[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.  


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