On Wed, Mar 29, 2006 at 03:45:28AM -0500, Brian Carling wrote: I never tried water-cooled power tubes... Wow - I just read this from WA5BXO, on another e-mail group:

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.