On Sun, 4 Jan 2009, Count Floyd wrote:

Just changed some tubes in my S-38 and the original 50L6GT was changed
to 30L6GT by my tube supplier. I know that it is the audio output
tube, but listening to it does not seem to make any difference. Is
there a difference? Should I pull the tube and use the 50?
Thanks

Yes.

The first number is the voltage of the filament. Most equipment would
have tubes starting in 6 or 12, indicating 6 and 12 volts, but in some
equipment they had no transformer to drop the voltage down to filament
voltage level, so they put the filaments in series. Since there were
only a handful of tubes in the unit, they used tubes created for the
purpose, which had higher filament voltages.


The 50L6GT requires a 50volt filament, and thus uses up just under half
the AC line, making it easier to drop the rest of the voltage. It was
common to have rectifier that used 35v on the filament, which then
totals 85 and only about 30 volts more were needed. That can be
made up of 12 and 6 volt filament tubes.

By using a 30L6GT in the radio, you've caused all the other tubes
to see a higher filament voltage. At the very least, that will cause
a shortened life for the tubes.

Michael VE2BVW