Apologies for the time lag. It took some time, typing and drawing to make my earlier stuff make sense in this context. I've attached a PDF containing a strawman design that I think would work.
This design is intended to address the task of quick, simple rebiasing of tubes, each tube to match the others and to match a desired cathode current. The approach is to have one bias pot per tube, and one indicator light per tube. The indicator is blue for not enough cathode current, red for too much, and green for just right. It's possible to bias a two-tube amp this way in about 10-30 seconds, by actual experience.
There are two design approaches in there. One is to just set a reference and have three comparators per tube run the LEDs. The other is to condition the cathode resistor (Rk) voltage so that a microcontroller can read it and do (ideally...) more intelligent things for you as well.
Comments welcome. I did not include a lot of thoughts about possible errors and offsets, precision, and so on unless it really had a bearing on the circuit.
Tube Bias Indicator Strawman
Moderators: pompeiisneaks, Colossal
Tube Bias Indicator Strawman
You do not have the required permissions to view the files attached to this post.