[Amps] a common topic here tspa

[R.Measures]

On Apr 7, 2005, at 3:26 AM, Ian White G3SEK wrote:

> R. Measures wrote:
>>> No, I'm simply saying that the Eimac tube guys that I knew and worked
>>> with said a lot of what passes for "truth" is a bunch of nonsense.
>>> In their experience, every case of "parasitic problems" in 8877 amps
>>> could be resolved by properly grounding the grid
>>
>> Did the 8877 design team use improper grid-grounding that resulted in
>> the "oscillation condition" they observed that caused gold to 
>> evaporate
>> from the grid of the DUT?
>>
> That seems to be conflating two different stories, on the basis of
> information from an individual Eimac employee which the company
> subsequently said was incorrect.

Eimac's Reid Brandon told Paul Pagel, the *QST* Technical Topics 
Editor, that Mr. Willis B. Foote was not authorized to release the 
information on the finding about gold-sputtering in the 8877 during 
testing at Eimac.
>
> The "oscillation condition" in question was not a VHF parasitic as we
> know it,

Agreed since the pattern of gold-evaporation indicates that the RF 
current was in the UHF region.

>  but can only arise in extremely large tubes that are big enough
> to have their own internal VHF resonances.

It's obviously not VHF.  Since the resonance of a 3-500Z's grid 
measures c. 90MHz, my guess is that an 8877's grid probably resonates 
above 600MHz when it is grounded by an Eimac grid-grounding collet.
*Has anyone ever measured the grid resonance of an 8877? - (my dipper 
craps out at 300MHz)
>
> What causes gold evaporation is overheating - that much seems obvious.
> What caused the overheating in any particular case is entirely open to
> question.

The grid's base metal is Mo (molybdenum), which melts at 2617ºC.  The 
grid is plated with Au (gold), which boils at 2966ºC.  If the entire 
grid structure was overheated, why isn't the Mo melted in tubes with Au 
boiling-blisters?

>
>
> -- 
> 73 from Ian G3SEK
>
>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org