[Amps] Matching GU-74Bs

[Michael Tope]

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..............