[Mondial]

I would first check the coupling capacitor between the plate of the first audio stage and the grid of the output tube. If this cap has not been replaced, change it immediately.

Any leakage through this cap will upset the bias on the output tube, causing excessive plate current and eventual damage to it and the output and power transformers.

If this cap has already been replaced and is good, then as Arran has said it could be a bad resistor, other cap or gassy tube in the audio section.