DESCRIPTION:
For many years, Variacs have been used to gradually warm up electronic devices such as radios and amplifiers, giving components such as the filter capacitors the opportunity to gradually return to functional status. Detractors, however, claim that this process is a waste of time with tube-rectified circuits, because the rectifier tube does not begin producing DC high voltage until the AC input is quite high, around 90 to 100VAC, and at that point the DC high voltage will be produced in a rapid surge rather than a slow, gentle (0 to full B+) increase.

NOTE: Due to the fact that some solid state components can be damaged by operation at reduced voltages, this process is recommended ONLY for vintage, pure-tube circuits.

This video demonstrates the B+ output of both a 5Y3 and a 5U4 rectifier tube in response to AC input voltage that is gradually increased from 0 to 120VAC, and provides some insights as to the efficacy of slow Variac warm up in tube-rectified circuits.