......
>>The vacuum in power grid tubes is high.
>
>Yes, so it's all the more easily affected by very minor contamination.
>
Contamination inside an 8877?
..... ... ...
>>According to one chem. data book, the boiling point of gold is 2966
>>degrees C, at a pressure of one atmosphere. In a high vacuum it says
>>that slow evaporation is possible at lower temperatures, however, it says
>>nothing about the boiling point in a high vacuum.
>
>That's right, it wouldn't, because the concept of "boiling point" ceases
>to apply. You have to think in terms of vapor pressure (which is not
>significantly affected by the external pressure or vacuum) and the
>effect of that vapor on the vacuum conditions.
When one sees gold blisters, it seems likely that boiling took place.
... ...
>>True, although the additional power needed to raise an object in a
>>black-chamber-vacuum to a specific temperature is a function of T^4. In
>>1986, an Eimac engineer said that gold-sputtering was due to an
>>oscillation condition.
>
>Sorry, I don't see how those two sentences are related.
According to Stephan's Law, far more joules than are available from the
driver would be required to elevate the grid to such a temperature.
However, an oscillation condition could boil off thin layers of gold - as
the 8877's designers reportedly concluded.
>
>Sure, I've seen them - I check in your web pages regularly because they
>are always interesting! The post-mortem evidence is not in dispute; all
>the questions are about the interpretation of the cause of death.
If the naysayers are so sure that the cause is not parasitics, why have
they not sent me a kaput tube for disassembly and photography? If gold
is evaporating from an arc, why is the evaporation uniform, and why is
there no arc mark?.
>> 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.
>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?
cheers, Ian
Rich...
R. L. Measures, 805-386-3734, AG6K
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