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Ming Da MC3008
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Old 10th November 2017, 04:54 PM   #11
artosalo is offline artosalo  Finland
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There is a bias adjustment potentiometer (33k) at the grid of 300B.
If you want to decrease the idle current of 805, you must decrease the positive voltage at the grid, not increase.
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Old 10th November 2017, 05:03 PM   #12
kevinkr is offline kevinkr  United States
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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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Old 10th November 2017, 05:09 PM   #13
jazbo8 is offline jazbo8
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@juancho - it’s against the Forum Rules to start multiple threads on the same topic, your threads have been merged.
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Old 10th November 2017, 05:53 PM   #14
juancho is offline juancho  United Kingdom
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Thanks for the helpful replies. The minimum dissipation I can currently adjustable to is 126ma whereas it should be around 95-110 region.So I need to reduce dissipation by 26-30ma. I kinda thought INCREASING the zener value was the way to go.
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Old 10th November 2017, 06:29 PM   #15
kevinkr is offline kevinkr  United States
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See my reply above, yes increasing the zener value as I stated is correct, but there is a better way to do it. (Some tweaking to avoid significantly shifting the operating point of the SRPP may be necessary.)
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