Cheers, personally the safety aspects of working on Valve Guitar amps cannot be stressed enough, which is what I was originally getting at, I've had some pretty close calls when working on Valve circuitry in the past, and know from first-hand experience just how dangerous it can be if you're not careful, but as long as you follow certain procedures you can minimize the safety-risks involved.
Whenever I work on any piece of electronic equipment that's mains-powered, I follow this procedure to ensure my personal safety:
1, First thing I do is turn off and completely disconnect the piece of electronic equipment from the 240V AC mains outlet, I usually do this by pulling the 3-Pin mains plug out of the wall socket after switching the wall socket and the piece of equipment off, if it uses an IEC "Kettle-Cord" I completely disconnect that from the equipment I'm working on.
2, I use my Digital Multimeter set to read DC Volts on it's highest DC Volts range to check for any residual voltage in the circuit, I attach the black multimeter probe to a suitable grounding-point in the piece of equipment, and I put my left hand either behind my back or in the back pocket of the shorts or trousers I'm wearing, and I hold the red multimeter probe in my right hand while I probe for voltages, this ensures that I don't get any dangerous shocks going across my chest, if I do accidentally happen to get a shock.
3, If I do get any significantly high voltage readings, I use a resistor of a suitable value and a couple of alligator-clip leads to discharge any power-supply filter caps to ground while monitoring the voltages with my multimeter.
4, Once I am satisfied that there are no dangerous voltages present in the equipment I'm working on, I consider it safe to work on, then I commence working on it.
5, Once I have finished doing the work I Double, and Triple-check that I have done everything correctly before I plug the equipment into the mains wall socket prior to testing the equipment for correct operation.