driver biasing question

[Malcolm Irving]

I think the balance adjustment should be on the plates.
On the cathodes, the adjustment might be too sensitive, as a small change in voltage there will have a big effect on the plate current (and hence the output balance).

EDIT: In the above I was thinking of balancing the DC. But as this is a driver stage, I guess you are looking more to balancing the magnitude of the two output signals. If so, I think the answer is still the same. Adjusting the plate loads alters the relative gain of the two sides. Altering the cathode resistances changes the bias of the two tubes, which will only have a secondary effect on the gains.

[catalin gramada]

Thanks Malcom. I tried to use both using a unbalanced factory tube. Found the adjustmenst are not complementary in any point, means I did not found any position to match both AC and DC in the same time. In this case seems the balance individual cathodes bias resistors have no sense and can use a shared one ? With balanced tubes it did not any difference: individual or shared , bypassed or not_-get consistent equal gain and bias- is simply amazin. but I want a solution to ensure the possibility to use unbalanced tubes ( more easy to find). Thanks

Edit: plate adjustments and potential dividers loads in power grids, maybe ?

[Malcolm Irving]

Is there any reason why you want DC balance? The outputs are coupled to power tubes via coupling caps which would block any influence of DC imbalance. Or am I misunderstanding something?

[catalin gramada]

Edit: I will do some test this days and post some scope scans for both situations AC balance & DC balance. Thanks for help