|[AMRadio] RE: Testing Transmitting Tubes|
k4kyv at hotmail.com
Thu Sep 23 19:50:46 EDT 2004
>Yes, but there are practical issues. This is certainly not a speedy method.
>Changing tubes out of a working rig puts a good tube in jeopardy of a
>mis-handling accident. How are results interpreted for a single tube
>changed into a push-pull final? You have to keep a different transmitter
>around for each tube type that you want to test (hmmmm, maybe this is an
>advantage, not a disadvantage....). Etc. Etc.
I once built up a panel with meters, and variable bias supply and plate
supplies, to simulate actual operation. The problem is measuring the peak
emission capability of a tube. I have seen tubes that seemed to have plenty
of emission at normal voltages go belly-up on modulation peaks.
When I test tubes that have been sitting for a long time, I go through the
standard burn-in process with periods of filament-only, filament with
reduced grid drive, filament with full grid drive plus reduced plate
voltage, and finally full filament, grid and plate voltage. Once I have
determined that the tube appears to be reasonably good, I run the plate
voltage and grid drive so that the tube operates at about 95% of its rated
plate dissipation for about 4 hours, then observe the tube again. I usually
do this with the test panel, not in a transmitter. I have seen apparently
soft tubes test normal after the 4-hour burn.
I'd say it is best to check each tube using several different methods to see
every possible aspect of the tube, and then you have a fairly good idea of
its condition. There is no magic test to plug a tube in and instantly get a
"good", "fair" or "poor" evaluation for sure. I have seen tubes that tested
extremely high, but went completely soft after only a few hours of
I also have one 833A with a bright shiney spot burnt on the plate, and the
glass envelope is actually sucked in over that spot. Obviously someone let
it run at extreme overload for a substantial length of time. The glass has
a silvery-smokey appearance on the inside of the envelope. Yet that tube
operates just as well as any of the best, well cared for ones in my
And yes, I have lost tubes due mishandling. However, it is still a good
idea to "rotate spares" occasionally to keep t h e tubes from gassing up, so
you always run that risk. I just try to be extra careful when handling
tubes, including removing them from the sockets and removing grid and plate
caps. It is very easy to break a tube at the grid or plate cap if you
stress it too much.
Ideally, you would need a pulsed power supply and oscilloscope or some kind
of accurate peak reading instrument, so you could operate a tube at its
maximum rated voltage times current without exceeding plate dissipation and
burning it up. It would then be simple a matter of comparing the observed
plate voltage/plate current/grid voltage curves with the charts in the
manufacturer's published specs.
I run pushpull triodes in my rig. To test one tube, I use a "neutralising
dummy" in the other socket. It consists of a defunct tube on which I have
cut off one of the filament pins. I insert it in the socket opposite the
one I want to test. I have found that you have to back off the loading of
the transmitter. Otherwise one good tube tries to run the full plate
current of a pair of good tubes. I back off the loading until the plate
current registers one half the rated plate current. That way, I can run the
tube in the final (final running at half power), and check the envelope and
trapezoid patterns on the scope, and be sure that I am really checking the
tube in question, without the influence of its mate.
Of course, the same problem exists with a parallel tube amplifier. The idea
is to make the amplifier work with just the one tube you want to test at its
If you don't have a defunct tube to use as a neutralising dummy, just
disconnect one of the filament leads to the socket and use a good tube as
the dummy. Just make sure the filament by-pass caps stay in place so the rf
circuitry is unaffected.
I try to keep a log of each tube, and match them as closely as possible in
the final. I use a Staedtler Lumocolor black permanent ink overhead
projector marker to write a number on the tube for positive identification
later. I usually mark the number on a plate or grid cap or base of the tube
in addition to the glass envelope just to make sure it doesn't rub off, but
I have tubes with still legible markings that I marked over 15 years ago.
The heat doesn't seem to affect the permanent ink of that brand of marker.
Most of the tubes I keep around and most concerned with testing are the same
ones I use in my various transmitters, so the availability of a test unit is
not a problem.
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