[Amps] a common topic here tspa

[R.Measures]

On Apr 7, 2005, at 5:42 AM, Joe Subich, K4IK wrote:

>
> Rich Measures writes:
>
>>
>> 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?
>
> The discussions we had related to bad design in general and not to
> any specific test fixtures they may have designed.  I would, however,
> expected them to have designed the first test fixtures using a belt
> and suspenders approach and then later evaluated reduced cost designs
> for stability/sensitivity ... that is, after all, how much real world
> development is done.
>
> In any case, and this is my last comment, I have read your "Foote
> letter."
>
> 1) You are 'leading the witness.' He specifically refers to your letter
>    (21 Jan 1986 which is not reproduced) suspecting parasitics.

My investigation of the 2, 8875 amplifier turned up something odd -- i. 
e., both VHF parasitic suppressor resistors appeared to be in mint 
condition, yet they had increased in R by factors of over 4x.   I felt 
it was important to tell Mr. Foote about this unusual phenomenon.  
After Eimac autopsied both tubes and discovered that both contained 
gold melt-balls, Mr. Foote telephoned me and related the strange 
finding of the 8877 development team.  Near the end  of our 
conversation, Mr. Foote said that the timing of this episode was 
perfect since all of the men who were on the 8877 development team were 
being forced out the door and that in a couple of months the story of 
their finding would disappear with them.
>
> 2) The tubes in question are 8875 not 8877 - there is a big difference.
>    Don't confuse the two.

They are similar in design except that the 8877 has c. 3x the current 
and dissipation capability.
>
> 3) Foote only confirms that the grids were overheated.  "An oscillation
>    condition" (note: he does not say "parasitic oscillation")


If the MLA-2500 was a power oscillator, it therefore could have no 
parasitic oscillation and you would have a point, Joe.

> is but one
>    possible cause of grid overheating.  Severe overdrive is also a 
> known
>    pathway for grid overheating - particularly of the fragile grids in
>    the 8873/8874/8875.

The 8877's grid weighs c. 2oz.  The amount of energy needed to heat it 
to the boiling point of gold would be prodigious.
>
> 73,
>
>    ... Joe, K4IK
>
>
>
>

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