Here it goes.
First of all if you run across one of those great finds hope that no one plugged it in to see if the tubes lit. Chances are they did, but maybe for not very long.
The first thing to do with an old piece of gear is to check the tubes. That might be the reason it was taken out of service to begin with. The second thing is to make sure you get all working tubes back in their original places so before you do the first thing mark the tubes so you know where they were placed.
Usually a bit of cleaning is in order and a large soft bristled paint brush is a great dusting tool to start with. Get rid of much dust as you can.
Bake the piece. I know it sounds strange but remove the bottom cover, check for strange deposits of wax or capasitor oil and if none is present then pop it in to the oven upside down. If you can set at at 175 that will be good. The upside down is to allow any moisture inside the transformers to escape. Do it right side up and steam can get trapped and later short out your transformer. The reason for this is that carbon resisters will slowly get moisture ridden and go out of tolerance. This is to get them back in so a couple of hours will take care of that.
Needless to say, carefully look power cords and replace if need be. Check the fuse, you want it to be original ,ie, old.
Now you are ready for the variac and if you are warming up a preamp or tuner you are set to go if everything is ok at that point. Power amps you need to remove the power tubes. I am not real clear on why you don't want power going to all the circuits but at this point you are worried only about the power supply and especially the large filter caps in it. If you have a doubt about the caps abiltity to take a charge you can remove all but the rectifier tube and leave it plugged into the variac at 30 volts for 12 hours or more and put a volt meter on it and watch to see if the voltage falls quicky or slowly when the amp is turned off. Slowly is what you want.
Start your warm up at 40 volts. You can hike the voltage 10 volts at intervals of at least an hour. If you take days to this it is better than rushing it. Also with the power tubes out of any amp you don't have to worry about having a load on it.
10 volts at at least one hour intervals till you get to 90 volts. At that point walk off and leave it for 12 to 24 hours. After that you can hook your preamp and tuner into a system and proceed with the 10 volts per hour. For an amp or reciever that has an amp in it you need to hook it up to some speakers (power off please) and put your power tubes back in, let it go for an hour and then finish at 10 volts per hour till done.
Now that your gear is fired up and you can hear it you listen for hums and pops. No hums mean that your caps are ok at this point. Pops can mean a short, bad solder connection or a bad potiometer. You can check out the pots by moving and wiggling the knobs. Pots and cleaners next time.
