how do i make a 50V power supply?

I presume you want + and - 50 volts DC for an amplifier project. If so, you don't want a transformer that puts out 50 volts AC. When you rectify and filter the transformer output the DC voltage will be near the peak of the AC waveform, which would be 70 volts. In practice there will be some loss so the DC will be a bit less, but still far from 50 volts.

Your best bet is to get a packaged ready-to-go DC power supply, such as those available from hypex. Many other manufacturers also offer power supplies. You need to know how many amps it must supply as well as the voltages.

Can you use a variac (variable transformer)and monitor the output.

I assume that you want 50VDC? How much current needs to be supplied? Does the voltage need to be regulated? Will the current demand be constant or will it vary? What about sonics? Poor choices made in designing a PS can destroy the sonics of the gear it's used in. Without more information no one is going to be able to help you. Besides voltage and current requirements and the possible need for regulation there are issues such as: noise (how much ripple will the following circuits tolerate), transformer ringing, switching noise generated by silicon rectifiers, etc. Also, many if not most audio circuits, analog output stages in particular, include the PS in the AC signal current loop (a capacitor acts like a short to an AC signal). The filter cap in the PS will most often be in the signal path and affect the sound of an amp. It would help if you'd indicate what type of equipment this PS is going into.

thanks for your help guys im really new to audio im only 15 so everything is a little ove rmy head i bought "the audiophiles project sourcebook" and there is a power amp schematic in there that i wanted to try out. the problem is that it only says the voltage needs to be between 40 to 55 volts i really appreciate your help im going to read up on the power supply section in the book more and i will probably be able to understand more of the information you gave me

the power amplifier is complementary-feedback BJT audio power amp
the book says that it is arated for 120-W RMS output power into 8-ohms its designed to operate fromdual-polarity rail volatages between 40 -44 V the input intensity is approximately 800mV input impedance is 12 kohm

also in the book with the first power amplifier it shows has a pretty simple PS schematic that uses 24 V.C.T. @ 2 amp transformer however it doesnt state what the voltage that perticular amplfier needs to operate so i have no idea if the amplfier i want to build runs off the same type of PS.
if any of oyu have the book you could probably help me a lot more because im very new to this
its THE AUDIOPHILES PROJECT SOURCEBOOK by Randy Slone and the amplifier i am looking to build is on page 154

thanks for your help if u can even give me any lol