For cathode-biased amps such as Fender Pro Jr. 15 watt
amp with 2 6V6's, or VOX, he thinks a variac might not work. A variac can take a 100 watt Marshall plexi down to 5 watts, when
you pull 2 tubes and run the variac at 55 volts. A variac works just with solid state rectifiers, not with tube rectifiers, because with
a tube rectifier, the heater voltage gets reduced too. He offers the THD Hot Plate attenuator for $249. A variac is also good for
amps that are meant for 105 or 110 volts, rather than 130 volts; 130 would damage the amp. He suggests using a variac and
attenuator in conjunction. For a 15-watt amp, he recommends an attenuator.