Hi All!
I am in the process of rebuilding a Central Electronics 20A and seem to
have an undocumented modification. As far as I can tell all of the mods
noted in the BAMA site are installed and complete. There are however two 5
amp diodes in series with the anode end grounded and the cathode tied to pin
1 of the 6AL5. The junction of the two diodes is connected thru a 33pf cap
to the junction of C42 & R77. None of this makes sense because the #1
cathode (pin 1) of the 6AL5 is only normally at -.5 volts and couldn't
provide enough voltage to forward bias the diodes. The junction of C42 & R77
is at -100 volts in stand-by and ground for transmit. Since the undocumented
33pf cap is polarized with the cathode end at the C42-R77 junction, I
suppose it could charge up to the -100 volts and serve some sort of spike
suppression when switched to transmit.

Anyone familure with this mod? Can you explain it?

Thanks
K3PID Ron H.

Ron,

None of that makes sense to me . Pin 1 (cathode) of the 6AL5 would
be a modest DC level that varies with speech level and actuates the VOX
relay tube. And 33pf is too small to couple anything but a very sharp
transit spike from the junction of C42 and R77 as the rig switches to
standby.

I would remove those added components.

My factory wired 20A that I acquired (used, of course) in 1964 is still
my main station exciter and has no modifications whatsoever. I had to
replace R45 several years ago as it's value creeped up and I really had
to holler in the mic to get the VOX relay to pull in.

John