>Rich Measures wrote:
>>>> The event that causes the damage apparently happens suddenly.
>>>>- IMO, overdrive tends to dislodge patches of cathode coating, without
>>>>damaging the gold plating on the grid. {photograph in September 1990
>>>>article "Parasitics Revisited".}
>>>
>>>That's my problem: how can you know these things?
>
>>By looking at the photo. The cathode coating is flaking off, the gold
>>plating is not.
>
>All you know for sure from the photograph is - quite literally - that
>"The cathode coating is flaking off, the gold plating is not." That is
>ALL you know for sure. The evidence is not sufficient to prove
>conclusively WHY it happened.
>
For an 8874, the rated drive is 27w. The drive level for the tube in the
photograph had been c. 100w. {Drake T4xc driving a pair of 8874s} A
grid/cathode short eventually occured. If 100w drive shorts the drive,
how could one ever apply enough cathode drive to boil gold off of the
grid?
>>>We can only see the
>>>after-effects, and have no direct evidence about the causes... it's all
>>>a matter of interpretation and "IMO".
>>>
>>If overdriving killed the tube, why is the grid in mint condition?
... ...
>You seem to have a real problem in separating evidence (the physical
>condition of the tube, which everybody can witness and agree) from your
>own theories about the causes.
>
>Read my fingers: what you see under the microscope does NOT include the
>cause of the event.
>
That is essentially what the French surgeons (who did not wash their
hands before operating) told Pasteur when he showed them some parasites
under a microscope.
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
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