Heat causes drift in the oscillator circuit. Once temperture stabilizes drift will stop. Probably not much you can do to prevent drift? Tubes such as 12SA7, coils and capacitors make up the circuit. Drift controlled capacitors are available but would take experimenting.
Norm
:Does anyone know what would cause station drift in an early 1940s American-5 superheterodyne? I need to retune twice during the first 30 minutes the radio is on. Then it stabilizes. The drift isn't a lot, but it moves off the center of the signal enough that the sound is fuzzy and distored. Otherwise, it works fine.
:Thoughts? Thanks.