purpose of R29 & R30

[dhuebert]

What is the purpose of these two pots in the circuit. I am stuffing the board and would like to leave R29 & 30 off if possible due to the difficulty of sourcing them locally.

Don

[mesherm]

Those pots balance the tube bias voltage of each push pull pair.

[dhuebert]

So with reasonably matched tubes I should be able to ignore them?

[mesherm]

You would be no worse off than with a stock ST70. You can eliminate the 1 meg resistor also.
Are you making an amp from scratch or replacing a stock driver board?
If you are scratch building then you could just double the number of 10k bias pots, one for each EL34 and achive the same effect.

[dhuebert]

I am scratch building, but I am not all that fussy about exact matching. Match biasing the tubes only works at idle, what happens along the rest of the transconductance curve is anyone's guess. Besides I'm not one of those golden ear types, I like tube amps because of the high coolness factor.


Don

[Shannon Parks]

Don, you can leave them off along with the 1M resistors.