[Amps] a common topic here tspa

Joe Subich, K4IK k4ik at subich.com
Thu Apr 7 08:42:30 EDT 2005


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. 

2) The tubes in question are 8875 not 8877 - there is a big difference. 
   Don't confuse the two.  

3) Foote only confirms that the grids were overheated.  "An oscillation 
   condition" (note: he does not say "parasitic oscillation") 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.   

73, 

   ... Joe, K4IK 
 



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