Hi Gordon,
What's the purpose of the 2200 ohm resistor from the bottom side of the 10K pots going to ground?
And do the 10 ohm cathode current sensing resistors necessarily have to tie in at the same ground point as indicated in the diagram?
This is what I wanted to do with my Altec 345A amplifier, having the capability to bias each output tube.
(The idea was brought up by a local retired AE.)
The amplifier's original negative bias circuit had one 5K pot to adjust bias for all four tubes, with the cathodes all tied to chassis ground.
I went and put 10 ohm 1/2 watt resistors to ground for each EL34 cathode with test points, for measuring bias current above chassis.
And I then was going to source four 5K bourns trimpots for each grid of EL34. Mouser catalog indicated a panel mount clip-on bracket for the little 3/8" trimpots.
They would have mounted nicely right in a oval chassis punchout or two.
I ask Mouser for the panel mount feature to the Bourns trimpots. They don't know nothing about them.
They contact Bourns and ask about them, then they contact me.......Bourns says no......lame....
I got tired of figuring out what I was going to do, or how to custom fabricate four trimpots mounted on a piece of perfboard. Then figure out how to mount the assembly to the chassis and still be able to bias the tubes from the top of the amplifier. I'm still gonna figure something out....
I went ahead and just went with two 5K bourns 2 watt precision wirewound pots, mostly because it was a drop-in mounting in a larger oval punchout on the chassis.
Of course that will only bias each pair.
When you measure across each 10 ohm resistor to a pair of tubes that you can only bias as a pair, you can really see where individual bias becomes a nice option.
After reading your writeup, this makes me want to follow through with the individual bias idea.
I'm pretty much set-up for it........
What were the size of those Radio Shack 10K pots that you used?
Thanks,
Mike