My tech told me to build one of these when testing these old amps.
https://2.bp.blogspot.com/-ahvfxs8G...ABTI/Ar6sz358jHg/s1600/light+bulb+limiter.jpg
This was the simplest picture i found without looking very hard. You can also chop an extension cord in half, attach both black wires to either side of a lamp, then reconnect white to white, green to green. Plug it into a gfci on your first attempt incase you screwed something up... 150watt incandescent gives a dim glo when everything is working fine, lower wattage bulbs will glow brighter sooner. You can also use this as a compression effect with lower wattage bulbs.
Hell, I just plug em in.