By the way, don't plug tubes in the radio while it's powered through the
line cord. When powered by the line, the filiment supply is derived from a
dropping resistor in series with the filiments and a hum smoothing
capacitor. If a tube is pulled while the radio is line powered, the
capacitor will charge up to the B+ voltage (usually 90V). The cap will have
enough charge to burn out at least one, and maybe even all of the filiments
if a tube gets plugged back in. The same thing can happen if you get
intermattant connections at the filiment pins. Most of the replacement
tubes are a few bucks each, but if it uses a 1L6, you'll be spending maybe
$30. Or a 1R5 may sub. I have a couple of battery sets. I've put zener
diodes across the filament supplies.

Frank Dresser