07-19-2014, 08:34 PM
Seems like the 78 is gassy or has grid emission. When the voltage rises it may reach the point where the gas ionizes and causes the control grid to go positive.
Tubes can develop some weird problems. I had a 39/44 tube that did something similar. Radio would play fine for a few minutes then gradually reduce volume. Found AVC line went from normal negative condition when receiving signal to slightly positive as it warmed up. Replacing the tube cured the problem.
Tubes can develop some weird problems. I had a 39/44 tube that did something similar. Radio would play fine for a few minutes then gradually reduce volume. Found AVC line went from normal negative condition when receiving signal to slightly positive as it warmed up. Replacing the tube cured the problem.
