[AMRadio] Re: ceramic tube tester


H.L. hl at sfsu.edu
Fri Oct 10 18:36:45 EDT 2008


Brett:
Many thanks for your response and assistance. You found me out!! I DO 
have several hamfest specials.  Quick question: I have a very nice 
6.00-Volt filament transformer, and a source of variablle 0-2000  DC 
Volts and could add SG voltage and grid bias voltage. If one builds a 
breadboard tester with these components, would it be possible to 
determine arc-over problems as well as weak emission of these ceramic 
tubes?

73 Hal KK6HY



>----- Original Message -----
>
>Message: 3
>Date: Thu, 09 Oct 2008 20:32:34 -0400
>From: "Brett Gazdzinski" <Brett.Gazdzinski at verizon.net>
>Subject: Re: [AMRadio] ceramic tube tester
>To: "Discussion of AM Radio in the Amateur Service"
>	<amradio at mailman.qth.net>
>
>
>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
>transformer.
>
>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....
>
>Brett
>N2DTS
>
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