[crackle]

I thought I would just add in case you were not aware. There is a capacitor which connects to the grid of the output valve from the preceding stage. These often go "leaky" with age, and allow a positive DC voltage to build up on the grid of the valve.
If this happens then you can sometimes say goodbye to one or more of following, the output valve, output transformer, rectifier and the mains transformer.
If in doubt it is advisable to change the capacitor, they only cost a quid or so.
If you need any more information about this just ask.

Mike