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

Patrick Jankowiak recycler at swbell.net
Mon Dec 13 21:03:33 EST 2004

It's to drive the screen grids, so since they run at 500VDC or 
more in AB1, it could reasonably take 1000V G2 to G2 to fully 
bring each tube to peak plate current. I think I read that the 
"120W 807's" needed about 400V G2 to G2.


I just don't see a pair of ANYTHING as needing 1000 grid volts of 
in Amatuer service...   or am I reading that wrong?

Patrick said:

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.

Man, my 250TH's (triode), in Class B, only require around 400vpk 
g-g drive,
for full output at 3kV.  And 400vpk is a bunch for driving 
modulating tubes!

73 = Best Regards,


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