SE Pentode, regulated screen voltage?

[Yvesm]

. . .
Note also that under some conditions, a good 6L6's Ig2 can *reverse*. If you have a screen-only supply, it is wise to have a bleeder for 10%-20% of nominal Ig2, because simple supplies won't hold-down when current flows INto them.

This was common in plate tuned RF amplifiers, specially during tuning when the plate can see very exotic loads.

If you care about that, you may also use a shunt regulator which will absorb the reverse current if any.

Yves.

 

[6A3sUMMER]

A generalization about pentodes:

Audio amps often use plate loads that are medium impedance.
And that requires more plate current to get a specific power out.
An output transformer matches that medium impedance to the loudspeaker impedance.
And it does so across the audio band of frequencies.

RF amps often use plate loads that are high impedance (at one frequency).
It is relatively easy to match high impedance to low impedance (i.e. a pi network)
And high impedance requires more plate voltage to get a specific power out.

The more the plate voltage, the more the secondary electrons come off the plate.
The suppressor screen was supposed to 'suppress' these electrons from reaching the screen.
A true tetrode tube does not have a suppressor screen.
Look at tetrode curves, you may see a kink in the plate curves (negative resistance).
And even some pentode/beam power tubes refer to this effect (Kinkless Tetrode = KT)