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

Geoff/W5OMR w5omr at w5omr.shacknet.nu
Fri Dec 10 07:29:39 EST 2004

> Patrick,
>       For a "legal" AM plate modulated ham rig, running the tubes you
> mentioned as you mentioned is way over kill. Heck, ~ 300 watts or so of
> audio is all you will ever need to modulate the a rig designed for 375 watts
> carrier output. If I had 4-400's I'd go AB1 and run it as a tetrode. Sure,
> that requires a bias supply, and a screen supply. The drive will be simpler,
> and a lot less swing, enough simpler to justify the extra complexity of the
> extra power supplies. A 12AX7 driver would do fine.
>       As for your question, yes it's workable. I would use negative feedback
> however to linearize those tubes a bit since distortion will be fairly high.
> The hardest part of your plan is getting undistorted drive from the driver.
> Maybe use one of your 2 X 8005 PA amplifiers driving a quality audio output
> transformer in reverse.
> Regards,
> Jim

I just don't see a pair of ANYTHING as needing 1000 grid volts of drive
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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