Why are amplifier transformers sized the way they

Are you talking about power transformers, or output transformers of tube amps?

A large power transformer will stay reasonably cool when the amplifier is driven hard. It also will continue to deliver full voltage under load. A large transformer benefits continuous audio power capability. Peak power demands should be met by the power supply capacitors, with the transformer having lesser importance.

A large audio transformer implies use of heavy gage wire in the windings, and plenty of iron in the magnetic core. This avoids magnetic saturation at high power, thus reducing distortion.

Running a very large transformer reduces the potential for distortion, provides a lower impedance path for the AC, can deliver more current as needed in a more timely basis i.e. improved transient response under load, etc... There are no drawbacks to using a larger transformer than what you need other than the added cost and weight involved.

On the other hand, using too small of a transformer can result in reduced power potential, increased distortion, poorer transient response under heavy load, etc...

There was an old saying pertaining to amplifiers. That is, the heavier the amp, the better it was built. In many cases, this is the truth. Then again, one can skimp on a transformer and heatsinking and use double wall thickness chassis parts and achieve equal or greater mass. If you took the fancy "sculptured" faceplate off of some amps, it would cut their weight down quite measurably. Sean
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Sean, I have a quick question for you, if you don't mind. I need to buy a sub panel box. I am confused as to which brand/model/features that I should look for. My current thought is just about any that has a solid copper bus. Is there anything else I should know? Thanks, Brooks

I am not familiar with all of the various laws or codes across the country, let alone in other parts of the world. I would look for robust construction using heavy gauge metals, good conductivity and plenty of slots for potential upgrades / added breakers. Other than that, you'll really need to discuss the specifics with an electrician or someone in your area that is familiar with local codes. So long as you can make and maintain a solid connection with low series resistance and adequate current potential, you should be okay. Sean
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Sean, thanks. The electrician that I spoke with said the only thing that I needed to be concerned with is having enough slots.

I read a comment of yours in another thread about keeping all breakers on the same phase. Does this apply to the sub panel or just if you are running the wires from the main box? If is applys to the sub pajel, then it would seem that I would need a box with twice as many slots as I need because every other one would be unused. Thanks for all your time and advice.