Mouser Electronics is one place.
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Search on Ebay - plenty there for cheap money. you want something like single phase, 120v, 10A or 20A. If you run the gear at least once a year or so, no need to reform caps with variac. If you left it idle for 5+ years, then maybe. Some old types of caps are no longer available new, but anything can be replaced, it's just a question of whether or not you can live w/ it not being the original "Mcintosh" part.
If you do not use any type of electronic gear for an extended period of time, powering up with a variac is never a bad idea. This is especially true if the devices are old and / or exposed to very high voltage levels ( like tubes ). So long as the gear is getting a good workout on a somewhat regular basis, it is not really a big deal. However, the older the gear & the less it is used, the greater the potential for damage.
As with most electronics, high heat levels are a major problem for capacitors. Items should not be stored in an area that is exposed to high temperatures ( like an attic ) or someplace that has a heat duct blowing directly on the items. Newer capacitors can deal with this a little better than older caps, but none of them really "like" this kind of treatment.
If you have other tubed gear and / or buy older used gear, a variac is kind of a handy thing to have. Just make sure that you fuse the input to the variac ( from the AC wall outlet ) and the output which feeds the gear being powered up. It is quite possible to smoke a variac when powering up gear that is not "healthy". Everything might be fine until you hit a certain voltage point and then "poof".... As you might guess, it's cheaper to blow a fuse than to smolder the variac. Sean
J: I would fuse the variac for an amp or two LESS than what its' rated output is just to be safe. If you have a 7 amp variac ( somewhat common size ) and try pulling 10 - 12 amps from it, it can smoke the variac without tripping a 15 amp breaker. If you blow the fuse on initial fire-up due to the in-rush current filling the capacitors, you should be able to simply replace the fuse and power the device back up again. The initial surge that blew the fuse should be enough to reduce the load on the variac to the point that it can function normally from that point on. If you blow a fuse the second time, try it one more time and be careful. If you pop a fuse three times in a row, the device being powered up is probably defective or is EXTREMELY current thirsty.
Here's a link to Mouser Electronics. Hope this helps... Sean
Some variacs come with fuses. At a hamfest this fall, I picked up a sweet old Tektronix unit, 10A I think, with Triplett meter that can be switched between incoming and outgoing line, 2A and 10A fuses built in, adjustable dial (of course), removable power cord, 2 outlets for output, pilot light, etc. And of course the reknown tektronix build quality. Grand total: $5. Gotta love stuff like that. I won't even go into the tube or scope deals that were to be had.