[R-390] Need straight poop on 6C4

[Flowertime01 at wmconnect.com]

Craig,

Any good 6C4 will work in an R390.
However, like every thing else all tubes were not created equal.
Some are just better than others.
JAN marked are OK.
RCA Sylvania brands are likely OK.
Other brands can also be good.

Buy what you can get and then swap them into the same tube socket and compare 
them for signal strength and noise. Some very strong signal tubes are also 
very noisy.

Plug them all in, and line them up on the bench from best to poor. Save a few 
extra. Plug the best ones in to the receiver. A brand new sleeve of five 
tubes will have each tube perform different. If they are all very close, have high 
output, low noise you get lucky. Sooner or later you will want some more 
tubes. Buy 5 for you receiver that needs 3. 

Some time later (6 months at 24 x 7) the tubes will be different. Do the 
comparison test again on all the tubes and run the best you have. While doing 
6C4's also do the 5814, 6AK5, 6AK6, and 5759 tubes.

Roger KC6TRU