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#1 2007-12-11 17:34:24

nricco
Member
Registered: 2007-07-26
Posts: 20

One for you amp gurus...

So I have a weee problem. This only happens to the ampeg, none of my other amps.
When not touching the guitar strings, I have a earth hum. The ampeg I have is an old american one and thus does not have an earth cable. Whn using stomp boxes, this is accentuated. As I mentioned noen of my other amps seem to have this problem. Do any of you know how to fix this? Why is it a lot worse with the stomp boxes? And there has been talk on the forum about not knowing when the caps needed replacing, and simply replacing them after a while.... I was under the impression, that you would know that they need replacing. So there over to the panel of the knowledgeable.

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#2 2007-12-13 17:55:26

paully
Member
From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: One for you amp gurus...

Hard to say exacly. Do you have access to a multi-meter? If so, with the amp off for a couple of hours and unplugged from the wall, plug in the guitar as normal with it's volume pot(s) turned full up. Using the meter's ohms-resistance lowest setting range(X 1), touch the probes together and adjust the meter to read zero. Then touch one probe to a string, and the other probe to a known ground point on the amp (like the metal handle of the on/off switch). If the meter does not completely deflect to zero, then you have a dirty or bad earth ground somewhere in the input stage.

Does the noise go away when you unplug the cable from the amp with the amp still on? If it goes away, the grounding issue is deeper into the circuit. FWIW, you should have a tech convert to a 3 wire AC cord. The old 'deathcap' 2 wire systems can not only cause noises, but can actually be dangerous. You've probably already tried this, but flip the amp's polarity switch and see if it helps.

Older amps may need re-capping. Not a bad idea if it needs it. Your symptoms don't indicate this. Stomp boxes require a clean ground also. A lot of them are designed for increased/modded output. Could be why they make things louder, as they also need a solid ground reference. When you touch the strings, YOU are effectively becoming the clean circuit ground. This is NOT a good thing. Have a tech troubleshoot unless you are capeable.

The first thing I do when I pick up an old piece of gear is to open it up and touch up every component's solder points. This is particularly importand with ground points. This proceeds voltage and component checks, tube checks, etc.

Good luck and be careful, Paul


WADAYAKNOW.. For the first time in my life, I'm wrong again!

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#3 2007-12-13 20:02:48

mojoman
Member
Registered: 2007-11-28
Posts: 36

Re: One for you amp gurus...

nricco wrote:

The ampeg I have is an old american one and thus does not have an earth cable.

Do any of you know how to fix this?

Why is it a lot worse with the stomp boxes?

To answer your questions.

1. You need to get that cable replaced ASAP. Therein lies your problem (swapped polarity), and a potential for an even bigger problem down the road, your own death.

2. Replace the cable with a modern earth ground cable.

3. The 60 cycle hum is being amplified by your pedals.

Last edited by mojoman (2007-12-13 20:04:19)

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#4 2007-12-13 22:25:14

nricco
Member
Registered: 2007-07-26
Posts: 20

Re: One for you amp gurus...

thanks for the replys.
Now i guess we've established that it's an earth issue. The problem is that for the mean time i'm stuck on an island on the edge of the world.. no amp techs. The polarity switch is a long
NO-NO. The polarity blew up ages ago. it turned out that the polarity switch was still expecting 110v. while the new
transformer was on 240. Please keep in mind that Ampeg amps are rare in europe. Is ther some one out there who can describe an idiots way of adding an earth to the amp? I mean how hard can it be>? live to live- neutral to neutral- earth to ? The mains transformer in from mercury magnetics in LA, I could call them. they made the friggin transformer, they should know how to earth it.



t

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#5 2007-12-14 00:50:51

paully
Member
From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: One for you amp gurus...

The easiest way to connect the chassis is with a heavy(#10 or heavier) length of stranded wire, screwed to the chassis, with the other end fastened to a known earth point. Clumsey way to do it, but it'll work. 

If the 240 is floated, and the outlet has a third terminal (apart from the 240 legs) that IS grounded, just replace the whole wire using a standard 3 conducter wire; black to one side of the power transformer's primary, white to the other, and green to the chassis. Make sure the 240 outlet's third terminal is actually connected to earth first, otherwise you're right back where you started. I'm guessing that the deathcap blew, so you should probably 86 it and any related switch.

Touch base with Mercury first just to be sure.

Best, Paul


WADAYAKNOW.. For the first time in my life, I'm wrong again!

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#6 2007-12-14 02:39:26

VT22
Member
Registered: 2007-11-26
Posts: 21

Re: One for you amp gurus...

Fix that cable, once i loaned my amp to a friend and he burned a lip on a mic because of it, electric shock on the mouth must be bad, other time i had to throw the guitar in panic into the air, it was a dirty landing by the way, then after giving the amp to someone capable to fix it, he just realized that the amp was ok, american version running at 110V properly grounded, the problem was in a conversion box that i use to convert 220V into 110V since i'm in europe, the damn thing had grounded IN plugs but no grounded cable into the wall, so he just replaced that cable by a 3 wire cord and closed the ground circuit. All the hum went away after that, the electric shocks too. Also have a memory man pedal that runs at 24V that was impossible to touch before fixing this ground problem.

Last edited by VT22 (2007-12-14 02:52:26)

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#7 2007-12-14 15:12:29

paully
Member
From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: One for you amp gurus...

nricco wrote:

...I mean how hard can it be>? live to live- neutral to neutral- earth to ?

>>>>>>>>>>>>>>>>>
There is no neutral in a 240 VAC floating setup. Both legs are 120, floated(no ground/neutral)to produce 240. Be aware of that. NOTHING gets grounded except the chassis.

Thought I'd edit this while I'm thinking about it. 240 floating is the way it's delivered here in the US. Other counties may use a neutral leg/hot leg system, but with 240 VAC (as opposed to 120, again, our standard). IOW, wired identically, but using different line voltages. Either way, a regular 3 wire cable should work, as long as the outlet has a dedicated ground.

Best, Paul

Last edited by paully (2007-12-15 23:19:16)


WADAYAKNOW.. For the first time in my life, I'm wrong again!

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