Hey,

Im trying to calculate the plate resistance of the 6S6B tube to find the suitable output transformer specs.

Can someone please take a look and tell me, if my math is correct?

I drew some lines on the grid curves

I calculated it as follows: the red line is the load line. 120V B+ with a 100k Ohm plate resistor limits the current to 1.2mA.

Idle state is the green line which is around 37V and 0.8mA.

Then I drew the blue lines which represent a 5V voltage change in each direction.

So for a 10V change in plate voltage the current change is 0.7mA.

10V/0.7mA = ~14k Ohm plate resistance.

This in parallel with the 100k Ohm plate resistor equals around 12.2K Ohm Zout.

Is my math correct? Is that the way to calculate it?

And is it normal, that the more negative you go on the grid the higher the plate resistance gets? Under the same B+ conditions.

On the -2V grid curve I got around 16.5K Ohm.