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[AMPS] parameter shift in Tetrode

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Subject: [AMPS] parameter shift in Tetrode
From: jtml@lanl.gov (John T. M. Lyles)
Date: Tue, 20 Feb 2001 15:39:32 -0700
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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