>> Subject: 2 pcs returned 8875 S/N G8AD-241 and F8VD-428J, for evaluation.
>> Reference: Your letter dated 21 January 1986; EIMAC RPA #SC-2303.
>>
>> Dear Richard:
>>
>> Your letter about parasitics is quite interesting, and it appears your
>> two tubes have had the same trouble. The emission was poor on test, and
>> consequently other test results looked bad. The tube engineer then cut
>> them both open for an internal examination.
>> Both have been badly overheated internally, the apparent result of an
>> oscillation condition. The grid in these tubes is gold plated and if
>> overheated the gold vaporizes off, of course, and some of it inevitably
>> lands on the oxide cathode, and that poisons emission.
>
>What you always conveniently leave out is that in YOUR initial report to
>Eimac you said you "thought the tubes oscillated, and that overheated the
>grids".
** No. I said that I suspected that the 8875s had sustained a parasitic
oscillation because both parasitic suppressor resistors had changed from
100-ohms to c. 400-ohms so quickly that no external change was visable.
I made no statement about the grids. Eimac subsequently autopsied the
tubes and observed the gold-loss damage to the grids.
>
>The response to YOUR claim you suspected there was an oscillation, from a
>guy whose job it was to handle customer letters,
** W. B. Foote was chief specs engineer as well as a customer letter
replier.
>included the sentence:
>
>> Both have been badly overheated internally, the apparent result of an
>> oscillation condition.
>
>If I take my truck into Ford with melted main bearings, and say to the
>service writer "I think I ran it out of oil"...and if that truck has a
>busted oil pump....Ford's response would almost certainly be "apparently you
>ran it out of oil".
>
** ______
>It very interesting how you manipulate answers to fit your fascination with
>oscillations. But the fact is it is impossible to tell WHY the grid was
>overheated, but the particular amplifier you selected has NO grid current
>protection and some models did not even include a grid current meter!!!
>
** The RF power available from a Ham transceiver seems likely to provide
enough BTUs to raise grid temp to the 5370?F that is needed to boil gold.
cheerz, Tom
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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