|[AMRadio] modulator tubes|
John Coleman ARS WA5BXO
wa5bxo2005 at pctechref.com
Tue Mar 28 22:10:41 EST 2006
I once used a pair of 4cx300 for modulators (weird tube but they
were free). I remember them having the capability of working into low Z
loads in AB1 operation. That is why I wanted to try them. I had a 1:1
modulation XFMR and was running 2000 volts at 400ma with a pair of 813s
in the final so the modulators were going to see 5000 ohms. I remember
now! I used 4 (not a pair) of the 4cx300s to get the current swing I
needed. Obviously I had plenty of audio.
I remember now that in experimenting, I had to wind small bare
wire, weaved back and forth around the tube's funny little toes that
stuck out side ways, and I brought the wires down to an octal base of an
old tube such as 6SN7s (wished I had not busted all those tubes up). I
then immersed the tubes upside down in cups of water and hooked it all
up in place of the old 805s. I remember now that I had to get screen
supply voltage from something but I don't remember, oh yea, the final
813s used a separate screen supply and a choke in the screen circuit so
I used the same supply for the modulators. Boy did I have wire running
all over hells half acre. At any rate the 4cx300s biased off nicely and
ran for a few minuets at idle. I shut off the power and felt of the
water and it was barely warm. So I fire it back up and whispered in the
microphone. It had very pretty audio on the scope. So I said FOOOuur
in the mike and the water in the cups instantly went to a boil!!! It's
a wonder the bubbles and steam didn't cause an arc to the screen
connection that was just above it. I hit the plate switch off and all
was OK. I did eventually build a modulator using those tubes and a
little SS speech amp to drive them. But I never had the right sockets.
I continued to use the octal lash up and suspended the tubes on a
Plexiglas supports which air blew threw from the pressurized chassis.
The story might have been funnier if something bad had happened but
thankfully it didn't. I did contemplate a special coffee maker /
modulator but was a little afraid. A shake of the head made that idea
go away pretty quick.
I eventually went to four 813s for modulators for the same
reason. Not that two would not give me enough audio but that the
modulation XFMR I had didn't have enough ratio. So the four tubes
worked into the lower Z really well.
More information about the AMRadio mailing list
This page last updated 14 Dec 2017.