[crackle]

A further note on the subject of the grid coupling capacitor.
You can often tell if the grid coupling capacitor is leaking badly by measuring the voltage between the cathode and the grid of the OP valve. The grid should almost always be a negative voltage (less than zero) in relation to the cathode. The nominal value of the grid voltage is often given in the specifications for the valve.
The service sheets often give this voltage as a value for the voltage on the cathode in relation to the receiver chassis. Under ideal conditions this would be the same as the grid to cathode voltage. But leakage through the coupling capacitor will still bring the grid voltage up, from a negative one towards zero and even in bad cases to positive, or in other words the same voltage as the cathode. This is when the valve will pass excess current and can cause damage.
I hope you don't think I am trying to tell you you are doing things wrong, you may already understand all this, but it took me a long time to appreciate the effects of leakage in these audio coupling capacitors.
Mike