A Variac is special type of transformer that supplies a variable AC voltage. A variac usually has a big control knob which when you turn it gives a voltage between 0 and 240V out. Why would you want that then? Well with valve amps having lethal voltages all over them it is useful to have a way to turn them on slowly for certain types of problems. Actually the same is true for transistor amps too – just to a lesser extent.
The bench variac that I have built is based on a 7 amp Zenith Variac with a steering wheel on it, an analogue ammeter for keeping an eye on current drawn by the amp under test and a digital volt meter for keeping an eye on the voltage and a double pole isolating switch to keep me safe.
I almost always use a variac to power on an amp, here’s why:
- You can prevent damage to an amplifier from turning it on – especially useful on amps with unknown history – electrolytic caps lose their strength over time, it is argued by some that they can be restored to an extent by turning them on slowly.
- When a fuse would normally blow or smoke would come out. A variac enables you to turn on gradually and identify the physical source of the fault.