[AMRadio] 3-500Z vs 4-400A

Patrick Jankowiak recycler at swbell.net
Thu Dec 9 21:38:43 EST 2004

Ok, in the name of versatility,

suppose there's this simple transformer coupled push pull 
modulator chassis, set up for 4-400A's in class B. (yes but read 
on) In the chassis, the socket terminals which would go to the 
4-400 control grids have a 20K resistor between them and the 
screen grid terminals. The idea is to limit the control grid 
current of the 4-400, and let most of the audio drive the screen 
grids. Kind of like the old 120W class B zero bias 807 modulator 
from the ARRL and RSGB handbooks. I won't nit pick about exactly 
what class it is, probably more like real-world AB2.

Ok. so I drop in a pair of 3-500Z's. Of course the 20K resistors 
would not matter when the 3-500Z was inserted, as all 3 
non-filament pins are connected to the triode's grid.

I throw a DPDT switch, to change taps on the driver transformer 
from ones supplying 1000V peak grid to grid (to drive the screens 
of the 4-400's as triodes), to those taps supplying a lesser 
voltage appropriate for the 3-500Z's.

workable? Interesting? nuts?

Best regards,


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