Help fixing ground hum?

It sounds like your outlet might be bad. Throw the circuit breaker, then take the faceplate off and check the wiring.

 

It looks like your amp's grounding is bad, either in the amp or on the wall outlet. Try the amp in a different room.

If that does not help, see if the amp has a "ground switch" on the back panel, this may be in the wrong position (if it has three positions best leave it centered).

I take it for granted that the volume and other controls are the same as when it last did work without hum.

 

That's kinda short notice.

There are a couple things to look at. Like several people mentioned it might be the household wiring.

If the amp itself hums it may need a cap job. In particular the MusicMaster Bass amp has filter caps rated at 300v on a 290v supply voltage. That's awfully close to the bone. It's no big deal for the supply voltage to be 5% high at the wall... except that it increases the supply voltage around 15v putting it just a hair over 300v. There is a little extra built into the caps but not so much you can run them over their rated voltage and whack them with voltage spikes.

If you're selling it today you probably need to see a tech today.

One little thing that may save your bacon: Swap the tubes if you can. If you're lucky that may solve the problem.

 

I don't begin to evaluate an amp's problems with hum with a guitar plugged in. I want to first know what the amp does sitting there by itself. What does the amp do before you plug in?
And...yes, some electric/acoustic set-ups can cause hums....but we don't know at this time what kind of system you have there in that Elec/acoustic guitar. IF the amp has the hum without the electric/acoustic plugged in, then your problems are with the amp, ime. IF the amp is quiet until you plug in that elec/acoustic, then I might think that the guitar is causing the problem. What kind of pickup is in that acoustic? Piezo transducer? Magnetic soundhole pickup?
Do you have an electric guitar...not an acoustic..that you can plug in? IF so and if the amp is quiet without the elec/acoustic plugged in, then what does the amp do with a pure electric plugged in?

 

Make that three possible sources of hum.

Bad ground in the household wiring.

Bad filter caps in the amp.

Guitar that just plain hums.


A basic cap job on these involves replacing the filter caps and maybe the cathode bypass caps. One cap is on the board. The other cap has two sections and hangs off the board. I always thought that was silly so I recently fabricated a one off circuit board that puts all the caps on the board. One off... only one exists and it's not in my posession.

The filter caps are about 20uf per section at 300v. Increasing the voltage rating is a good idea because 450v caps don't cost any more than 300v. The MMB has a solid state rectifier so the first couple caps can be increased to 33uf or 47uf @ 450v.


But, but, but...

That may or may not be the problem. If you're selling the amp have the buyer come over and bring his or her guitar. It may not hum at all if you buyer plays a guitar with humbuckers.

Even if it does hum... will your buyer consider that to be a problem? I'd rather show up and get a good deal rather than the ubiquitous "Just checked over by a reputable tech and given a clean bill of health" with the also ubiquitous $300 receipt as proof.

See... my labor costs me shredded chicken chimichangas washed down with Turkey Hill ice tea. Chicken breasts are on sale for $1.79 per pound this week. Turkey Hill costs $1.50 for a half gallon. I can buy a wheelbarrow fulla chicken and a bathtub fulla ice tea for $300.

 

Ime, when a filter cap stops filtering the AC hum, that hum is present at all times....without anything plugged in and even with all controls turned to zero the hum will still be there. Again...if the hum is not there with nothing plugged in, I am looking somewhere other than the amp for the problem. I once repaired a 5E3 clone that had a hum problem....a filter cap was bad, a tone cap was bad, and the input ground was bad. All of those hums were present with nothing plugged into the amp. Each time I replaced a bad component, the hum in the amp would change. After replacing the bad filter cap, the hum changed. After replacing the bad tone cap, the hum changed a bit. I then found and corrected the bad input ground---which had never been made....and the amp was quiet as a mouse. Kicker...I told the fellow before I started work that I was hearing 3 different hums!! LOL...I have no idea how that came to my ears or my thought process...but it proved to be correct. Probably that was just a smart remark that turned out to be correct, right? This fellow had come around for a year and a half wanting me to correct the problem from afar...and he was on the internet for all of that time trying to get someone to correct the hum from afar. I was tired of telling him that the hum could be caused by a variety of things.....and that I would be glad to fix it for him. He finally relented and spent a little bit of money to get a really fine amp working for the very first time in its existence!
IF you want to to do some investigation and if there is a music store near you, take the amp with you to try out a guitar. Tell the people at the store that you are looking for a humbucking guitar and you want to hear it through your amp.Try an inexpensive Epiphone L.P. or SG with humbuckers. Plug the amp in, let it warm up, and assess its noise level. IF the amp is quiet, plug that humbucking-equipped guitar in. Without making contact with the strings or bridge, there will be a small 60 cycle hum. Touch the strings, that hum will go away. IS there any hum left??? IF not, then the hum that you are experiencing with that elec/acoustic is not of the amp's doing. IF the amp hums when you plug that humbucking guitar in with your hands on the strings, then the amp has a problem.

 

Unplug guitar,turn amp on.If it does not hum,It's not the amp