Variac?

[Notimetolooz]

Do you know how to use the Search function on this forum?
Actually I just did an advanced search and didn't find as many threads as I thought I would.
Anyway, first step is to try to predict what the largest current draw will be on anything you are likely to work on in the near future. Most radios and TV in the later years have a wattage rating number on them. Divide the wattage by 120V and that will give you a rough idea of the amperage draw.
Most home radios would draw less than 2 amps. Most TVs, jukeboxes and large stereos would draw less than 5 amp. Some HAM radio transmitters might draw 15 amp or more.
I my opinion a ammeter or watt meter should be used with a variac.
In use the voltage is slowly increased from zero, if the current or wattage climbs too high or too fast that means there is a problem.
There are a few tubed based TVs that have what could be called a "soft start" circuit where a relay does not activate until the voltage climbs to a certain point. Those can be a problem, the increase in voltage should not be increased too slowly or there will be problems.
Powering up a solid state unit may cause problems, so it may not be a good idea.
A low cost alternative would be to use a "Dim-bulb" tester.
https://www.antiqueradio.org/dimbulb.htm