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[Amps] 3-500Z parasitic wows

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Subject: [Amps] 3-500Z parasitic wows
From: (2)
Date: Thu, 22 Aug 2002 06:39:06 -0700
>  I have been around a lot of amps .....   Every amp that had a parasitic
>blowup episode used a 3-500Z  . I'm talking TL -922 , SB-220 , AL 80 , AL
>80a ,AL82,  SB-1000 , Drakes, Viewstars & more ... If it has a 3-500z
> WILL someday hand grenade,  I had two go off next to me , my friends
>SB220 & AL80.... blowing parts everywhere & dreaded grid to filament
>shorts.......I have no confidence in that tube. Every time I use an amp with
>a 3-500z in it , I am waiting for the big bang .....

I too have had stentorian parasites at 110 - 120 MHz with the 3-500Z.  
However. there is a method to reduce the VHF gain of the tube -- 
providing one can tolerate a 2% power reduction at 29MHz:   That is:  
Reduce the Q of the hidden VHF tuned-circuit between the tube/tubes and 
the Tune-C.    This can be accomplished with lower VHF-Q suppressors.
-  The 3-500Z has a reputation for parasitic oscillation and it has 
0.15pF of feedback-C.  However, 2, 8877s have 0.2pF of feedback-C and 
they have roughly twice as much VHF-gain.  Thus, 8877s are hardly 
fool-proof. . . .  Murphy was right. 

cheers, John

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,  

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