When I plug my Ephiphone Sheraton II into an amp, I get a very loud humming or buzzing sound from the amp. I can never put the volume higher than a minimum setting. I also have an acoustic electric that barely hums when I connect it, so I know it's not the amp. I'm thinking there is a problem with the ground in the guitar, but how can I fix it?

You have a bad ground somewhere. Try different cables and check the effect channel port if it is tight and secured.

Most Amps will buzz or hum when there is a bad ground. It could be because you're plugged into an outlet with some other piece of equipment on it like a motor. if it buzzes on every outlet then there's probably a ground issue in the amp. You can try cleaning your 1/4 jacks with cleaner cause sometimes they'll get cruddy and make for a bad connection. I wouldn't recommend opening the amp and messing around with anything since you could zap yourself. If the amp buzzes withouth even having anything plugged in, then yeah, it's probably the amp with the bad ground. If it buzzes with instrument plugged in, then maybe it's a plug on your instrument since they have to deal with movement and can go faulty after a while. Good luck!

Or a bad solder connection on the ground wire on the jack, or shell/sheild on the guitar.

Here is the list you seek.
http://www.gibson.com/en-us/Support/TechSupport/Wiring Diagrams/

If this ties to a keyboard, make sure the keyboard and this are BOTH plugged into the same receptacle. You should send a higher level from the keyboard (turn volume up to around 70% or so) and use less volume at the input gain of this amp. Use more at master volume if the amp has one if needed. If the hiss is there with NO input cables plugged in and input gains at 50%, that is objectionable, the amp probably needs servicing. Unused channel gains should be clear down.

Most likely it is wired like most Gibsons. The volume knobs are the two at the neck end, the tones are the two nearest the rear. The lower two are volume and tone for the bridge pickup, the upper two are for neck pickup. Switch down is bridge or lead pickup, switch up is for neck or rythym pickup, and centre position is both pickups one, where all knobs can be used together.

i) Disconnect one lead of loudspeaker & insert an open-ended guitar cable into headphone socket.

ii) Put a dc voltmeter across open end jack then turn Frontman ON for a few seconds.

iii) The reading should be about 0volts - if the voltage is high (27v), then it is likely that the PA chip TDA1514A has blown.

iv) If,however, the voltage is correct in iii) , then reconnect speaker & put guitar cable into the POWER AMP IN socket & connect other end to guitar...Turn on briefly, if there is still hum then fault is in the output stages & will require chassis removal to solve.

Adjust this using the EQ on the amp and to a lesser extent the tone controls on the guitar.

Often people boost the bass way up causing this problem.

You say the "tubes are fine" I take it that you have either replaced them with new or used a very sophisticated tester on them. A heater-cathode short might cause the symptom. There are a bunch of tests I would want you to run alnd get back to me with the results.

Heat up the amp but DO NOT play anything into it for the usual failure time or more and they start to play i... did hum occur WITHOUT playing? With all volume controls down does the hum occur?

The right tools is an oscilloscpe to fing out if the filter caps are good. These are C31, C33, C35, C36.

With hum present in failed stater, plug in a grounded plug into the power amp in jack... if hum is stiil there, check the +/- 16 volts regulating Zener for ripple. get schematic here:

http://elektrotanya.com/fender_hot-rod-deluxe_sch.pdf/download.html

Can you remove the pickups and get your hands inside? Since wires are soldered to the jack and connected to the controls etc, you may have to remove all of the electronics.

Make sure you have a ground prong on your power cord. Most input jacks nowadays are closed circuit. They have a switch built in that won't allow any sound without the cord plugged in. You may have a damaged jack that will give you a loud hiss, a loud hum or crackling depending on what volume your controls are set at.