Vintage tube amp resurrection with Variac?

@OP, I'm not a tech and that's what makes my post worthwhile.

Knowing what I DO know is enough to say that rather than screw around with your old amp, I'd have it looked over by a reputable tech. If the manufacturer is still around... even better. I bet the old PS and coupling caps are dried out and need replacing. Other parts may need to be replaced too. Here's a quote from TLS49's article:

"You should probably consider replacing the capacitors on an old piece of equipment even if it doesn’t hum. Old filter caps will fail sooner or later, so you may save yourself some time and trouble later on."

Seems like good advice to me.

I'm bringing back Fisher, Marantz, HH Scott, McIntosh equipment brands that are up to 40 years old vintage.

With tubes inserted that I'm confident that are good I connect several voltmeters connected to each bias pot or resistor to make sure that none of tubes will significantly drop or increase bias current during voltage increments. There's a slight risk of damaging tubes, but it's part of business give or take. Blind replacement of DC supply caps (assuming they're all bad) before-hand usually never hurts and prevents from tubes being destroyed especially when I'm dealing with client's equipment that needed be restored.

Old coupling caps usually create less of a problem if any. quick test for short is usually enough

Now about increments and waiting time. I don't think that it's necessary to wait 30min per 6V increments. If I did that, I'd have to charge client at least a grand or couple depending on unit complexity and keep unit for a month or couple. I simply slowly bring it to the point when something begins to fail or to the operational voltage AFTER REPLACING ALL DC CAPS. Voltmeters or DMM testers I connect to each tube bias pot or if self-biased to the bias resistor to measure proper voltage. From voltage as you know you can figure the current and if you can't you'll need to go via basics of electrical circuits.
Current = Voltage/Resistance

Similar methods I apply to the vintage solid state equipment. I would usually use NTE replacement transistors instead of original or NOS to run these tests. I wouldn't be worried if I fry $1 worth power transistor instead of $15 one.

Lots of good, albeit varying, advice. My technique was based on the recommendation of a well known manufacturer who's name I have forgotten. I will say that technique is not one you would want to pay someone to do, but then if you are paying someone they will use what works for them. Given the amount of time these units were stored extra caution is certainly advised!

There is a simple way to do this using an incandescent light bulb. You remove all tubes and put the light bulb in series with the AC power and turn it on.

The light bulb will light up, which means most of the power is being dissipated by the bulb because the caps are likely shorted. However, if they need to 'form up', the bulb prevents them from heating up by limiting the current. If the bulb goes out the caps are formed up and OK.

Time-honored technique BTW; but if this is an amplifier you intend to really play a lot don't screw around- just change out every filter cap in sight... If there is a selenium rectifier change that out too.

If using a variac on a unit with the tubes installed care needs to be taken if the item is tube rectified.

If it has a slow start rectifier installed it will NOT see a slow ramp up from the variac, but will eventually quickly see higher voltage, which is not good for reforming caps.

There are some quick start subs for a GZ34 (think I once used a 5Y3, but not certain as it's been awhile).

A local tech once used a SS (plug in) rectifier on one of my vintage amps with a GZ34 (he made made it in a manner of minutes), but I've seen them for sale online as well.