[AMPS] Late 1980's 8877 "cathode dam" episode

[richard w. ehrhorn]

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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