[R-390] maintenance

[Tim Shoppa]

Scott adds:
> 1/2 of some of the 12AU7's are gone.  Measuring 20 on 1 side and 90 on 
> the other

My experience with much simpler (fewer tubes, less "excess
gain") radio is that I often get the radio with the original factory
(e.g. Hallicrafters-branded tubes in a Hallicrafters or
Daystrom-branded tubes in a Heathkit) tubes.

The radio works fine with the tubes.

I pull the tubes and run them through the transconductance tester
and every single tube is below the acceptable number, often two or
three times low.

I put new tubes in and the radio works fine too.
Maybe the AGC action is a little different with the new tubes.

Now a 390 or 390A has so so many gain stages (if not labeled
gain then the mixers still provide gain). There is no shortage
of raw gain available from the tubes. Many of the stages are
operated with considerable degeneration and you will simply
not notice if the tube is considerably low in transcondunctance.

Tim.