I am going to attempt to explain some of the basics of tube amplifiers in different sections, primarily to help dis-spell some misinformation that is out there, and explain our choices in parts and operations. I am going to stay away from the heavy math parts, and the overly technical.
Power transformers are integral to the overall operation of an amplifier. At the most basic, a transformer transforms one voltage to another (or sometimes multiple voltages). For most tube amplifiers, wall voltage comes into the first side of the transformer, also known as the "primary" side. On the output side, or secondary, we have a few voltages. In most tube amplifiers, there will be a 5 volt, a 6.3 volt, and some sort of higher voltage, normally referred to as B+ voltage.
The 5 volt is usually used as a filament, or heater, voltage for a rectifier tube. Most rectifier tubes used in tube amplifiers run off 5 volts. This filament voltage is what warms up the internals of a tube to make them function properly. Usually this voltage has enough current, measured in amperes or amps, to handle the tube being used. The current can be thought of as a supply, or reserve that the tube can pull from. With rectifiers, having more current to supply the 5 volt supply is perfectly fine. The thing to avoid is not having enough of a supply, as this will overheat the power transformer and could cause it to fail.
The 6.3 volt is the filament that normally supplies the preamp, driver, and power tubes. Pretty much all of the other tubes in the amplifier. This has a similar function as the filament in the rectifier section. The difference here is that too much extra current will cause the voltage to be too high. The really basic explanation of how a tube works is that when electricity hits the filaments, they warm up, kind of like a toaster. When they warm up, they heat the inside of the tube to make electrons transfer from one part to another. If you put too much or too little voltage in the filaments, it makes the tubes not function at their optimum. Too high and the sound is harsh, and grating. Too low and it is mushy. Right at the correct filament voltage the tube will have the greatest fidelity and frequency response.
The B+, or high voltage is dependent on what voltages are desired down the schematic. Each amplifier model may want a different B+ voltage. In designing an amplifier and choosing a transformer, there are voltages that are desirable ranges for different tubes to sound good in different functions. Consistency from one to another is very important.
We pick our transformers to make sure that we have exactly the voltages and current supplies we need. We use Mercury Magnetics in all of our builds, as we believe in the quality of their product, and they work closely with us to make sure we are getting exactly what we want. There are a number of other transformer manufacturers out there. Some good. Some bad. Just like anything, really. A number of companies will use generic transformers that will work in a schematic, but the current supplies or B+ may be off. The companies know that it will technically function, and they can offer them out cheaply. Most budget transformers fall into this category. A lot of lower end amplifiers benefit highly from having transformers changed to have proper voltage and current supplies inside. We prefer to start out with the best.