Question about screen current in a plate modded tetrode

[KZ5A]

This is about my RCA BTA-1R1 conversion.   I'm changing the PA from a pair of 4x4's to a 4x1 (just because I want to). 

The original PA screen circuit was a resistively self modulated screen via a 50k 225W tapped resistor from the un-modulated 3KV HV.   This poor resistor was severely cooked.   The 4x1 shows to have around twice as much screen grid current as the pair of 4x4's, so I consider continuing this approach to be impractical due to too much heat and load on the HV supply.

So, in the interest of staying near the spirit of the original design, I'm planning on converting the PA screen to "reactively" self modulated using a 10H choke feed with regulated 500VDC developed from the LV (800V) supply.

My question is... how much is the current draw from the regulated supply going to vary during modulation?   Will it switch from sink to source on audio cycles or will the choke filter all that out so the supply sees a steady avg current draw?  Or?

73 Jack KZ5A

[K1JJ]

Hi Jack -

In class C plate modulated service the screen voltage does not need to be regulated since the average current draw is pretty steady.  You may see the screen current meter fluctuate slightly under modulation, but not much.  This also applies to the screen choke technique.  It will help to use plenty of filtering to keep hum low and keep the supply stiff.  But electronic supply regulation is not needed.

A tetrode's screen (and grid) in linear service does need to be regulated, however.


It is helpful to be able to vary the screen voltage somewhat with a Variac to accommodate the different loading requirements of various imperfect antennas. As you know, plate loading has a big effect on screen current and being able to vary from 400v to 600v to get the proper 140ma of 4X1 screen current for a given desired plate loading is an advantage.

T