After a long, long sleep: ML-335

I would probably just plug it in and let er ride! But…..

I think GS5556’s suggestion is the better one and you should do that. 

The big issue with old amps is the caps going dry, leaking, etc. and that’s what the variac should help you with.  That is, they'll help you blow a fuse before you do too much damage.  When those caps go bad they can take out the rest of the amp with them. 

Before using though, open it up and take a look around the biggest caps you find. They’ll look like soda cans with screws. Check them for any sign of corrosion or leakage. If it looks like the Coke or Pepsi has seeped out even a little, or the cans are bulging have it serviced first.

Do the same for any of the smaller caps too.  Inspect them for obvious bulges or leaks and at any signs of degradation take them to be serviced first.

Yeah, I would power it up with some care. At a minimum, make a dim bulb tester. But a Variac is the way to go. Capacitors that have been sitting so long, may not like the surge of power that the transformer will be dumping into them. Would really suck to have a cap go bad and then have it take out other components in its wake. 
 

I was gifted a Marantz 7T that had been sitting for an unknown amount of time not being powered up. Before I switched it on, I replaced the power supply caps with some cheap ones as insurance. That, and as I don’t have a Variac, I used  a dim bulb tester as “insurance”. 
 

Clearly, this is an important piece of kit seeing as how it came from your father.

The cost to have it shipped and given a once over by a qualified tech would be money well spent I would think.
 

It’s a bit irresponsible to suggest firing it up as it’s not a land mine…..  People shouldn’t comment on things they don’t know or understand. But that’s easy to do when dealing with someone else’s stuff.

Have it serviced, don't touch it before that. It is very valuable. Have it looked over and it will serve you well for a long time to come.

To use a Variac, you need to bypass the control circuit .