Ah I thought you meant measure it.I was not suggesting you check the actual voltages in the amplifier, just that you take a look inside the amplifier with no power and check the voltage rating of those capacitors which will be written on the capacitor body.
Have put it on tonight having put in and taken out all the left hand valves a few time and so far no crackle!
It is being taken out of the rack tomorrow for works on the rack so may well have another look based on all the suggestions above, thanks guys