You gotta love the Internet. I googled "variac troubleshooting tubes" and the first hit was "Adams Dynaco Stereo 70 Troubleshooting Tips" :
http://www.electronixandmore.com/adam/dynaresto.html
"If you have acquired a Stereo 70 in unknown condition, there are several things you should do before trying it out. Unless it was working when you acquired it, you shouldn’t assume that it is working, and there are several parts that should be replaced even if it you think it works. Do not just plug it in and turn it on, or there could be catastrophic results. WARNING: some parts of this amplifier use voltages in excess of 400V. If you do not feel comfortable working on a device which employs such high voltages, have someone who is repair it for you. Do not work on the amplifier while it is plugged in. Even while turned off, high voltages can be found near the power cord. Before doing work near the filter capacitors, make sure they are discharged by shorting them with an insulated wire or screwdriver. Also, make sure that the solder joints you make are good. If not, they could cause problems. If you do not know how to solder, take the amplifier to someone who does."
etc....
