[R-390] R390 NON A IF deck mods

[2002tii]

Dan wrote:

>V508 normally a 5749 (6BA6) has been replaced with a 6AU6 ???
>Seems to reduce the IF deck gain BUT makes the S-meter zero pot MUCH
>easier to set !!

The 6BA6 is a "remote cutoff" pentode, meaning that it takes a 
relatively large negative grid bias to cut off plate current.  The 
6AU6 is a "sharp cutoff" pentode, meaning that its plate current cuts 
off with a much smaller negative grid bias.  Remote cutoff tubes are 
generally used in stages with AGC control, to provide a proper linear 
AGC action.

In any case, all of the tubes on a common AGC bus should have the 
same cutoff characteristic, so the IF gain is distributed properly 
between the stages.

Best regards,

Don


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