Very special radio you have there. It’s the first time I see a table radio with both AC and 6V DC vibrator power supply. I can see that when operating on DC, the filaments are fed directly from the 6V battery and the B+ comes from a standard vibrator power supply. Now when operating from AC, a primary winding added to the transformer (27 ohms winding) makes it act as a standard power transformer for the B+. But you need 6V to feed the filaments. That is taken out from the windings on the vibrator side of the transformer. The poor quality of the schematic on Nostagia Air makes it difficult to analyse the wiring, especially around the plug at the lower left side. My guess is that something in that area, may the vibrator itself, is cutting filament power after a few minutes of heating and leaves you with a dead radio. Try the radio cold without the vibrator. If it does not come on at all, you have a cle to the problem.