I have a fender princeton chorus that I bought, I'd say at least 10 years ago. So it's not exactly new, and I think I've even had the same problems before and got it fixed, but I'm not even sure. If I did it was some time ago and since I've rarely played the electric in recent years I've kind of neglected the amp a great deal. But now I've got some important performances coming up in a very short time (namely for a local theatre production of The Rocky Horror Picture Show (I plug when I can)) and I sure could use some quick advice.

Here's the problem: The amp seems OK at first when I turn it on. After playing for a short time (10 - 20 min.) it develops a loud buzzing hum (which in the distortion channel is simply unbearable; in the clean channel it is present but not terrible). The more I continue to play the worse it gets.

The questions: What is the problem or the possibilities? How much could I expect to pay to have it repaired? Is it something that a complete layman who's good at following directions, like myself, could possibly fix himself and save some cash?
What you really need is a new amp.
There's no such thing; there never was. Where I am going you cannot follow me now.
we're done here

But boys will be boys and girls have those eyes
that'll cut you to ribbons, sometimes
and all you can do is just wait by the moon
and bleed if it's what she says you ought to do
A common problem I've found that can cause this is cracked solder joints on the pots and jacks. You have to pull the circuit board and reflow the solder pads. Because the pots and jacks are connected to the front panel and circuit board they get stressed when the amp is moved. The reason why i takes a while to show up is heat in the amp causes expansion.
A gradual increase in buzzing or hum as an amp warms up is a strong indicator of a bad power supply capacitor, or at least a bad or weak solder joint on a power supply cap. The heating causes the expansion that compromises the connection after several minutes of run time. The big caps are the most suspect. Try to inspect their connections carefully, a magnifier is helpful. You may not be able to see much. But you can "re-flow" the solder joints with a low-watt iron and see if the problem goes away. But a word of caution is in order. You can electrocute yourself from the lethal voltages stored in the caps! This has happened to a number of technicians who were just "going in for a quick look" A cap, especially a large power supply cap, stores electricity for days, weeks, or even months after the amp was last plugged in, and you must bleed all the caps prior to doing anything else inside. You must do this before touching anything inside the amp, as a precaution. Make sure you are standing on a rubber mat, or a thick rug or carpet, and always keep one hand away from the amp at all times. You don't want a cap to discharge a lethal amount of current from one hand, through your heart, and on through your other hand! Like it is often advised, learn to be a "one-armed" technician, for safety's sake. But don't be scared of working on your amp, either. Here's what you do: get an alligator cable, which is basically a length of insulated wire with an alligator clip attached at each end. Next, obtain a one-watt resistor with a value between 470 Ohms and 10,000 Ohms. With the amp unplugged from the wall outlet and the power and standby switches in the "on" position, ground one end of the cable to the metal chassis of the amp and clip the resistor to the other end of the cable, so that you can use it like a test probe. It's a good idea to use a cable that has insulation over the ends of the alligator clips, but if not you can hold the end with a pair of insulated needlenose pliers. You must next touch and hold the resistor probe to the positive terminals of each cap, one at a time, for about a MINUTE EACH. This may vary, but why take a chance.? I'll assume you don't have a meter to monitor your results, so this is seat-of-the-pants work. You are "bleeding" the residual charges that were still stored in the capacitors after the amp was operated. The resistor is limiting the amount of current flow, so that you don't get a spark when you touch the contacts. Once you have done this, however, you must still treat the circuitry as if lethal voltages were present, and remember the one-arm rule! You may then, CAREFULLY, re-flow all the capacitor connections with your soldering pencil. this may be a bit of a scavenger hunt, and you want to check the connections on the other end of the wires that are connected to the caps, as well. If you are finally satisfied with your procedure, then try the amp and see how it goes. Hopefully, you solved the problem. If the buzz or hum persists, it is likely that one or more of the caps has a bad internal connection, or is failing to the point that it will only work until it warms up. Then they need to be replaced, and it's often a good idea to replace the entire complement of power caps at the same time, or you may be having to repeat this over & over as each cap slowly wears out. You may notice an improvement in tonality, sparkle, low-end response, and overall volume when you replace old, worn-out caps. They don't have to be actually blown to affect the sound quality. This is damn near the number-one procedure when doing an amp restoration! The replacement caps don't have to be the exact same capacitance or voltage values, just get the same or larger values, as close as you can, but never use any with lower values. For example, you can replace a 400-volt, 22-microfarad cap with a new one that is 450 volts and 25 microfarads, but don't use 18 microfarad or 300-volt caps. The capacitance value is more about the circuit design & operation, while the voltage value is more about ruggedness. A higher voltage rating is good for longer life, while a radically different capacitance value may screw up the circuit design parameters. Many designers have it imposed on them to cut costs, and go with the lowest-voltage caps that will survive the operating conditions, to save a few bucks. There are reasons for paying big bucks for a real boutique amp. Nobody is cutting costs and limiting the life expectancy or sound quality at those radically high price points. You really do get what you pay for! I hope you get the idea that a big part of a capacitor's job is to eliminate AC hum from the signal path. Caps do other useful things, too, such as coupling circuit sections together that have differing voltages on them, allowing the guitar signal to pass freely to each part of the circuit while preventing the power supply voltages from being connected or "shorted " to each other. They are also a must-use component in tone shaping circuits. However you did not exactly ask to have all this stuff explained in fine detail, so I apologize for my digressions. I want you to consider the idea that working on your amp can be as much fun as playing your guitar through it, that's all. You can learn a lot of easy, cheap amp mods, and enjoy the experience of having a dramatically improved sound, that you tailored to your exact wishes! Best of luck, thanks for reading this, and remember to do it safely.