Regards to the creeping Harris amplifier with 2 x 4CX1500B's:
Once I was testing a 4CX1500A tetrode amplifier, class C, 1500 watts
output at 100 MHz. My assistant and I left a rag or duct tape over
the blower hole under the tube socket, as we were changing a tuning
element and didn't want to drop any screws into the blower scroll.
Being a prototype, we hadn't gotten to installing an air flow switch
yet. Oops. Well, you know what happened already. We turned it on, and
were perplexed as the output power dropped and the efficiency fell,
for a few minutes. Then we smelled the almost burning chimney. We had
thoroughly warped that tube, changed the interelectrode dimensions.
It was still working when we shut it off, although the nickel plating
was quite tarnished. I still have that trophy somewhere in my junk
box. Uggghh.
I'm not suggesting that the amplifier in question is drifting due to
severe overheating, however, some sort of dimensional change or bias
change is about all I could guess would do this. Clearly, something
related to high duty operation as opposed to SSB is causing a drift,
and that usually means something thermal. But Murphy's law can always
make a fool out of me, so!!!!
Good luck and let us know what you find.
John
K5PRO
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