[AMRadio] ceramic tubes

Brett Gazdzinski Brett.Gazdzinski at verizon.net
Sun Oct 12 18:19:30 EDT 2008

Re: ceramic tube testerI think the 4cx300 tubes are even cleaner (lower distortion) then the 4cx250 tubes.

I have tried various loads, and the 4cx250b and 4x150a tubes seem to not mind the impedances being off somewhat.

In my book, the only real downside of these type tubes is the forced air cooling.

I have a blower that pressurizes the underside of the chassis, which also helps cool anything under there, and have a variac on the blower, its a big blower turning slow so its not loud.

I have variacs on the modulator power supply and the RF deck, and can play with impedance a lot, but I cant say I hear much difference between various settings.


  ----- Original Message ----- 
  From: John Coleman 
  To: 'Discussion of AM Radio in the Amateur Service' 
  Sent: Saturday, October 11, 2008 6:53 PM
  Subject: [AMRadio] ceramic tubes

  Hi Brett:

              Yep, in spite of the fact that you see these type of tubes mostly in HF to UHF service they work very nicely as modulators.  I was given 4 of the 4CX300s a long time ago and a buddy of mine and I split them up and each built modulators using a pair, with 2500 volts on the plates in class AB1 service.  They worked very well into low impedance loads far below 10K ohms.  In fact if the plate load Z is too high they will draw too much screen current so it is important to monitor the screen current on these tubes.  I began to think that there plate current swing had no saturation point that I could reach.  

              These 4CX300 tubes had very strange sockets.  The connections were in little rings with tabs up and down the ceramic post.  I wrapped small wire back and forth around each ring and twisted it tight then soldered all of the wires into a broken octal tube base.  I first tested the tubes as modulators by turning each upside down into a coffee cup of water. So that the water just covered the plates.  Then I turned it all on and biased the tubes to the proper plate current.  I let them idle for about 5 minutes then powered down and felt of the water.  It was not even up to body temperature.  So I powered back on and began to modulate.  Boy was I surprised at the level of current swing for the amount of drive.  But when I said a big FOUR into the microphone the water foamed and boiled immediately.  My hand was poised over the plate switch so there was no damage.  I went to work figuring out a mounting scheme and air system and used those tubes for quite some time.  But I really don't know now what ever happened to that modulator.  I think it may have been trashed by my 2nd ex-XLY.  I'll blame her for it anyway HIHI.




  -----Original Message-----
  From: amradio-bounces at mailman.qth.net [mailto:amradio-bounces at mailman.qth.net] On Behalf Of Brett Gazdzinski
  Sent: Friday, October 10, 2008 6:03 PM
  To: Discussion of AM Radio in the Amateur Service
  Subject: Re: [AMRadio] Re: ceramic tube tester

  No, I don't think you can test for flash over other than using the tubes.

  I have a LOT of old hamfest specials, and pick 2 matching resting current tubes in the modulator, and they work ok for a few weekends, then flash.

  I pick some more matched tubes, same thing.

  I found 4 NEW 4x150a tubes at an old TV repair parts store (long gone now) and have been using the 1st set for about 6 years now without problems.

  The 4x150a tubes seem to work exactly the same as the 4cx250b tubes, they look the same, work the same.

  If you look in old handbooks, they list the 4x150a as a 150 watt tube, but its also in the 250 watt tube with the 7034, a later revision of the 4x150a?

  Anyway, if the tube looks exactly like the 4x250b, it has to have the same plate dissipation, both are forced air cooled, have the same fins, shape, size, etc.

  The older early 4x150 tubes looked different from the 4cx250b type tubes.

  Mine came in sealed green cans marked 4x150a/FAA, cost me $25.00 each.




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