grounded grid Tetrode/beam tubes, control grid current portioning/limiting

[Opcom]

This isn't a ham amp but it's a good example for studying the techniques for managing the control grid current in GG applications of beam tubes/Tetrodes.

I'm interested in how a small resistance can help balance out the current between the 1st and 2nd grids so that the 1st grid does not burn up or is protected from excess drive.

Burning the control grid can be an issue with some tubes like the 4X150, and even tubes like the 4-125...4-1000 if the plate voltage is proportionally low. Even the sweep tubes shown in the diagram have their limits on control grid drive.

estimation of peak power levels:
First stage 1 tube ~15W input
second stage 2 parallel tubes ~40W input,
third stage 6 parallel tubes ~150W input
output about 600W peak (extreme ICAS..)

In this amp I can see the first two stages, at fairly low drive power, use a smallish resistance of 470 Ohms, and the last stage uses 3.9K. Note there is only the one resistor, despite the number of tubes - and the largest driving current would be through the screens.