Hi. I need to build an AC power supply, to power from one to eight tube pre-amplifiers. Power requirements of each (what the factory ps produces) is 18VAC, 100mA, 15 watts. I would like to be able to have each output meet those specs.
Well, that'll be interesting to do, because 18 volts at 100mA multiplies out to 1.8 watts -- where do the other 13.2 watts come from?
If there were a typo there, and you really just need 18VAC at 100mA eight times, then look for a power transformer that delivers 8 x 100mA =
800mA at 18VAC. You could either use a pair or Rat Shack filament transformers, one 6.3VAC and one 12.6VAC to get 19VAC which should be close enough, or you can look in the catalogs of Digi-Key, Mouser, Herbach & Rademan, All Electronics, or just about any other internet retailer of electronic stuff for the correct part.
Should you go the Rat Shack route, you should connect the primaries (120V) side in parallel, and connect the secondaries in series. The result will give you either close to 19V or it will give you 6.3V. If it gives you 6.3V then you have connected them up in "buck"; you need to reverse the connections of _one_ of the transformers to get the right voltage.
Ground your case, make good connections, observe all due electrical safety precautions, don't kill yourself, it's not my fault if you do, etc., etc., etc.