Technical advice please...

[echofinger]

So I got this new guitar toy that allows me to set up in stereo, but... When I connect the second channel up I get an annoying buzz out of both amps.

Roomie suggested that I try pluggin' both amps into the same power circuit in the house we're livin' in, and that took about half the noise away. However, the problem persists and it's irritating enough that I want it gone.

Sounds great in mono, or just one channel through either amp.

Any tips would be great, cause I haven't the slighest clue where to start on this one.

 

[beandip]

Re: Technical advice please...

hmmmm.....are you using alot of reverb? or any at all?

 

[echofinger]

Re: Technical advice please...

Eyah, after I posted I figured I shoulded splained things a bit more, so here goes...

I am pluggin' into a signal processor, and using the stereo out jacks on the back.

One channel is gettin' plugged into a 100 watt combo amp, and the other into a 100 watt half stack.

I am experimenting at this point, so there are no other effects or what not in the signal chain, and I was not using any effects in the processor, just plain old guitar traveling the signal path.

For the record, both amps pre-amps were set round 2 or 3, and main volume low as well, as I was afraid of destroying something.

And again, I only have trouble when I plug 'em both in. SO... what gives?

 

[Lewguitar]

Re: Technical advice please...

You have a ground loop.

If each amp had a two prong plug to plug into the wall, you could reverse one plug from one amp and the problem would disappear. But amps these days are not made to do that nor do they have funtional ground switches. But basically, it'd be nice if you could reverse the ground on one amp.

Since you can't, the next best thing to try is to take one of the guitar cords you are using to connect the device to one of the amps and disconnect the ground or shield from one end only.

Open up the plug on one end of the guitar cord if you can and unsolder the braided shield and tape it up to insulate it.

That might break the ground loop and solve your problem.

Lew

 

[echofinger]

Re: Technical advice please...

Thank you Lewguitar.

Ummm, seen as how we are discussing disconnecting grounds...

I would like to know if there is any chance of death or dismemberment here, or, you know... aforementioned signal processor bursting into flames.

Obviously I dont know much about electricity

What happens if i make a power cord for one of the amps with the ends reversed? Are amps built in such a fashion that current can only go one way? lol oh lordy...

Thanks for your time!

BZZZZZZZT (oops)

 

[echofinger]

Re: Technical advice please...

I know there is someone in here who knows amps...

I got 2 people telling me I can make a custom power cord with the positive and negative ends reversed, and the amp will work. So, I am heading to home depot to acquire a new end for my power cord.

You have 45 minutes to save me from destroying my JCM 800


### Boots thread ###

 

[Lewguitar]

Re: Technical advice please...

All vintage amps made before 1970 have two prong cords...the three prong plug came into use about that time and at that point, ground switches became inoperable. Many, many artists use thier vintage amps with two prong cords in the studio without being electrocuted! And many remove the third prong (the ground) from thier three prong plug, file down one of the remaining prongs so they're both the same size so they can remove the plug from the wall and plug it back in upside down to reverse the ground and eliminate hum in the studio. ALL of my vintage 50's and 60's amps have two prong cords...I can't stand HUM and I want my ground switches to work.

But YES: you do increase the risk of being shocked. We used to get shocked regularly back in the old days when we'd touch a microphone with our lips and have our fingers on our guitars when we'd play in funky old bars wired by someone who didn't follow code. The problem would be that the ground of the PA needed to be reversed compared to our guitar amps.

Generally, it's not a problem at home or in the studio and NO: you won't blow up anything.

Lew

 

[some_dude]

Re: Technical advice please...

If I could make a suggestion,

Plug your guitar into the preamp on the JCM800, plug that into your processor, and plug the two stereo outs on the processor into the power amp in (or effects loop return) on each amp.

It won't cure your ground loop, but it's another idea to experiment with.

Personally, I'd cut the shield like Lew suggests. But if you're going to cut the ground on the power cord, make a custom ground lift cable. Just buy the male and female extenstion cord ends from the hardware store, and wire them together using only the hot and neutral wires (leave the ground disconnected). That way, whenever you need to lift the ground, you just use the lift cable, and your regular cable is still grounded for normal use.

You can also build a custom power cable for reversing the polarity, instead of cutting the ground terminal and filing the hot termanal so it's reversable. Just use the same male and female ends as above, but connect the hot and cold normally on one end, and reverse them on the other.

 

[proxy]

Re: Technical advice please...

I have a fender from the 70's that is 3 prong and has a switch to reverse polarity.

I would see if you can get a device that would go between the amps power cord and wall so you don't make any permanent changes.

 

[Lewguitar]

Re: Technical advice please...

I have a fender from the 70's that is 3 prong and has a switch to reverse polarity.

I would see if you can get a device that would go between the amps power cord and wall so you don't make any permanent changes.

You can buy a three prong to two prong adaptor from most good hardware stores. They're usually gray plastic and often have a green wire attached that you'd attach to the screw that holds the wall socket cover plate to the wall.

What I've done is cut off that green wire and wire and toss it, and then file both prongs to the same size so I can reverse the plug it in the wall socket if I need to.

This is usually fine at home or in the studio...tho it's safer not to gig that way. The risk of shock is always there on gigs.

Lew

 

[echofinger]

Re: Technical advice please...

Humble thanks to all who posted!

 

[Gearjoneser]

Re: Technical advice please...

Yeah, Lews solution with the gray adapter plug is the easiest way. You're not the first guy who ran into the ground loop problem, using two amps. It also helps to have all of your gear going to a common power strip....NEVER plugged into different sockets.
I've personally bridged the gap between two sides of a room by placing my lips on a mic, or touching a mixing board.

 

[Indie P Bass]

Re: Technical advice please...

Yeah, Lews solution with the gray adapter plug is the easiest way. You're not the first guy who ran into the ground loop problem, using two amps. It also helps to have all of your gear going to a common power strip....NEVER plugged into different sockets.
I've personally bridged the gap between two sides of a room by placing my lips on a mic, or touching a mixing board.

I had this problem with my Vox/Marshall setup, I was using a Boss AC-2 switcher and it didn't ground channel A, no matter what two amps you use or where they were plugged in. It was elaborate and electric and active, I traded it in for the DOD switcher (which is literally three inputs and a button), problem solved instantly. It was odd. I guess, my advice is, try simpler setups.

 

[Jeff Seal]

Re: Technical advice please...

.....AC Isolation Transformer!....

for a number of reasons, not the least of which is most Marshall's don't take too kindly to running without the earth lead.......it's a floating power supply but has no coupling cap for ground reference......Plug one amp into the wall as always, the other plug into the Iso trans... Should fix the problem!

They're about $40- $80...well worth it!!!

Jeff Seal