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Ming Da MC3008

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Calling All Tube Experts

Here's hoping someone can help! I'm sorting out a Ming Da 3008A amp. These are monoblocks which use a 300b driving an 805 output tube. I've attached the circuit diagram. The amp uses a novel bias circuit as shown using a Zener reference and supplying positive bias to the output tube.

As supplied the amp runs its output tubes too hard at 135ma when 95ma is the point to aim for for decent tube life. My question is: if I want to increase the positive bias I assume I lower the Zener reference to give less negative bias to the 300b and also to raise the positive bias for the 805?

Is that correct?
Thanks
David
 

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Note that the circuit is not as obvious as it seems, one side of the bias pot goes to a positive supply (lower cathode of SRPP) and the other to a negative bias supply, in fact the negative bias supply needs to increase slightly. The easiest way to do this would be to reduce the value of that 8.2K resistor slightly and add some resistance in series with the positive end of the pot.

Maybe try 6.8K in place of the 8.2K resistor and add 1.5K on the other side, this should not upset operating points in the preceding SRPP stage.

The other option would be to INCREASE the value of zener regulating the bias supply, but I would not try this until all other avenues are exhausted as it will affect the operating point of the SRPP a bit.
 
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