3-500Z Problem
Some time ago in a chain of TS 850S driving a TL922 linear amplifier, an
'incident' occurred during initial switch-on and tuning into a dummy load.
There was a catastrophic failure of the transceiver, but the state of the
linear amplifier was at that stage unknown. The transceiver failure was
eventually located, and turned out to be due to the short-circuit of one of the
front-end steering diodes, and when this was replaced the transceiver operated
normally again in all respects.
The TL922 was now tested in the same configuration as before, and functioned
normally up to about half power. But then the plate current began to increase
uncontrollably. The circuitry was examined, and it appeared that the Zener
bias diode had failed, but there were signs that one of the grid chokes had at
some time experienced a flash-over, although it appeared to be otherwise sound.
The Zener diode was replaced, and once again the chain was tested. Again there
was plate current instability at some level. Then the suspect grid choke was
replaced also, but tests again indicated instability at about half-power.
By using a tube of the same filament rating in one socket, but with no plate
voltage applied, a test was then made of the amplifier operating with only one
3-500Z in the other socket. The behaviour of the amplifier was that one tube
produced about the right output power, but the other showed distinct
plate-current instability at a much lower power. Now the question:
Is it your experience that a 3-500Z (Eimac) can appear to be quite sound in
respect of filament emission, but exhibit plate-current instability under some
drive conditions. In essence the question amounts to what now needs to be
done. Should one or both tubes be replaced by new ones, or should one look for
some other fault in the circuitry (which appears to be all in order)?
If one or both of the tubes has had an internal flash-over, and something (the
grid?) is damaged, can anything be done to salvage the tube(s)?
Any help you can give would be appreciated.
de Mike
G4VQH
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