Re: ceramic tube testerNo, 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
----- Original Message -----
From: H.L.
To: [email protected]
Cc: Brett Gazdzinski
Sent: Friday, October 10, 2008 6:36 PM
Subject: [AMRadio] Re: ceramic tube tester
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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