Just a brief anecdotal observation relating to Bob Marston's post:
In a commercial MRI amp which used both a YC-156 and a 3CPX800A7 (both
oxide heaters), we started seeing tubes going soft at 18,000 - 20,000
hours minimum. It did appear to make a difference how much they were
actually used to produce power. Apparently, if the maximum current
capabilities of these cathodes are "pushed", you can actually strip
oxide from them. If you really want to push them for excess power, the
better bet is to raise the anode voltage, protect the amp properly from
the then-expected higher rate of arcing, and let the cathode current
"loaf". On the other hand, a YC-156 in ham service (i.e. under, say
10kW PEP) won't be anywhere close to pushing the max cathode current.
Some tubes ran 30,000 hours, and some are still out there with unknown
hours. I left the business before any of the tubes had 80,000 hours on
them. I can't speak for the subject of turn-on or turn-off. We ran the
heaters on regulated DC supplies that limited current to 30% more than
spec, and never once did we see an open heater. The failure mechanism
was always lack of emission.
Dave W8NF
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