Intermittent buzzes and hums coming from my speaker
 

I have an amp that sounds fantastic when it is working properly. It has a 175 watt, 8" speaker. A few days after I got it, I noticed a midrange hum that varied in volume from hardly noticeable to quite loud. I took it to a local repair shop and tried to replicate it for the technician, but it disappeared when tested in the shop. (Much like the old proverb about going to a mechanic with an automoblile problem -- it goes away when the mechanic is there).

Anyway, they kept it for a week and the head guy at the shop had a close look. He called, said the amp sounded amazing and he could find no problems. I came and picked up the amp, thinking maybe it was just one of those things.

I took it home and played it for an hour or two. The problem appeared to have resolved it self. Then suddenly I hit an open G string (at practice volume) and mwaaahhhhh. A completely new hum. Much more noticeable and only on the G. Different from the last hum which was most noticeable on the E and A strings. I shut the amp off. I was busy so I could not make an MP3 recording of it immediately. I came back a few days later with my H2 Zoom recorder to document the hum and...it's gone again. The amp sounds great.

I have been a musician for a long time and have owned many amps. For the first round of mid range hums I hooked it up as an extension cab to another practice amp, same hum, so I think I have ruled out a problem with the amp's electronics. When it's behaving it sounds great at all volumes, so I cannot see how it could be blown.

If someone can please give me an idea of what may be wrong with it I would appreciate it.

Job 1 is to find a reliable way to duplicate the problem. Try varying temperature first, such as running it on the floor in a cold room, then running it located high over a heat source or in direct sunlight. Keep a log book. When you find something you think triggers the problem, run several more instances of the same test to verify the result wasn't just a statistical outlier.

I think it could be a bad solder joint, that doesn't show up until it is warm, and get vibrated, as the metal expands from the temperature, the frequency that sets it off changes. Had a similar thing with an Ampeg head that only misbehaved after being on for about an hour.

Thanks, Craig; Fox. I will try the above. The first round of hums began as soon as I turned the amp on, so if the two are related, I don't think it is a solder joint. Also, I hooked the cabinet up to another amps extension output and got the same effect, so I 'think' I have ruled out the head.