tube...
Now you have disimilair clipping points....or uneven headroom....
Feedback can fix some of this but...it still is not correct...
I prefer to keep the bias voltage matched....but instead turn down the heater voltage on the stronger tube.... I have adjustable heater voltage on one valve socket....and i place the stronger valve at this socket..
Make sure to really measure the true PLATE current in SERIES with the plate wire.....not CATHODE current measurements or shunt by-pass methods...

Chris
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Old 22nd August 2012, 03:41 PM   #4
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Individual bias for each tube is a little more complicated, but that's the way I would go.
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Old 23rd August 2012, 06:02 PM   #5
Sherman is offline Sherman  United States
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Thanks for the input! I'm redrawing the power supply to provide individual bias.
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