Ground loop?

[Toxophilite]

I hope I'm not being too much of a pest but I have yet another question


My two B763 deluxes I just built in Hammond AO-43 chassis sound good and are quiet


However when I plug my space echo into them, there's a loud hum, like a ground loop


They amps are quiet by themselves, add space echo = hum!


And get this, the space echo doesn't even need to be plugged in to the AC, I can unplug it and still have hum, as long as it's plugged into the amp/amps


When I plug my space echo input my super reverb  I have no hum!!!


To me this sounds like some sort of ground loop probelm




What does my super reverb have that my two AB763 deluxes don't??


Could this be an AC phase reversal. When hooking up the PT to the switch and plug etc I assumed that the two wire set up was the same as the modern setup for AC  Black = hot  White = neutral
Is that right??

[sluckey]

What does my super reverb have that my two AB763 deluxes don't??

Do all three amps have a three prong AC power plug that has the green wire connected to chassis?

[archaos]

Interesting, and do you have a schematic of the amps you built ? What are their ground planes ?

[Toxophilite]

All three amps have 3 prong plugs with earth (green) tied to the chassis (as does the space echo)
I tested the continuity of the chassis grounds on the new amp when I 1st installed it



The amps are AB763 deluxes


-THere is an earth ground


-then a ground for all the filter caps which the phase inverter, 6V6 cathodes, and bias ground are tied to via another 'node'


-and then all the preamp grounds(including the cathode by pass caps as this seemed to reduce hum) are tied to the a ground near the inputs

[Colas LeGrippa]

don't use the ground on the space echo, plug it in an adaptor without ground. Your guitar will do the rest. Many times, when using more than one amp, theres is a gnd loop causing hum.

Colas

[archaos]

All three amps have 3 prong plugs with earth (green) tied to the chassis (as does the space echo)

In that case, I'd say : "What do your AB763's have that your super reverb doesn't??"

You indeed create one/several ground loops(s) each time you connect your space echo either to one or both amps, and this is normal. Still what remains curious is that some hum occurs even with the fx disconnected from the wall outlet. I have sometimes noticed that when causing a ground loop with the wall outlets inducing hum, the latter may persist even after the ground loop is broken, i.e. even if one single piece of gear remains connected to the AC. And I still can't figure it out...

[Toxophilite]

t is curious
Further experimentation with a different power bars and an isolating 4 plug unit meant for computers with everything plugged into a different powerbar (though all essentially coming from the same outlet) reduces the hum down to the normal space echo noise hsssss


However I've never really encountered this hum before and I'
ve used the Space echo in conjunction with my super with another amp(stereo echo) for years and years!!


Is there something I can do to my builds to help prevent this?

[archaos]

Is there something I can do to my builds to help prevent this?

You could try a tool like this : http://www.ebtechaudio.com/humxdes.html with your FX for instance.

[sluckey]

This can be a common problem with effects units. Some have a ground lift switch.

Hoffman recommends to clip the power cord green wire on his Revibe unit to break the ground loop. This floats the chassis of the Revibe unit but ground will still be maintained from your guitar to the amp through the instrument cable shields. Hum is eliminated and saftey ground is maintained.

I didn't clip mine. Just get one of these...

[Toxophilite]

Cool, some options!
Thanks
THat ground lift unit is $99 up here!!






I'm pretty sure I wired the power correctly, I copied what was there previously
Black hot
white neutral and added green to chassis
However is there anyway to safely test this just for peace of mind??


Can OPT phasing do this sort of thing ?
I wired mine like the original one was on the primary
On a fender deluxe they have the brown and blue wires going to opposite 6V6 sockets than the original hammond AO-43wiring(though with EL84s of course)


On the secondary the original seemed to have green to ground and blk to hot??
I instinctively wired it the other way
THis has worked with the negative feedback etc (I tried reversing it ..ow!! SQUEAL!)


I'm just curious if the OPT can be a part of this problem or if it's purely a power supply thing

[shooter]

is there anyway to safely test this just for peace of mind??

make sure your guitar cable is GOOD, the "lifted ground" relies on a good ground from guitar to effect to amp.

I'm just curious if the OPT can be a part of this problem

Pretty sure has nothing to do with grounding, only the squealing

[sluckey]

However is there anyway to safely test this just for peace of mind??

You're not listening.

[Toxophilite]

I listened to my computer and it didn't say anything!!!


My apologies for my seeming inattentiveness


i understand the need to lift the ground on the effects units(space echo, revibe etc) and I am very appreciative of the suggestion as it does work...when using one amp


I did at one point try plugging the space echo into a separate power bar that at some point in the past had the ground pin clipped off the plug and it had no effect, However when I try it with just one amp it works quite nicely!

I didn't realize till now that using both amps seem to make the problem worse and a little more complex


I'm using both simultaneously, that was the whole idea of this double build


The guitar is plugged into the space echo which has separate wet and dry ouputs(the only model that came that way) The dry goes to the 1st singe channel amp and the wet goes to the 2nd 2 channel amp, as well a little of the dry sound from amp one is fed from the line out into the other channel of amp 2 to preserve stereo when the echo is turned off(I've been using a setup like this for years, sounds great! however i used to have my super and a Pa head and cab as my second amp and I'm going for a smaller setup over all.)


Anywho, with just the dry plugged into one and the echo into another, both amps plugged into one power bar and the space echo plugged into a ground lifted one..
The hum is still quite loud


It would seem that it  is  ground loop interaction between the two amps as well


i just tested this by plugging them both into the same power bar and running the line out from one into the other and there's a good bit of the same low hum
I then plugged amp 2 into the groundless power bar and no hum




What I was curious about was if one amp could be out of phase with the other?
And how do I tell which one is right ?
Is it simply a matter of flipping the leads at the jack at the amp(I have those 3 prong computer style jacks on both amps)


Or is the only solution that one amp be ground and everything lese connected be floating and getting a ground via the 1st amp?


Sorry if asking this is a pain in the @$$ or if  I'm missing something very obvious

[sluckey]

Try bonding the two amp chassis together with a heavy gauge wire screwed to each chassis. Does that help?

[Toxophilite]

Hmm
I tried that with a Heavy duty alligator clip and the hum pretty well stopped

[Glennjeff]

What I was curious about was if one amp could be out of phase with the other?
And how do I tell which one is right ?
Is it simply a matter of flipping the leads at the jack at the amp(I have those 3 prong computer style jacks on both amps)

 is the only solution that one amp be ground and everything lese connected be floating and getting a ground via the 1st amp?

I may have misunderstood BUT:

Why using 3 pin phono jack sockets (Tip Ring Sleeve ) for mono ??? , make sure ring and sleeve are connected, thus converting them to mono.

I've had these cause all sorts of unexpected problems in the past.

[Toxophilite]

Forgive me if I wasn't clear
All the jacks and chords are mono


I'm using 2 amps to achieve a kind of stereo echo sound


No stereo jacks or chords are being used


thanks

[Toxophilite]

If I connect the chassis together and eliminate hum(or at least most of it)as was the case
Am I getting rid of the ground loop, or at least making it smaller?
I'm curious as to what exactly is happening
I have been trying to read up on it

[Ed_Chambley]

Why don't you install ground switches in your amps.  You can actually make power cords to switch polarity.  I have a few I painted yellow so I would know.  I also have one of those outlet test plugs to check the wall first.  I suspect your Super Reverb and your old PA head had them and that is why they do not hum.

I use a wet and dry setup very similar and have played in places where they came in handy.

[shooter]

On one of my builds I did a 1950's sorta thing where I installed 2 "AC line out 2 prong" rescepts on the amp after the fuse and power switch where you can plug in your "accessories" but it completely relies on a GOOD patch cable for grounding everything, including the musician!  I even build 2 prong male 3 prong female plugs for the guy.  I don't do that anymore! read thread on liability!!

[Toxophilite]

My super has a 3 position ground switch. The odd time I'll use it and it does something good, however the times are few and far between.
I've been thinking about these options and trying to decide which is easiest and requires the least extra gear.


I was also wondering about a plug in mono ground cable between the amps


What sort of existing cable could be used for ease?


I don't want them permanently connected as I plan to separate them by 2-6 feet on stage...I could see myself stumbling through between them and bringing down the works like slapstick routine! reminds me of the one time I tried wearing spurs on stage....

[Toxophilite]

Hi
THanks for the reply


Actually if I have the amps on  different power strips I don't get the hum


Same power bar = hum     + connected chassis = no hum


Different power bars  = No hum


These are power bars hooked to the same outlet


The amps are connected via a late 70s silverface style line out
Strangely I realized that the line out and the input on the secpond amp SHOULD be providing a chassis connection.
I was using a short , but heavy duty alligator clip to connect the chassis


the patch chord connecting the chassis is 6 ft long


Out of curiousity I tried a short 1' long patch chord and lo and behold it cut down the hum alot!!
As the amps will be in different cabs 2-6' apart that's not an option but it is telling I think


What do you think??


I think S.Luckey was primarily suggesting I lift the ground on the effects unit with the adpater as when connected the unit will get it's earth from the amplifiers. I don't think he's suggesting I un ground my amps

[sluckey]

The picture I saw was  three prong adapter to to a two prong rect.  If the metal tab is not grounded then third third prong, the ground circuit is not completed.   I respect Sluckey's opinion and he has great builds.

Well, at least Toxophilite understands what I was talking about.

It does not do the forum any good, if I point out an error without attempting to provide a solution.  I have a attached a pdf, that describes grounding techniques.  The article, which I have lost the author and source to, appears to be valid.

Nice reference with a lot of good info dealing with grounding inside a single amp. However, it doesn't address the issue at hand.

[HotBluePlates]

Ughhh... See the picture below.

Each item with a 3-prong power cord has a ground connection to an outlet. But the interconnecting cables between each piece of equipment also connects the grounds from one device to another. So there are ground loops between each of the pieces of gear.

The loops may not cause hum... Or maybe they will. A lot of the time, it depends on the outlets the devices are plugged into. Specifically, neutral/ground may not be exactly 0v relative to earth at every outlet, or even 0v relative to another outlet. When you plugged everything into a single power strip, you both shrank the size of the loops (because the wiring in the wall between outlets is also part of the loop when plugging into different outlets), and also likely ensured each neutral/ground was nearly the same voltage (because they're all connected in parallel in the strip).

Your proposed arrangement will always result in large (area-wise) ground loops, because you want to place everything far apart. When you changed nothing except using a short patch cord, hum dropped because everything was closer together, yielding a smaller loop area and reducing hum. Other various trials reduced hum for various reasons, but they're not worth going into here because it's much faster to just solve the problem.

What Sluckey advised was to break the loop by disconnecting one device's ground wire from the outlet, in a reversible way by using a 3-to-2 adapter (If these cost $99 where you are, you're paying at least 20 times too much, maybe more). The ground connection through the interconnecting cord still provides a path to the wall outlet ground.

Better/safer would be to use amps designed/built from the ground up with true safe ground-lift switches and no need to disconnect the power cord safety ground. But you need to plan the wiring of the whole amp to accomplish that from the start of building, because it's a lot of @$$-pain to retrofit it into an amp after the fact.

You could also spend much more money on various isolation transformers to break the ground at the power cord or at the input end of the amp, but they won't work any better (aside form safety) than what Sluckey suggested.

[eleventeen]

Excellent explanation/illustration, HBP!

[Toxophilite]

Hi
THanks for the detailed information
That's explains things  a little more thoroughly


The units S Luckey suggested aren't $99 they're likely more like 2-$6


 it's the fancy unit the other fellow suggested that costs $99 up here

[mresistor]

Would any of you people please spell out MOV?  Not every being in the universe knows what it is.
Thank you.

[Toxophilite]

On my amps there are two grounding points for the circuit
One near the inputs for the preamp grounds
The rest go to an attachment point at the rectifier base (the original ground point of the AO-43
The earth ground is on a leg of the PT right next to the rectifier