What you might use with your ST-35 in place of the variac is some type of "soft start" device which delays the application of B+ high voltage until the tubes have warmed up. It is useful though not absolutely needed with the ST-35 power amp and the SCA-35 integrated amp. Both amps have solid state rectifiers and apply the B+ high voltage almost instantaneously. Some say that the application of high voltage before the tubes have warmed up can cause "cathode stripping" on the output tubes and reduced life on these tubes. Personally I have never seen this and the fact that there are still many ST-35's and SCA-35's out there working fine gives testament to the fact that maybe this issue is over hyped.
You can make your own "soft start" AC outlet by wiring an outlet so that one side of the AC line has a thermistor in series with the line. The thermistor acts then as a current inrush limiter slowing the rate at which the full 120 volts AC is applied. The thermistor used should be capable of handling 5 or 6 amps continuous.
Dynaco amps with tube rectifiers have a built in delay with the use of a GZ34 tube. This tube is indirectly heated and takes 15 seconds or more to warm up before applying high voltage.