Bias test points.

[wozt]

I want to use 2 6v6's in push pull with adjustable grid bias with test point(s). Is a 1 potentiometer and 1 (shared) 1ohm cathode resistor OK, I'm struggling to see why each tube should have its' own test point. Thanks.

[Tom Phillips]

Even if you only use one bias voltage adjustment pot you will want to have the ability to measure the current flowing through each individual tube. Then you will know both the bias set point and the balance between the tubes. I.e. verify that they are a reasonably well matched pair.

[wozt]

Yes, mist has lifted, DOH thanks

[Chuck H]

Right. What can happen is, if the tubes are grossly mismatched or there is a problem with one it may still be possible to set what looks to be the correct net current with no indication that anything is amiss. That said, I've just tacked in a resistor temporarily on a shared cathode lead to check bias on amps many times for just straight up "bias adjustment". On my cathode biased amps I don't trouble with test points either. There's already a resistor there! If I want to check how close the tubes are I just plug in one at a time to see that they're drawing similar current by themselves. Obviously this doesn't allow me to test anything without removing the amp from the cabinet. If I were adding test points as a permanent externally accessible feature I would certainly allow for individual tube readings.