I’m revisiting the individual bias adjust kit from Dynakitparts (that appears to be the same as the biasing setup in the DIYTube ST35). I played around with some different resistor values (on paper) for the two parallel branches. I think I came up with a change to the resistor combination that would expand the overall biasing range.

Disregarding the 10Ω resistors, the original values in the two parallel branches are 470Ω || (1KΩ + 10KΩ pot). If I did the math right, this makes the original range of adjustment run from ≈ 320Ω to ≈ 451Ω.

Again disregarding the 10Ω resistors, if I change this to 620Ω || (390Ω + 10KΩ pot), I get a range of ≈ 240Ω to ≈ 585Ω.

I suppose this is all well and good, but I am wondering about the corresponding change in power dissipated in the pot, which is rated at 0.5W. I made an attempt at calculating the quiescent power dissipated in the pot assuming a total resistance for the two branches adjusted to 400Ω with 35mA total cathode current. With the two original fixed resistor values, the pot dissipates around 0.04W. With the changes to the values of the two fixed resistors, power dissipation in the pot increases to 0.10W. Again, this assumes I got the calculations right.

0.10W dissipated in a 0.5W pot under quiescent conditions is not a concern. But, I don’t know how to calculate the power dissipated in the pot with the amp running full tilt. Could the pot’s 0.5W rating be exceeded with the amp running at full power?

Thanks.

Peter