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bias resistor to use for a PP 300B?

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jonusgrumby

Member
Joined 2006
I'm finishing up my 300B PP amplifier. I designed this amplifier. It uses grid bias, with approximately -90 volts grid bias and 435 volts on the plate. The output stage uses 10% cathode feedback so the grid drive voltage is on the order of 250Vp-p. My breadboard used 47K resistors on the grid and I had no bias problems. I have a cathode follower drive so the 47K load is not a problem, but I notice that most other circuits use a much larger value. I have yet to find a recommended value in the 300B data sheets I've found.

I know that 47K is much lower than what you could use on a EL34 circuit, but 6550's specify 50K when used fixed bias. When I read about output tube bias problems frequently the grid bais resistor value is pretty large, and I want to avoid that.

I'd appreciate any thoughts or experiences.
 
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gingertube

Member
Joined 2004
Tom,
I've worked on some Chinese Push Pull 300B Amps using fixed bias with Rg1 values of 220K but that amp had 2V drop across that resistor indicating 10 uA of grid current at idle.
My view is that 220K is too high for fixed bias but probably OK for cathode bias.
Better designs I have seen (but not done any work on) usually used 100K or even 47K as you have done.
I would stick to the 47K that you used in your breadboard.
Cheers,
Ian
 
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