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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 21:16:17 -0700
>At 03:57 PM 8/22/02 -0400, WA2BPE wrote:
>>...Had this happen to me with an old SB220 destroying one tube; luckily (??),
>>they were about to be replaced anyways.  Later investigation showed that the
>>resistors in the parasitic chokes had increased between 5 & 10x their 
>>value.  I was "fortunate" - only the tube arced with no other damage.
>Which "this" is that?  A parasitic, or a random grid/filament short, 

**  In my opinion, there is no such thing as a random grid/filament short 
unless there is too much filament inrush current or the tube is operated 
horizontally for more than several months.  

>arcing due to a mismatched load, or a Rocky Point arc?  

**  RCA's Rocky Point Investigation was somewhat specious  since there is 
no such thing as disappearing gas and vanishing "barnacles". .  Later, 
General Electric's G. W. Fyler seems to have gotten to the root cause.  [ 
Fyler, G. W. ''Parasites in Transmitters'', Institute of Radio Engineers 
journal. Sept. 1935 ]  :
-  Fyler's Conclusion:
"... If necessary, the plate or grid parasitic circuits should be damped 
with resistance. ... . ..."

>There are lots of 
>things that can happen with a manually tuned amp, and I'd be more inclined 
>to look at other things to blame besides a workhorse tube that is probably 
>in more amateur amps than any other...
**  Agreed.  The fault lies not in the 3-500Z.  As Eimac's W. B. Foote 
told me:  the fault of parasites is not with the tube manufacturer, it is 
with the amplifier's engineer.  
cheerz, Pete 

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

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