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[AMPS] Mixing Tubes in an SB-221

To: <amps@contesting.com>
Subject: [AMPS] Mixing Tubes in an SB-221
From: measures@vc.net (Rich Measures)
Date: Thu, 2 Apr 98 16:30:05 -0800
>Here's the problem.  I have an SB-221 with one dead Eimac 3-500Z and one that
>seems to be working okay (both tubes have been in the amp since 1982).  The
>bad tube seems to have 0 cathode emission.  

Zero emission meaning that the filament is not lit?  If the filament is 
not lit, the solder on pin 1 or pin 5 may have melted and dripped out of 
the pin.  Resoldering the pins using rosin flux usually fixes the 
problem.  Oiling the fan helps.  Sometimes, one needs to tighten the 
helper-spring tension on contacts 1 and 5 on the sockets. 

>Emission on the good tube seems to be okay.  What is the implication of 
purchasing a
>single, cheap tube toreplace the bad Eimac?  What problems can I expect?  

A friend recently purchased two, new Chinese tubes from RF Parts which 
proved to have roughly the same Mu (130) as Eimac tubes.  The biasing 
requirement was very similar.   

>probably a good idea to replace both tubes at the same time, but I'm trying 
to squeeze
>some more life out of the amp before I have to invest too much money.
>
>FWIW, I suspect the bad tube was damaged when parasitic oscillations ran
>through the thing a couple of years ago.  

 Bent filament helixes, possibly.  However, this would not cause any drop 
in emission.  

-  later, Chris

cheers
Rich...

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


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