[AMRadio] ceramic tube tester

Brett Gazdzinski Brett.Gazdzinski at verizon.net
Thu Oct 9 20:32:34 EDT 2008

I always tested them by measuring the resting current.

At 2000 volts on the plate, 350 on the screens, and a bias voltage to give 
100 ma on new tubes, I would plug a mess of tubes in and see what they 
rested at.

A pair of 4cx250b's is 500 watts of plate dissipation, same as one 3-500z 
type tube.
My guess is max of about 125 watts of carrier in AM service.

According to the book, they don't like plate modulation without separate 
modulation of the screens, usually done with a separate winding on the mod 

They make great modulators though, a clean 600 watts of audio out in AB1, no 
driving power.

They have the very interesting chaicteristic of not needing any screen or 
bias voltage changes with 1000 to 2000 volts on the plates.

>From 1000 to 2000 volts on the plates, and a 500 ma peak current equals a 
very reasonable plate load impedance.

Beware broadcast pullouts, this type of tube suffers from the cathode 
chemical boiling off and coating the inside of the tube, which causes arc 
overs after long run times.
When they arc over, they get pulled out of service and sold at hamfests....


----- Original Message ----- 
From: "H.L." <hl at sfsu.edu>
To: <amradio at mailman.qth.net>
Sent: Thursday, October 09, 2008 7:30 PM
Subject: [AMRadio] ceramic tube tester

> I finally got my 1966 TMC linear amplifier operating, but with a modulated 
> AM (MCW) input, my Wattmeter indicates an anemic 25 Watts into a dummy 
> load with a total PA plate current of 200mA (nominal current reading 
> according to the manual) for the two parallel 4CX250b's. The manual claims 
> 200 Watts CW! Perhaps one or both tubes are weak. Perhaps a resistor or 
> capacitor has changed value. I just don't know yet and may have to put it 
> on the bench again.
> If anyone knows of specific tube testers that will test ceramic 
> transmitter tubes, such as the 4CX250b or the 4CX350a, please share them 
> with this list.  Many amplifiers and transmitters use these tubes.
> 73 Hal KK6HY
> P.S. I am still interested in TMC transmitter and amplifier original 
> prices to share with the group.  So far, no one has responded.  TMC 
> certainly seems to have kept their prices well hidden.
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