I just got a note from Alex UR4LL that is out of GU-74Bs. When I went
searching the internet for alternate suppliers, I found a tech note from
Alpha indicating that use of anything except "matched pairs" of tubes
voids the warranty for the 91B. This seems strange since the tubes are
run in parallel. It would seem that the anode current sharing might not
be exact in an unmatched pair, but that it would be way off before there
was an impact on performance. Also, what would be the danger to the
amplifier. Seems like the worst thing that would happen is that the tube
with more gain would run hotter than the other tube, but that would only
be a danger to the tube, not the amp. Moreover, seems like you could
detect a really bad mismatch by comparing the exhaust air temperature
coming off the anode chimneys. I found a thread on QRZ.com where W8JI
dismisses "matching" as mostly a "sales ploy". It seems pretty rare that
Tom isn't spot on with his technical advice, so I am guessing that is
what Alpha is up to with their tech note (or perhaps their overarching
concerns is people plugging in dud tubes rather than mismatched tubes).
They are advertising matched pairs of 4CX800As tested and warranted for
1 year for $675.00:
http://www.rfconcepts.com/s.nl/it.A/id.4266/.f?sc=15&category=15030
Here is the tech note:
http://www.rfconcepts.com/Alpha-Support_2
I could see how checking each tube individually (perhaps by running it
by itself in the 91B at reduced drive) would be a good idea just to make
sure the tube isn't bad, but beyond that is it really necessary to run
"matched pairs", and if so, what is the matching criteria and how close
is close enough?
Thanks,
Mike W4EF..............
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