Vintage tube amp resurrection with Variac?

I have acquired some vintage tube equipment that has not been turned on for several decades. I bought a Variac on eBay having read many times to be sure to "bring up" old tube gear "slowly on a Variac". What exactly constitutes "bring up slowly"? Is it some sort of staging perhaps? If so, what voltage at each stage, number of stages, how long per stage. Or does it mean to continuously, but very slowly, crank the Variac from 0 to 117 volts?

Showing 4 responses by czarivey

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.
With tube rectifier things get simpler because tube rectifiers reach operational voltage slow enough to catch problem with or even without variac.
With variac set to operational voltage and voltmeters attached to the bias pots or resistors and one attached to the rectifier output you can monitor gradual voltage increase. Once bias on any of the bias pots goes beyond tolerant, you can bring back variac to the noted voltage or below or turn amp off and inspect circuit elements around the fault bias pot or resistor.
Dekay I mentioned earlier replacing all DC supply caps before even getting onto the variac.
In my environment I have to pay rent and make sure that investment in rent is returned. Mostly I restore items I purchased from estates or items purchased in bulk by pound.
Replacing DC caps takes me less than hour on most tube units.