[David G4EBT]

I've attached an example below.

It's part of the Bush DAC90A circuit showing the audio coupling capacitor to the grid of the UL41 output valve from the anode of the preceding stage.

Its role in life is to allow AC (audio) signals to pass to the output valve for amplification, whilst blocking any DC from getting to the grid. If it 'leaks' in the electrical sense, it allows Dc to pass to the grid. If that happens, excess current will be drawn by the output valve, which will pass through the primary winding of the output transformer, which - particularly in the DAC90A - are known to be fragile. It's therefore a good example of what can happen if - as is often the case - someone can't resist the temptation to 'switch it on to see if it lights up'. That's often followed by 'it lights up but I can't hear any sound'. You won't if the output transformer primary has burnt out.

Given that DAC90As generally command prices far above their worth as a radio, and they are quite costly to restore in any event, in terms of replacement capacitors, (and often, the likely need for a rectifier valve and output valve), unless 'that' capacitor is changed, it will become even costlier and more complex to restore. Particularly dispiriting for newcomers, who seem to favour the DAC90A - maybe because they're plentiful as hundreds of thousands were our on rental from about 1948 into the late 1950s, and are seen by some as 'iconic'. (As an aside, on the DAC90A - and other sets that have one - the mains suppressor capacitor across the AC input should be snipped out and either left out or replaced with an X Class capacitor. If left in place, it may self-destruct with an almighty bang. If the suppressor capacitor is removed and not replaced, it's no detriment).

Hope that helps a bit.