What you said, or reranted is not wrong in some instances. One notable exception is audio amps that use VFETS for outputs, which are depletion mode and WILL fry if you try to run the up slow on a variac, at least in the Sony ones. I don't know of any others but to be tolerant of low AC input it would take a quite special design with abnormal conditions in mind. I worked for a Sony Signature dealer and ASC back before their product turned to real **** and I can tell you that i n most cases, abnormal conditions were not on their engineers' minds.

Dim bulbs and variacs come in handy when you have repaired most regular solid state amplifiers. Once you drop $35 worth of silicon in them for a fried channel it is nice not to have all that short out in the first millisecond..

With most tube stuff, honestly I would just plug it in. Possible exceptions would be if it has selenium rectifiers. Even moreso a high end tube amp with a big old 5U4 but using a selenium rectifier for the negative bias to the outputs.

Bottom line is you can't just make a blanket statement about working on all equipment. And with the advent of the SMPS, there are even less assumptions that can be made. You can usually use a DBT on like a DVD player or something like that and it will keep the smoke in in most cases. But the normal current drain is usually so low all a hundred watt bulb will do is keep you from burning foil off the PS board. Some short on the five volt line is not protected and if it is going to fry something it is going to succeed. Then, if you use a lower wattage bulb, some SMPSes won't start.

Before I got kicked off AK they were always talking about the DBT. And really it does prevent pretty much all board burning. You burn the foil of a circuit board it is not fun to fix, especially with audiophiles who are going to want your repair to follow the same path as the original foil. I have done that type of rework but really most of the **** today does just fine with jumper wires.

Lately I am dealing with a design defect in amps built by Apex for RSQ, KDS and Technical Pro. All a bunch of liars on the power specs, Technical Pro claims like 1,300 watts or something on an amp that actually puts out about 90 watts a channel. The defect is that the assholes thought the bias regulator would just hold itself to the heatsink. Well they don't, and now ater going through several options we just could not make happen I am simply gluing them. Got me some JB Weld and made some clamps out of a couple of coat hangars. How is that for factory service ?

Enough rant, back to the subject. The DBT and variac have actually lost some of their usefulness. They are not useless but it does depend on what you are working on. Old tube stuff I would just plug in. For a few seconds. then I would remove all the tubes ad plug it in for longer. then I would put in the rectifier tube and see what happens to the current draw, with all the other tubes out. Scope the caps, if all the other tubes are not in there and you got ripple you got a problem. Turn if off before you blow that 5U4, which is no longer available for $3. The shipping is more than $3.

I have had a pretty diverse career and even got a little bit into CNC machines but not much. I learned automotive electronics when they started with those ECMs and ****. We flipped cars, and the reason we made money was because the backyard mechanics could not understand electronic and My Goodwrench forgot there was an actual engine under all that ****. I beat them several times in troubleshooting. Three of them making fifty bucks an hour looking at a computer readout and can't tell you have a burnt valve or a fouled plug. And the idiots probably made more money than me.

If you think powering up old electronics is scary, try starting an old car that has been sitting for 20 years or more. That's why some people actually take them apart first.

Well not completely apart but they will do a compression check, change the oil, whatever they can coax out of the crankcase that is and put new oil in it, drain the gas tank, pressure check the cooling system maybe, but that can come later after the thing actually starts.

Everything requires a slightly different approach.