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previous: MatthiasM By the way, what sorts of proble... -- 1129020159 View Thread

Re: Good point, Mike.

10/11/2005 12:41 PM
MBSetzer
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Re: Good point, Mike.
Plus the math is correct, the single 50ohm would end up dropping the screens to the same slightly reduced B+ at idle which you would get from the separate 100 ohmers.  
 
If the tubes were very well matched, and both had equivalent screen geometry as well as equal dynamic screen, plate, and screen vs. plate characteristics, then a shared resistor would perform in a balanced way about as well as separate ones. Gassiness and other minor characteristics could come into play also, especially with modern EL84's or even NOS when you do not have a lot of tubes to select from, it is not very easy to find such outstandingly matched sets. Otherwise parallel operation emphasizes any mismatch in a similar way to how a shared cathode resistor on unmatched tubes overstresses the higher-dissipating tube at idle or the higher Gm tube when driving signal is present.  
 
to some techs 100ohms seems low, but that is what I like with EL84's, just need to select tubes which are tough enough to handle it if you are using higher B+. And still use at least 2W flameproof screen resistors, better yet 5W especially if it is shared.  
 
Mike

 
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