AR amp w/ variac

Some tube amps can oscillate when unloaded (some SS, too...) so I always used a power resistor on the outputs and a shorting plug on the input. Most ARC power amps include a relay circuit that lets the supplies come up before applying full line voltage; you may have to bypass the relay. You might want to use a shorting plug on the preamp and a load resistor on its output as well; I used 10 k to 100 k ohms just so it wasn't seeing an open circuit on the outputs. In the past I would bring the voltage up slowly over several minutes to maybe 60 ~ 80 % and then let it sit for an hour or two to help recondition the capacitors.

Make sure you test all the tubes first.

HTH - Don