That old ground loop hum

[sturoc]

Got my 150B back from rehab.
The local tech did a thorough job and I had him install a proper grounded 3 prong power cord.
Yes he did remove the death cap as well.
I plugged in the power cord and instantly there was hum thru the other amps that were on.
Although the 150B was not switched on yet.
Then my guitarist suggested I use another outlet i.e. a different circuit for power.
Which i did via an ext cord. Hum goes away.

Amp still off, I pick up the bass input cord to the 150b and a splitter box which was plugged into a mixer channel and...
WHAMMO a big 120v across my body . I was awake now !
Fortunately I have a good heart and have withstood being zapped before, once or twice.

So we miked the bass cab instead of splitting the bass output.
Little did I know that the whole time playing was a accident waiting to happen again.
We got thru rehearsal and I talked to my tech to make sure of what he had done and he was all good.
He explained to ck all power outlets and do voltage check across the two different path'ed input cables to make sure.

So today i bought a ground/phase tester for 5 bucks, to troubleshoot all outlets (they are wired properly) and the hum etc
Did the ck with VOM to make sure there was no voltage leaking to the grounds of the input cables and with the amp into the same circuit power outlet without using the ext cord - hint .
Then I ck'ed the ext cord and VIOLA there it was, the ground was lifted, cut or disconnected in the cord.

Using another cord and also with amp right into the wall outlet I ck'ed everything again : no voltage, Good.
One problem solved, the critical one !

Now with the 150b turned off but plugged into the same power circuit as the rest of the gear, which is on,
there is that good 60hz hum from the other amps.
btw: nothing is plugged into the 150b inputs at this point.

What is my solution here ?

 

[seamonkey]

Maybe the hot and neutral from the power is connected backwards in the amp.
Check the jack ground and cord ground and neutral are at 0 ohms.

Plug in through a GFI - it should trip to save your life.
Consult an electrician, or somebody qualified to work on line power.

 

[dannylectro]

+1 what Seamonkey said. And I'm an electrician. And don't use the amp again until you know what the problem is and have fixed it.

 

[dincz]

Then I ck'ed the ext cord and VIOLA there it was

There you go. Get a new viola

 

[Jerrold Tiers]

Maybe the hot and neutral from the power is connected backwards in the amp.

That will make NO difference in any even somewhat modern unit.... like since maybe 1935.....

The basic problem involved in shocks is nearly always the same...... a connection to earth ground in one piece of equipment, vs an amp or other equipment that has no earth ground connection from the chassis, plus has some leakage current. (And everything has some leakage current, especially newer class-D etc amps).

All somewhat modern amplifiers treat hot and neutral exactly the same.... and it makes very little difference which is which. Safety tests verify that BOTH are well insulated from the chassis.

Hum from earth connections comes from currents that flow when units are connected together by some signal cable (usually NOT a balanced line), AND are plugged into different outlets. Widely separated outlets usually have somewhat different (even if rather low) voltages on the ground conductors, and that voltage causes the current flow which you hear as hum in the signal.

The voltage and current can be caused by wiring in the building, OR by induction from large power transformers in some of the equipment.

The solution is balanced line connections, or plugging connected units into outlets that are physically close and on the same earth ground circuit. Ones in the same outlet box will be on the same earth ground. Any others you don't know about.

Cutting earth ground terminals on the plugs is unsafe, and is a "makeshift" solution which may or may not even work.

 

[warnergt]

...Amp still off, I pick up the bass input cord to the 150b and a splitter box which was plugged into a mixer channel and...
WHAMMO a big 120v across my body . I was awake now !

This is going to be the biggest clue to your problem.
Even with your power cord missing the ground connection,
this should never have happened. Did this problem go away
when you replaced the power cord?

There is not enough information to pin down this problem here.
But let me give you one easy and safe experiment you can
perform. Try plugging your amp into an outlet with a GFI
(Ground Fault Interrupter). Almost every outlet found in a
bathroom has one of these (although the GFI might not be
in the same room). If your amp trips the GFI, you'll have
another big clue to the problem. If it doesn't, you'll need to
keep looking.

And let me ask you a question. When you were shocked,
was anything else connected to your amp (besides the
power cord)? Were you touching anything else when you
got the shock? I'm wondering if perhaps there is another
piece of equipment with a problem and, for some reason, the
problem doesn't exhibit itself until you plugged in your amp.

 

[sturoc]

This is going to be the biggest clue to your problem.
Even with your power cord missing the ground connection,
this should never have happened. Did this problem go away
when you replaced the power cord?........
....When you were shocked,
was anything else connected to your amp (besides the
power cord)? Were you touching anything else when you
got the shock? I'm wondering if perhaps there is another
piece of equipment with a problem and, for some reason, the
problem doesn't exhibit itself until you plugged in your amp.

My writing can be a bit jagged sometimes though I tired to type clearly so all could understand the situ.

The amp is a 1970 Acoustic 150B the chassis is part of the ground system by design.
When bought, the amp needed some work so it went into my tech for the rehab. One of the things done right off the bat was to replace the non-grounded stock power cord with a grounded one.
I NEVER pull ground terminals off any power cords etc.

The shock problem can be attributed to the test of the extension power cord which showed no ground path i.e. ground connection is not there within the cord or it's plug /socket.
A second ext cord was tested and worked as it should.
Amp was also tested w/o ext cords but plugged directly into wall socket that supplies power to the other amps.

Again no shock/voltage across the amp's input cable and the other input cable going to the mixer ( which was powered on).

So we are down to the hum issue of when the Acoustic head ( powered off or on) is plugged into the same power source as the other amps.
and that is the baffling part for me:
they are sharing the same ground !

So would a solution be to get an ISO DI box or similar ?
But I'd rather find the source of this issue then just do guesswork that may or may not solve the real problem.

 

[eriky4003]

Sturoc,
Trying to follow but unsure if an instrument is being plugged into an amp giving you a 60 Hz hum. If so, you may want to try an ElectroHarmonix Hum Debugger which has worked miracles for me in my place of practice and live.

 

[sturoc]

No instrument plugged in at the time. The input cord is not even plugged in either.
it happens as soon as i plug the amp into the outlet-remember it is the same outlet the other amps are using as well.

there is a stock ground switch which my tech left functional but it does nothing to remove this hum

 

[Slevage]

Have a tech perform a dielectric test on the transformer.

This might also be called "megging" or "hi-potting".

It could save your life.

 

[sturoc]

Now that's an interesting test, John M !

 

[bgavin]

I carry an inexpensive Radio Shack outlet tester.
Plugged into the wall, it can tell you about bad wiring and missing grounds.

Start with a known-safe outlet.
If an extension cord is used, plug the tester into the cord.
That would have found your floating ground.

Take the amp to a different (known good) tech for a 2nd opinion.
Your big shock was a wakeup call.
Do not play Russian Roulette with your life, just to save face or a few bucks.

 

[Steve Dallman]

Those 3 light circuit testers do not reveal all possible wiring problems. There are two possible that don't show up (and don't ask me to remember all the possibilities.) Our guitar player uses a 65 Ampeg Reverbrocket and at two bars, it hums and causes mic shocks. But if he switches to a different outlet, there is no problem...and I've checked his amp. The upgraded 3 pin power cord is wired right.

A GFCI (and I've carried one for decades) will prevent a shock, but will require fixing the problem that caused the GFCI to kick out to keep it from continuing to kick out.

I had a GFCI continually kick out, and found the problem to be a ceramic rF suppression cap in a preamp that was slightly leaky. The preamp performed normally when not plugged into the GFCI, but I did replace the slightly leaky cap.

 

[sturoc]

bgavin,
if ya read the first post you'll see I did that right off. that's what told me the ext cord was bad.
Consecutive tests then showed no voltage across the input cord ( to the head) and the cable to the mixer input, with amp is plugged in, power on and off.

as for GFCI option not a bad idea.
But I have tested with voltmeter and the little 3 LED tester, across different outlets ALL power outlets were wired correctly and finding that the ext. cord used was shot does eliminate that issue of shock.

So let's concentrate on that hum problem since there is no voltage leaking at least not to cause concern now for life, limb and the pursuit of music !

 

[seamonkey]

Take it back to the tech, and see if he can reproduce the problem in his shop.

 

[Jerrold Tiers]

OK, misunderstood that the "little hummers" were connected other than by the power cords. If not, then forget the bit about different grounds, in this case, at least.

"As soon as the amp is plugged-in"..... I assume you mean it is not even turned on? if not even turned on, it is a puzzler, but there is a possible reason.

If it happens without being turned on, I suspect that the green wire and the neutral have become connected inside. (neutral and ground). That will cause a decent amount of current through the green wire ground system, and *may* cause hum.

If it has to be turned on, then it may be that the green wire and neutral got reversed. That would send the return current through the grounding system, again very well *may* cause hum.

Both of the above should trip a GFCI ("RCD" for non US folks). So find an outlet with a GFCI, such as a kitchen or bathroom outlet, and try it. if it trips the GFCI, take it back for sure.

 

[sturoc]

Okay J.T., will try that one with a GFCI as a power source. That's is an interesting theory of the leads being swapped.
TO refresh there is a GND switch on the front panel nxt to the power switch .My tech kept that in the wiring and told me it does function.
But it seems to have no affect on the hum.
I did speak with him this evening and he said if I wanted he could isolate the ground somehow, but keeping the original parts in place but not in use. He has done this on a few vintage amps. But it also alters the "original vintage" design et. He also adds that he has wired countless vintage SS amps to 3 prong grounded cords and with similar circuitry and there has never been a problem. And he is willing to ck things over again if I wanted.
This is just so you guys know what his responses are in our conversations.

One thing I should add is after playing thru this head the other night i was impressed with the tone and also the output:
I never got past "3" ( there are no #'s on Acoustic amp controls of this era) and we rehearse in a good size room !!!

 

[David Jayne]

Some amps have the primary side of the power supply transformer wired directly to the cord, bypassing the power switch. At least one of these, a GK micro IIRC, will actually give off a quiet humming sound when plugged in. I can't help but wonder if this is relevant. Maybe your amp works this way and it's transformer is somehow inducing hum into the ground leg(or something like that).