Variable transformers are used for several purposes with guitar amplifiers.
Tubes in amplifiers last longer if voltage is gently increased during power up. The filaments in the tubes are not stressed as much as when power is applied all at once.
When the amplifier has not been used for some time capacitors can fail if the voltage is not brought up slowly the first time. Using a Variac to slowly raise the voltage can prevent this problem.
Van Halen (and others) have used a Variac to produce 'brown sound' by operating his amplifier at lower than rated voltage.
The most popular models that are used with guitar amplifiers are:
3PN1010B Variable Transformer,
120VAC Single Phase 50/60 Hz Input;
0-140VAC Output;
10 Amps Maximum;
Manually operated;
Includes case, cord, plug, receptacle, lighted switch and fuse.
120VAC Single Phase 50/60 Hz Input;
0-140VAC Output;
10 Amps Maximum;
Manually operated;
Includes case, cord, plug, receptacle, lighted switch and fuse.
- or -
3PN1010B-DVM Variable Transformer,
120VAC Single Phase 50/60 Hz Input;
0-140VAC Output;
10 Amps Maximum;
Manually operated;
Includes case, cord, plug, receptacle, lighted switch and fuse.
Digital Voltmeter +/- 0.5%
120VAC Single Phase 50/60 Hz Input;
0-140VAC Output;
10 Amps Maximum;
Manually operated;
Includes case, cord, plug, receptacle, lighted switch and fuse.
Digital Voltmeter +/- 0.5%
Visit http://variac.com/staco_3PN10_20.htm for additional details