Bad Humm when using two amps together

[rschiller]

I am having an odd humm issue when I plug into two amplifiers simultaneously. I need a bit more headroom for a job. Both amplifiers are 1958 Fender 5E3 Deluxes. Both have been gone through and filter caps replaced. The amp I primarily use has had the Coupling Caps replaced w/ Mallory 150s and the other amp has the original Astrons which are slightly leaky. I do not think this the issue. Individually the primary amp is dead quiet and about as quiet as a tube amp can get. The other amp has slightly more background hum (leaky Astrons?) but is certainly in the range of properly working tube ampliifers.

So I daisy chained by plugging into INST 1 input and then a jumper cord from INST 2 to the other amp INST 1. A Loud hum which immediately goes away by turning one of the amps off. As this was not my ideal solution, given the amp with both Inst 1 & 2 plugged into would have reduced input gain, I made a splitter box. The box is a metal case though to be sure I used a shielded cable from the input jack to one of the output jacks. Same hum problem. Both amps are plugged into the same wall socket and the socket has been tested with an electricians circuit tester and is working properly. Both amps have the AC (aka death cap) removed and properly installed grounded 3 wire AC cord. Ground switches are disconnected as cap is removed.

Why the hum?

 

[Paul G.]

Ground loop. Disconnect the ground wire on one end of the cable you're using as a jumper between amps. Or you can get a splitter box with a ground lift.

 

[rschiller]

Ground loop. Disconnect the ground wire on one end of the cable you're using as a jumper between amps. Or you can get a splitter box with a ground lift.

Splitter box with ground lift? The box is metal and the switchcraft jacks ground to the box so a common ground. I can reinsert the jacks with plastic washers that would isolate from the metal box. And then no common ground internally? That is the jacks only wired hot?

I prefer the splitter box solution which lets me plug into both INST-1 on the two amps.

 

[Bob M]

Get a Radial a/b/y box. Problem solved.

 

[Chud]

AB-Y box with ground loop switch

 

[rschiller]

Ground loop. Disconnect the ground wire on one end of the cable you're using as a jumper between amps. Or you can get a splitter box with a ground lift.

Humm much worse w/ one cable ungrounded.

 

[rschiller]

Ground loop is because both are common ground to house AC; I properly added 3 prong to both. Putting a ground lift on one of the plugs solved the problem.

 

[2L man]

It can be dangerous to detach Mains Safety Earth! Most likely against electric regulations?

I have not yet test this but perhaps cutting the input signal cable cold / "ground" reference on other amp prevent the ground loop? Then signal get the reference thru Safety Earth. There should not come a loop anymore.

The signal reference path would come thru possible effects (which use batterys or isolated power supplys) to a "stereo effect" (which also use battery or isolated power supply). Then to possible other channel effect chain to the other amp SE and back to first amp (which input cable Cold is cut) thru amp SE.

 

[schmee]

This is the reason I quit trying to use 2 amps at gigs. Taking the ground off one cable end often does NOT stop the hum.
The only reliable solution I know of unless you like messing around at a gig setup, is to buy the Radial Tone Zone device.

Also, even if you stop the hum, make sure both amps/speakers are in phase.

 

[schmee]

Ground loop is because both are common ground to house AC; I properly added 3 prong to both. Putting a ground lift on one of the plugs solved the problem.

Sounds like one of those amps is wired wrong...? Neutral wired wrong? Or have you put a tester in the house outlet?

 

[Wally]

Humm much worse w/ one cable ungrounded.

The lifting of the ground should be done on the cable that connects the two amps. Is this what you did? The hum should have gone away because this breaks the ground between the two amp while maintaining the ground at the wall…..mean8ng that you then0layer would NOT be the shortest path to ground.

Ground loop is because both are common ground to house AC; I properly added 3 prong to both. Putting a ground lift on one of the plugs solved the problem. Yes, lifting the ground for one amp at the walls I’ll break the ground loop. However it places YOU the player in the shortest path to ground. Danger!

 

[Burning Fingers]

I have 2 x 5E3 amps and yep the hum was terrible when both were linked...even though I built them exactly the same as each other. My solution was an ACTIVE AB-Y switch box which allows me to use either amp or both amps without any hum at all.

 

[J-bass&Tele]

Have you tried a transformer isolated splitter or ABY?
Lehle P-split
Gigrig Humdinger
Radial Bigshot ABY or Twin City if you want a buffer.
Lehle Little Dual if you need stereo

 

[Peegoo]

Radial Big Shot ABY splitter box.

It has a ground lift switch AND a 180-degree phase switch.

It solves all kinds of problems--this being one of 'em.