For those who expressed interest and/or skepticism about the issue,
here's a reasonably accurate time line of events from our QA records.
Eimac puts 8877 heat dam design change into production -- ~08/86
(Completely unknown to us at the time)
First identified tube failure at MRI end-customer field site -- 12/86
First suspect tube routed back to Eimac via ETO -- 01/87
Meeting with Eimac -- 05/87
FIrst fix devised by Eimac; second meeting w/Eimac -- 07/87
Third & fourth meeting with Eimac -- 08/87
Initial Eimac delivery of "fixed" 8877's -- 09/97
Delivery of "fixed" 8877's to customers -- 10/87
Problem re-surfaces @ field customer(s) -- 03/88
FIfth meeting with Eimac -- 05/88
Second fix devised by Eimac -- 06/88
Initial Eimac delivery of 8877's with second "fix" -- 08/88
Delivery of "2nd fix" 8877's to MRI customers -- 09/88
Completion of field upgrade with replacement tubes -- 05/89
** NOTE: Because tube date codes approximate
shipping rather than manufacture dates,
they may vary ~1-3 months from the
event dates given above.
Largest MRI RF amplifier manufacturer's record
of 8877 usage by year:
In new amps As replacements
almost 100% due to
heat dam/g-k shorts
1987 500 575
1988 650 1600
1989 500 550
Statistical data on average end of tube life in our MRI amps
(corrected from earlier off-hand estimates). Excludes small
"infant mortality" humps for both types @ 2400-2600 hrs.
3CX800A7 18,500 hours (very sharp peak
between 18,000 - 19,000 hours)
3CPX5000A7 In excess of 30,000 hours. (Of
course tube costs $2-3k in qty.)
To save reflector bandwidth & QRM, if you have a legitimate
professional interest please contact me by direct e-mail
rather than on the reflector. Thanks & 73,
Dick W0ID
Amps Technobabble, SK & CL
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