Grateful Dead Music Forum

After about 10 years of playing unplugged and through smaller amps, yesterday I dusted off my 2x12 cabinet, MC250, and SMS Jerry pre-amp. I have the amp and the preamp plugged into a regular power strip, and there is humming galore when they are both turned on. (Even with no pedals.) I’m assuming this could be due to a multitude of reasons, but what are the first things I should be doing to troubleshoot? Is a power conditioner like a Furman something that might make a difference or no? I’ve never been sure about the history of the MC250 and have no idea if it’s been properly maintained so maybe that’s an issue as well. (Half thinking of unloading it and getting a Milkman… )
-Brendan

The simple things -

> do you hear the same hum/noise without a guitar plugged in?
> does the level change with the power amp turned down? When the SMS is not connected to the power amp?
> does the hum/noise change with the SMS level?
> does it change with different cables?

See if you can isolate to the main blocks Depending on age, electrolytic capacitors can go bad (the amp, mainly). The tubes in the SMS are likely OK but are easy to diagnose by swapping.

If your AC power is not causing problems with other equipment, probably not the issue and a conditioner may not help. Proximity to neon lights and various electronics can cause various noises and confirmed by distance from the source,

The MC250 is worth repairing if it is the source.

BoogieMK2 wrote: ↑Mon Nov 21, 2022 7:41 pm The simple things -

> do you hear the same hum/noise without a guitar plugged in?
> does the level change with the power amp turned down? When the SMS is not connected to the power amp?
> does the hum/noise change with the SMS level?
> does it change with different cables?

See if you can isolate to the main blocks Depending on age, electrolytic capacitors can go bad (the amp, mainly). The tubes in the SMS are likely OK but are easy to diagnose by swapping.

If your AC power is not causing problems with other equipment, probably not the issue and a conditioner may not help. Proximity to neon lights and various electronics can cause various noises and confirmed by distance from the source,

The MC250 is worth repairing if it is the source.

BoogieMK2 --

Thanks so much for this. When I get back from Thanksgiving I will test all of the above.

I totally agree about the MC250 and hopefully all is well with that beast! I may keep it now that I've seen this sweet rack:

https://jerrytonestore.com/store/p/cosc ... e-gig-rack

I'd check the simples too:
1) is the outlet grounded
2) are you getting enough voltage
3) does this happen with other, better quality, power strips

^ Yeah. Ive had a few times when it was one of the outlets and id forgotten we'd gotten a new appliance or something. One time it was the dishwasher was on the same circuit, so the sound would come and go, always at importune times when trying to diagnose this issue haha. I use a Furman rack unit for my power these days and i will never go anywhere without that thing. That really solved almost all my issues i was ever getting with noise. Pretty amazing and i found one for $30 used shipped.

Good luck and let us know how things go

If you need your McIntosh fixed up, you can send it to me. I have been working on those old amps a long time. I’m an amp tech in Olean NY. I refurbed my own MC50 and MC2105 recently. Along with a C32 and a C34V. Good stuff for sure. So well made.

Any update on locating your noise?

Hey I think it’s my single coil strat pickups causing the buzz/hum. It’s interesting; the middle pickup, which I prefer, of course has the loudest hum. I changed out the power supply to a Furman and that had no effect, nor did swapping in a Milkman The Amp for my SMS/MC250. My Franken-Jerry guitar with humbuckers is getting re-wired so it will be interesting to see what those do noise wise. I ordered an Electro Harmonix Hum Debugger pedal, which is supposed to get rid of the single coil hum. Should arrive tomorrow and I’ll update.

Another option is to shield the inside of the PU covers. This was done on Jerry’s Alligator guitar. Not only did it help fight hum, it allowed for the strings not to be grounded so Jerry didn’t have to worry about getting zapped by the mic.

Bill, tgo

Is there a dimmer near your setup? Dimmer switches almost always cause hum issues for me.

Hmm a dimmer switch. There are a couple that are probably in the same circuit. I should test that. But I think it’s my pickups. I got the Electro Harmonix Hum Debugger yesterday and it pretty much takes care of the problem. I haven’t decided yet whether I think it does anything negative to the overall sound.

RF interference caused by dimmer switches usually goes away if the dimmer switch is all the way on and tends to increase as the dimmer is dimmed.

Bill, tgo