If you're going to build amps, it's good to understand how to spec and source iron. So this will depend on what kind of amp you want to build. Since an 18 watt Marshall can be a few different things, I'd need you to narrow things down more. For instance, some 18w Marshalls have a tube rectifier and some do not. Some have a tremolo channel and normal channel. Some use a simplified single channel. Some do a sort of AC30/Marshall hybrid type design using an EF86 on one channel. Some modify the amp to have bass middle treble controls.
Unless an amp has solid state stuff going on, like SS reverb, SS FX loop or SS channel switching, then the things drawing current are the tubes. If using an old Fender style pilot light you can add that on too.
First figure out all the tubes in the amp. Write down their heater current draw. Add them up. Give a bit of room for extra so the power transformer isn't over stressed.
For instance, most 9 pin preamp tubes run 300mA at 6.3 volts. 150mA at 12.6 volts. Power tubes will vary, and you can find the number by googling the data sheet. Hint: always look up the data sheet.
Suppose you calculate you need 2.3A of heater current. Shoot for iron with 3A. Don't get an iron with 2A. 2.5A will technically work, but you want a bit more wiggle room than that. 3.5A is pushing it. 4A is too much.
Then calculate the B+ current. Preamp tubes vary quite a bit, but for 12ax7's it's easier just to calculate 1mA per triode. For power tubes, go with the maximum current draw. For the plates and for the screen grids. Add them up. Add a little extra for a safety margin. Suppose you calculate you need 100mA. Look for iron that can do 125-150mA. Don't go for 75mA unless you want to replace your power transformer quickly. Don't go for 250mA unless you want your B+ voltage to go too high and also stress your power transformer.
Suppose you want a rectified B+ voltage of 350. Here the magic number is 1.3. Really it's 1.414, but under load it's 1.3. 350/1.3=269.23. You want a transformer winding of 270-0-270. Or with no center tap, 540 total.
Write down what specs you need for the amp you plan to build. Then match them to a power transformer you can buy off the shelf. There's no need to buy some insanely expensive thing from Mercury. Hammond makes plenty of transformers, and one of them will work for your build. You just have to check the specs. EdcorUSA also makes quality iron. As does Pacific Audio and Sour Sound. I could of a few UK based ones if you're in Europe.
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