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.
Brett
N2DTS
----- 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.
John
WA5BXO
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] 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.
Brett
N2DTS
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