I have been working on a 6688-300B SE schematic with input from user dsavitsk here, and am at the point where I am happy with it conceptually, but I have one question. The plan is to run fixed bias on the 300B, and reading the WE datasheet, it states that the max grid resistance for fixed bias is 50kohm. I will be using most likely EH 300B's, and the grid bias scheme isnt exactly normal, so I have no idea if this is going to be a problem.
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Mainly to keep the cap size down, and give the driver a bit easier of a load. I have seen plenty of other schematics violating this, apparently without issue, so didn't know if it was all that important. Since it seems to be, I will change it. Make the CCS adjustable from 2mA down, and have a fixed 50k from the grid to ground.
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cetoole said:
I've been designing and building 300B based SE and PP amplifiers for about 14yrs now, and I have not had problems with grid resistances of up to 100K ohms.. Anything significantly higher than that with fixed bias really is asking for trouble. (I rarely use cathode bias in my output stage designs.) Typical 300B draw several or more uA of grid current depending on plate voltage and it can get considerably higher than this before the tube is considered "bad." I've seen north of 10uA in some and even with 100K this did not cause an issue, with 1M it definitely would. AND don't count on it being constant over the life of the tube!
You also comment on making life easier for the driver, but you probably are not thinking about the miller capacitance of this tube and you should. The CCS in the bias circuit is drawn the wrong way around, and had better be adjustable.. Also this scheme can only used with regulated B+ unless you like having your operating point wondering all over the place with changes in your line voltage.
Kevin has it right.
I did some work on an Audio Space Push Pull 300B last week. This amp has fixed biased JJ 300Bs in it. It had 220K grid 1 resistors which I thought was a bit high so I measured the voltage drop across the 220K, exactly 2.0 Volts.
That means 9 uA grid current at idle.
Follow Kevins advice and do not use any larger than 100K.
Cheers,
Ian
I did some work on an Audio Space Push Pull 300B last week. This amp has fixed biased JJ 300Bs in it. It had 220K grid 1 resistors which I thought was a bit high so I measured the voltage drop across the 220K, exactly 2.0 Volts.
That means 9 uA grid current at idle.
Follow Kevins advice and do not use any larger than 100K.
Cheers,
Ian
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