All assembly and testing and tweaking of electrically powered equipment should be done with care. Mains voltages (100-240 Volts AC) themselves are dangerous and almost all valve-powered amplifiers have higher voltages (250-500V DC). The latter can be stored within capacitors insude the circuit – even when the amplifier is switched off.
What this means, in practice, is that any device which has ever been connected to the mains and powered up should be treated as if they contain potentially lethal electric charges and then properly tested to make sure that any such charge has safely dissipated. Only then is the amplifier safe to work on.