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" <[EMAIL PROTECTED]>
Subject: Re: [AMRadio] ceramic tube tester
To: "Discussion of AM Radio in the Amateur Service"
<[email protected]>
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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