[AMRadio] modulator tubes

Brett gazdzinski brett.gazdzinski at verizonbusiness.com
Thu Apr 6 11:23:42 EDT 2006

I got a set of sockets for 4cx250b/4x150a tubes from fair 
radio about 20 years ago.
I found a design in an old QST about using those tubes in 
a 600 watt modulator and built it with minor changes.

Works great, some of those type tubes are low distortion
plus no driving power.
Another advantage of them is you can change the plate 
voltage between 1000 volts and 2000 volts without changing the 
bias or screen voltage.

The only downside is they need a blower.
I pressurize the lower chassis, which cools any
other parts that might get hot.

It works, its been trouble free for 20 years, despite the 
tube type adjustable screen voltage regulator circuit
out of the Bill Orr handbook. 

Bill Orr also had a water cooled setup of an RF
amp for mobile work, anyone remember that?
Water cooled tubes (designed to be) that used a fuel 
pump and a 5 gallon jug of water....

Now that's real ham radio!


> -----Original Message-----
> From: amradio-bounces at mailman.qth.net 
> [mailto:amradio-bounces at mailman.qth.net] On Behalf Of John 
> Coleman ARS WA5BXO
> Sent: Tuesday, March 28, 2006 10:11 PM
> To: 'Discussion of AM Radio'
> Subject: [AMRadio] modulator tubes
> 	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.  
> John,
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