Transformers are comprised of at least a primary and secondary winding. Often times there are multiple secondary windings, or even multiple primary windings, but for the sake of discussion, let's keep it simple and consider 1 primary winding, and 1 secondary winding. These 2 windings are wrapped around (usually) an iron core, or some other ferrous metal. Maximum breakdown voltage ratings are based on the insulation used. Maximum breakdown current is based on the size of the wire used. Where the magic happens in a transformer is ratio of winding turns. A 1:1 transformer has the same number of primary windings as secondary windings. These are used for isolation and in a perfect world, what you put into the primary should be exactly what you get out of the secondary. A 2:1 transformer has twice as many turn on the primary as it does on the secondary. "Turns Ratio" can be measured as Vin / Vout. (Of course we're strictly talking about AC.) So if you put 10VAC into a 2:1 transformer you should get 5VAC out. Cool right? So consider the following, a power transformer with a 120V primary rating and a 480V secondary. The Turns ratio is 1:4. Now let's assume there's a 6.3V filament winding. That turn ratio is 120/6.3= 19:1. Now... there's the matter of those pesky VA ratings. VA stands for Volt Amps which for all intents and purposes can be called Watts. watts = volts x amps. This rating is the maximum power you can draw off the transformer. Anything more and the transformer will get hot and your voltages will sag (be lower than you expected). So when you're sizing a PT, you need to make sure your voltages are at least high enough for your needs and the power rating is also adequate. If you're voltages are a little too high, there are easy ways to blow off the excess. There's no easy way to grab another 50 volts that's not available from the source. Typically with guitar amps, the VA rating of the PT is just high enough so that the power will sag but only a little.
these same rules apply to output transformers. Audio is VAC with varying frequency and amplitude. Sizing an OPT is based entirely on the tubes driving it and the speaker load it's feeding. Designing from scratch, you look at the desired output tube's spec sheet for watts and Zout. You select your OPT based on these parameters. This can be dumbed down to, "this OPT was designed for an amp with 2 6V6's so it will work in any amp with 2 6V6's."
>bandmaster output transformer is good in a Deluxe Reverb, is a Super Reverb OT better? A Twin OT even better still?
IIRC, the Bandmaster and the Deluxe are both 6V6 amps, the super is 2 6L6's, and the Twin is 4 6L6's. If bigger is better, then yes, but that's not really a tonal solution. Under-sizing a transformer usually ends up in catastrophic failure. Over-sizing usually ends up sounding dry because there is no core saturation. It's a fine line of just big enough. A super OPT should work fine in an amp with 2 6V6's, but the Twin won't because the impedance is is too far out of spec. Yes you can change speaker impedance taps and make it work, but I'm not gonna encourage you to attempt to cram a Twin OT into a Bleu Jr.
>a Marshall 18W power transformer would not work in a Fender Blues Jr.
It might could with major rewiring. The BJ is fixed bias and the PT has an additional secondary winding for bias voltage. The 18 is cathode biased so it's PT doesn't have a winding for bias voltage.
Topic: Power and Output Transformers (Read 3365 times)