Can a variac be used to maintain constant voltage?

I still find it amazing that the variac is showing a consistent 120 volts as set by my original dial setting.

If you have a descent true RMS DVM meter you should check the voltage at the receptacle the amp is plugged into at start-up. That will tell you what the true voltage drop (VD) caused by the inrush current draw of the primary winding of the power transformer of the amp.

Next voltage reading to measure is with the amp loaded. That is with music playing, volume set to where you normally listen. Play something with lots of dynamics.

Measure the voltage... If the voltage is fluctuating in rhythm with the music that will tell you the variac is voltage/current limiting. To verify the variac is causing the VD repeat the same test with the DVM at the wall receptacle the variac is plugged into.

Dynamics performance of the amp will suffer.
Your amp should sound best connected to a dedicated 20 amp branch circuit without being restricted, choked, by the variac.
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With your voltage only varying by around 1-2 volts, plug your amp right into the outlet. I would love to have my voltage stay that close to 120 volts. Mine will vary up to five volts, and sometimes more, the same same day. I live in a straight residential area. In other words, no commercial equipment running off of my part of the substation feeding the power to our house. My friends that even live in a different state have similar variations.

One problem with a variac can be when your amp draws a lot of current (when it puts out a lot of watts), the voltage will drop a lot more through the variac. Another problem a regular conventional variac will usually have a contact (sometimes with a motor type brush), that can add noise to your power. With voltage that stable, plug your equipment in without the variac. You will be way better without it.

I have a couple of variacs. One of them will put out about 130 volts, the other one will put out about 140 volts (if memory is correct) with 120 volts in. More voltage output yet, with more than 120 volts in. With my voltage running low, and a variac boosting it, it could output way too much when the voltage would go back to 120 volts, or worse yet, a lot more when my voltage goes to around 126 volts, as it does sometimes. That would probably damage something. Another good reason not to run one.

Variacs are mostly used for testing. One would be to correct the voltage to 120 volts for testing and measuring, another reason is to simulate what some equipment will perform like on 114 volts to 127 volts. This is what an outlet can give out, and still be within power guidelines set by government regulations. They can be used for testing and simulating, or temporarily correcting other voltage swings too. But still, they are mainly used for testing.

On a related point what is the difference between a variac and an autotransformer (which is what I have)? Thanks.

On a related point what is the difference between a variac and an autotransformer?

The Wikipedia writeup I linked to earlier provides a good answer:

From 1934 to 2002, Variac was a U.S. trademark of General Radio for a variable autotransformer intended to conveniently vary the output voltage for a steady AC input voltage. In 2004, Instrument Service Equipment applied for and obtained the Variac trademark for the same type of product.

So a Variac is a kind of autotransformer. Other kinds of autotransformers may not be variable, and may be designed for purposes other than controlling AC power. The "autoformers" used in the output circuits of some McIntosh amplifiers are an example of an autotransformer that is not a Variac.

Regards,
-- Al

Thanks all for your informative input. I guess a variac is simply not suitable for my purposes so I am selling it.