Bias four tubes with two terminals?

[ThePanda]

Is there a schematic or way to bias two output tubes at a time with one terminal instead of having one for each?

Looking to implement this in a 100W build with four tubes. trying to avoid drilling five holes (4 positive, 1 GND).

I could not find it by searching.

Thank you

[sluckey]

For one pair of tubes just connect the two cathodes together and use a single 1Ω resistor connected to ground for the pair. Repeat for the other two tubes.

[Helmholtz]

You could still use individual 1R resistors at each tube but connect the cathodes of each pair.
This would allow to directly read the average idle current per tube (without having to divide by 2).

[ThePanda]

You could still use individual 1R resistors at each tube but connect the cathodes of each pair.
This would allow to directly read the average idle current per tube (without having to divide by 2).

Connect them to one terminal so that they are in parallel?

[Helmholtz]

If you connect the cathodes of the pair the resistors will be wired in parallel.

[sluckey]

If you connect the cathodes of the pair the resistors will be wired in parallel.

That's right. Total resistance is now 0.5Ω, so you're back to math. Kinda defeats the reason for using a 1Ω resistor.

[Helmholtz]

If you connect the cathodes of the pair the resistors will be wired in parallel.

That's right. Total resistance is now 0.5Ω, so you're back to math. Kinda defeats the reason for using a 1Ω resistor.

No, the 0.5R resistance allows to read half of the sum of the 2 idle currents which corresponds to the averaged current per tube.
So no math required.

[R.G.]

Is there a schematic or way to bias two output tubes at a time with one terminal instead of having one for each?

Looking to implement this in a 100W build with four tubes. trying to avoid drilling five holes (4 positive, 1 GND).

Sure. make one hole contain a four pole rotary switch, and have that switch which tube you're monitoring in the other positive/ground holes. This preserves both individual biasing and "no math". EDIT: forgot to add that there exist very small 4pst rotary switches that will fit small holes.

Any time you parallel tubes for biasing, you either have to have well matched tubes in the parallel pair, or at least one of the tubes in the pair will be off, if not both. Reading each tube separately is the whole point of individual biasing.

There are probably games you could play by biasing both paralleled tubes completely off, biasing one to the target current, then biasing the second one and trying to hit two times the bias current of the first one of the pair. This assumes that the first tube bias does not wander when you bias the second one, and makes tweaking bias as they drift a b&tch, as well as requiring math. Although, frankly, multiplying/dividing by two is the kind of thing that is easy to do in your head.