bias 300b pse

[Princo]

Can anyone explain the theory of setting the cathode bias on the 300b output tubes on parallel single ended circuit using 300b, (as in wad 300b pse)

Its set at -40v but plotting this on valve data sheet makes no sense, im assuming that because its a parallel circuit the actual grid voltage curve used is -80v but i cant figure out why

[John Caswell]

This is because the 300B PSE is a mix of both fixed bias and "cathode" bias.
The fixed part comes from the 300B grids being directly coupled to the ECC82 cathode which are sitting at approx. -35V and the current drawn by the 300b developed approx 40V across the bias resistors. Add the two and you get approx -80V negative bias on the grid.
I put the term "cathode" in commas as the 300B does not have a "cathode" as the heater acts as the "cathode"

John

[Princo]

thanks john