I have an early 1950s am/fm Silvertone radio. Based on the chassis no., it's either a model 18 or model 20 (same schematic).The radio works fine on both am and fm when it's first turned on. After 20 minutes or so, the fm signals fade away, but am continues to work perfectly. After the radio has been turned off for awhile, the fm works again for another 20 minutes before fading away.

Any ideas about where the trouble might be or how to begin troubleshooting this poblem?

The 6T8 would be first suspect. It's a triple diode triode. 4 tests on a tube tester. The diode test will sometimes drop away after the same time the radio fades away. Swap the three 6BA6 tubes around. See if that made any difference too.

:The 6T8 would be first suspect. It's a triple diode triode. 4 tests on a tube tester. The diode test will sometimes drop away after the same time the radio fades away. Swap the three 6BA6 tubes around. See if that made any difference too.
:Interesting, Warren. I swapped around the 6BA6s, and it has been playing fine on FM for several hours. Thanks. Why do you think it makes a difference?

Sometimes old tubes have less emission than the other same number tubes in the radio. In one circuit the demand for emission can be more or less than the other circuit. A weaker tube will work fine in the lesser.

:Sometimes old tubes have less emission than the other same number tubes in the radio. In one circuit the demand for emission can be more or less than the other circuit. A weaker tube will work fine in the lesser.
:OK. I'll remember that. Thanks for your help.

If the FM wasn't used much when the set was new, and the set switches off plate current to the unused tubes, then the cathode of the unused tube may have been poisoned.

Sometimes poisoned cathodes can be fixed by sending a high amount of plate current through them, but not always.

A preventative measure is to bypass the switch that cuts B+ to the FM (or AM) circuitry with a 500K or 1Meg resistor. This resistor would keep enough plate current on the tube to prevent poisoning, and yet not make the circuits fully operational.

T.