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[AMPS] 3-500Z amplifiers and grid current

To: <>
Subject: [AMPS] 3-500Z amplifiers and grid current
From: (km1h @
Date: Tue, 17 Mar 1998 11:14:10 EST
On Tue, 17 Mar 98 03:18:37 -0800 Rich Measures <> writes:

>Only in relation to defective anode cooler welds, not to bent filament 

The welds caused infant mortality. The long term problem was real gas,
not the vanishing kind. I had several bad ones sent to me and they all
had that telltale glow...or worse. I was only able to recover one out of
maybe a dozen. 

>>I can only relate a few of the hooror stories of the hams that
>>bought '89 tubes for spares only to have them go bang when used 
>>years later. 
>>Actually I commend your honesty about the date code Rich...others 
>>have tried to hide it.
>By what mechanism does an allegedly defective tube damage the 
>suppressor R and blow the grid choke?

A big bang in an amp without a HV surge resistor.

>>I was buying 3-500Z's at 100 a time from Eimac then and even right 
>out of
>>the box I had a high failure rate. It was around 1992 or so that I 
>>got feedback from those ...many good friends...that had decided to 
>>their "spares". and having big bangs.  
>>It was also at that time that I started a serious evaluation of all 
>>Chinese versions. Today...1998 I only buy Chinese.   Failures are
>>virtually zero..
>>Rich..and others is my "opinion only"...that a huge majority of
>>Eimac 3-500Z failures are due ONLY to poor quality tubes...NOTHING 
>>I fail to see any failures with Amperex (1968 and later) or good 
>>(1994 and later) versions that even approximate the Eimac 
>experiences. I
>>stress the word ANY.
>I have tested Amperex and Chinese [Penta] tubes that failed from a 
>filament helix.  
>>I suspect that I know the source of those tubes and amp which is why 
>The two, new tubes came with the new TL-922.  
>>Bang..yes...parasitic..highly doubtful.
>Only if you can explain a plausible alternative scenario based on the 
>circumstantial evidence.  .  Your serve, Carl.  

Real proof would clinch it Rich. How about setting up a SB-220, TL-922,
etc with a spectrum analyzer and try to create the incident ?

Until that occurs I find it a real stretch to believe that a brand new 9
year old amp and tubes would harbor a dreaded parasitic at turn-on. 

73  Carl  KM1H

>R. L. Measures, 805-386-3734, AG6K   
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