I was advised to steer clear of the current crop of cheap variacs imported from china.
A high quality used Made-in-U.S.A. variac (5, 7.5, or 10 amp) from companys such as General Radio, Technipower, or Staco is a better value at under $75. These show up used on eBay, usually from estate sales and ham radio swap meets, and these are the ones I recommend.
To run a single tube amplifier or receiver, unless the unit is a very high power design, a 5 amp variac unit is fine, but anything less than 5 amps is not recommended. Tube preamps and tuners don't draw much power, so you can even run a preamp, tuner, and amplifier on one 5 amp unit with no problem. Certainly, you don't need a unit rated at more than 10 amps unless you have a special application. The older variacs are not wired with a 3-wire cord or socket. You may want to get one with a grounded outlet, for safety and ease of connecting a power strip to the variac, or if you intend to rewire your vintage tube gear with a grounded power cord.
If the seller does not state the model number, be sure to ask the rating of the variac. I purchased a monster 20 amp General Radio variac off eBay by mistake, thinking it was a 5 amp model, because from the photo it looked like the smaller model.
A 20 amp variac weighs about 50 lbs and can control the stage lighting for an entire high school auditorium stage. When I went to pick up this package at the Post Office, I wondered why the counter person at the Post Office was unable to lift the package and had to meet me at the door to the employee's only entrance. It is worth several hundred dollars, at least, and only cost me $40. This purchase has been my funniest eBay screw up, to date, that turned out all right in the end. It is not for sale.
