Valve / Tube Amp Repair / Servicing is DANGEROUSNote: For anyone who pooh-whos this and chooses to ignore what I am about to say, at least do one thing; ALWAYS ensure you have a RCB (sometimes called RCD) into which you connect your amps power supply - These units are similar to what is used in the garden with hedge trimmers - If a full short occurs then the mains 240VAC (or 115VAC) is automatically disconnected. All modern houses and public venues should be equipped with these in place of the old fusebox, but hundreds of thousands still use the fusebox as the only form of protection, which are not instant by a long margin. The bottom line here is better safe than sorry.
Unfortunately most people I come across have not even considered this fundamental safeguard, which costs around a fiver.
The thing I find so amazing is that they will happily remove the chassis and dabble about inside [innocently] thinking that only 240VAC is present - well it jolly well isn't. Simply put, and with particular respect to the output tubes/valves (EL34, 6L6, EL84, etal) they require a high Voltage Direct Current (VDC) to drive them and this can easily be in the 400 - 500 VDC range. What does this mean to you if you complete the circuit - one hell of a shock or even possible death. Also, with DC voltage you do not "shake" the same as if shocked with AC (Alternating Current), which runs at 50 - 60 cycles per second or Hz (hertz). With AC voltage you at least stand a chance of letting go, with VDC your muscles will "clamp-on".
Whenever I get an amp in for repair or a service, I always do 3 things before even switching on the power:
1. Check that if it has a mains EARTH connection that is in place and has not been disconnected. Amazingly some musicians will disconnect their amps earth wire to stop hum and then they wonder why they get shocks when they touch the microphone stand, which is earthed!!!.
2. Check that the mains fuses (and any internal ones for that matter) have not been fuse wire wrapped or tinfoil wrapped. Again, I have seen this cardinal sin being committed and why? "Well, the amp keeps on blowing fuses and this has stopped it" is a stock answer. Keeps on blowing fusees - doesn't that tell you something???
3. Make a visual inspection to see if there are any other peculiarities present e.g. discolouration due to high temperatures or a blown component.
If you are still intent on dabbling (and everyone has to learn), then find a friendly professional repair guy/gal who will allow you to watch the way they treat [with great respect] a valve amp when it comes in for repair/service.
This only touches on the subject, but at least it might help someone understand and think twice before removing that chassis.