Can a variac be used to maintain constant voltage?

Actually, a Variac is a variable Autotransformer, which can be, and often is, designed to step voltage up as well as down. That would be done as shown in the first figure of the linked writeup, where the input voltage is placed across a segment of the winding that encompasses fewer than the full number of turns. Variacs that are designed for 120 volt single-phase inputs are commonly designed to output up to 140 volts, and sometimes more.

One other benefit they might provide, in addition to those Kijanki mentioned, is some reduction of high frequency noise that may be present on the incoming AC. However, as Rodman indicated Variacs are not specifically designed for audio applications, and an audio-oriented power conditioner (which in most cases won't regulate voltage) or a power regenerator (which will regulate voltage) can be expected to be a better alternative, albeit at a considerably higher price.

Regards,
-- Al

The Monster AVS2000 voltage regulator is a giant automated variac like device. I had one. The rheostat inside has a mechanical wiper which is electronically controlled to maintain the 120 volts. regardless of the incoming voltage.

So it keeps the voltage output steady with variable incoming.

Somut,

Just curious why you think you need to run the amp off the variac. On the back of the amp what is the input AC voltage rating? 120V? 125V line voltage will not hurt the amp.

Depending on how big, (VA rating), the variac is the variac could be starving the amp of power.

Plug the amp directly into the wall and set back and listen..... Post back your results.

I stand corrected! The power supply I used at home(test purposes), would only maintain, or maintain at at a reduced voltage. The veriac(Ohmite) I had on the job could up to double the voltages, presented to it(but still directly proportional to the input). Gotta stop the early morning posting.

My own reason for having this variac in the first place is that prior to the Threshold, I was using a pair of vintage vacuum tube monoblocks running 8 el34's per side: when turned on the amps would spike to the maximum level as gauged by their ammeters and then settle back down to the nominal 6 amp running rate: so, rather than having to deal with this initial surge and its potential negative impact, I bought the variac to slowly bring up the amps to running voltage. The variac is selling for a few dollars short of $600.00 now and I do not if its worth keeping this expensive piece for any valuable audio uses -- i.e. reduction of high frequency noise as Almarg mentions above -- or sell it off to someone with a more suitable application. I still find it amazing that the variac is showing a consistent 120 volts as set by my original dial setting. I had a PS Audio Power Plant Premier at one time and the incoming voltage showed as high as 125 volts sometimes. So it makes me wonder if the variac is not in fact more useful in my current Threshold application as might otherwise seem.