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Grounding a guitar amp

post #1 of 14
Thread Starter 
I know this amp is not common for discussion around these parts, but I know that you guys know your amps inside and out.

I own a vintage (1968) Ampeg Jet 12II tube amp for guitars. I've been playing with it for the past three years, but I keep getting electrocuted.

After grounding out a tube amp through a $600 audio interface, I decided it was time to fix the problem with my amp. I can't afford the $150 it would cost to have a guitar tech do it, and I'm okay with the soldering iron, so I thought that I might do it myself.


Does anybody know the proper way of going about this? I already bought a heavy-duty extension cord that I can chop off the outlet terminals of to use the cable. I know to replace the old cable's AC wires in the same locations with the new cable @low wattage on the soldering iron. But... the new cable will have a ground. Where should I solder the ground to on the amp, and at what wattage?
Thanks guys.
post #2 of 14
ummm you should not be getting shocked from that amp ungrounded or not.
it sounds like it has bigger issues than not being grounded
post #3 of 14
Thread Starter 
Quote:
Originally Posted by heatmizer View Post
ummm you should not be getting shocked from that amp ungrounded or not.
it sounds like it has bigger issues than not being grounded
If the amp is ungrounded, with me touching the strings AND a ground source, the amp WILL ground out through me. I'm not the only person this has happened to, and two amp techs have confirmed this. I simply cannot afford to pay them to fix my amp, so I must do it myself. I remember something about grounding the plug to the metal frame of the chassis, but I would hate to do something stupid with 120V AC, and put myself at risk for more electric shock.

Edit - Just thought I'd point out, that the pickups on the guitar itself are grounded to the strings. So, that's where the grounding issue comes into play - me touching the strings is me touching a grounding point. When I touch another grounding point (i.e. a bass plugged into a grounded amp), that's when the amp grounds out through me and electrocutes me.
post #4 of 14
Thread Starter 
Here's an excerpt I found on a website...

Quote:
Failure of the guitarist's own amplifier places a lethal voltage on the "ground" side of the jack and thus on the strings. This scenario is usually only encountered on early vintage equipment. Even then such failures are extremely rare but when they occur they can place hundreds of volts DC on the "ground" side of the jack and the failure may be difficult to detect until the shock knocks you on your backside. This failure is basically impossible with modern three-prong equipment plugged into a properly grounded three-prong mains outlet. Never use ground lift adaptors on power cords! The shock path is from the amplifer, through the guitar chord to the guitar bridge and strings, to the guitarist's fingers. From there the shock path either goes through the guitarist to the floor through his feet or through the guitarist to a grounded piece of equipment such as a microphone. Even the first path through the resistance of shoes and floor covering may sometimes be lethal because the voltage potential is so high. The second path is easily lethal because the path from the guitarist to earth ground has very little resistance to the flow of current.
http://www.guitarnuts.com/technical/...fety/index.php
post #5 of 14
Start by replacing ALL the electrolytic capacitors. Be aware that the main power supply cap(s) may be isolated from ground on purpose, and need to stay that way.
post #6 of 14
Thread Starter 
I guess I should elaborate... The amp has NO ground - the power cable ONLY has two prongs.

I am considering an overhaul of the amp anyway - replacing the caps and tubes (the tubes do need replacing, and Ampeg recently started making the replacements again), so I might do this eventually.

I'm starting to wonder if I should just save up to have this done professionally.
post #7 of 14
looks like grounding it would only prevent you from being shocked - but it will not fix your amp. if you got a decent electrical system at your place, it should blow a fuse. in the us, this system seems to be called Ground Fault Circuit Interrupter.

edit: I meant, it should blow a fuse when you grounded it. no more shocking, but also no more electricity on a whole floor
post #8 of 14
Do you have a schematic for the amp? not the end of the world but it would help. Have a look at this ground scheme for a vox amp = it mostly deals with re-arranging the ground to get rid of hum but it will serve our purpose.

The first thing you want to do is make sure the power caps on your amp are drained of charge - 2 ways to do this, leave it alone for 48hours or so or bleed the power rails to amp ground with a suitable resistor. (please only do this if you feel comfortable working with high voltages as I am assuming that although the mains line is 120V , the amp is probably running 300V or thereabouts.) Use a DMM to measure the DC from the power supply to amp ground and make sure there is nothing that is going to kill you. Strip off your 2 pronged power cord and replace it with the 3 pronged cord, the hot and neutral will go to the amp psu as before, the ground can be bolted to the chassis. I would advise using a bolt so that you can then hook-up the amplifiers ground to the same point via a ground loop breaker ( a simple resistor in parallel with a capacitor - I use 100-200nF and 10R . Make sure the cap is rated X2 or similar) As a test before you power up, it may be worth plugging your bass in and using the DMM to check for continuity between ground and the strings.

On a different note, I think it is a good idea to overhaul the amp at the same time - get rid of old caps that are probably leaking etc. While you are at it, add a fuse into the supply in case of a short. good luck..dB
post #9 of 14
Quote:
Originally Posted by balou View Post
looks like grounding it would only prevent you from being shocked - but it will not fix your amp. if you got a decent electrical system at your place, it should blow a fuse. in the us, this system seems to be called Ground Fault Circuit Interrupter.

edit: I meant, it should blow a fuse when you grounded it. no more shocking, but also no more electricity on a whole floor
you won't blow a fuse or trip a breaker when you get shocked. The current the flows through the human body is only milliamps if that. A GFI requires 5mA in most cases to trip, however we're not dealing with ground paths of the same circuit so the GFI will never see this current difference anyways.

If the amp never had a ground on the input line cord, then it was probably designed to run on a floating ground. If it's enough to give a tickle, it's not out of the ordinary. replacing all the caps and regrounding all seem a little extreme to me. You may acutally hurt the amp if you just through a ground on it. Remember that a "ground" is a zero voltage reference for the circuit, and not necessarily at the same potential as earth ground.

The amp was most likely designed for guitars in which the strings and pickups are not electically connected (which I find strange anyways). If this were the case, then only the pickups would be at the higher potential.
post #10 of 14
Thread Starter 
Quote:
Originally Posted by Bjornboy81 View Post
If the amp never had a ground on the input line cord, then it was probably designed to run on a floating ground. If it's enough to give a tickle, it's not out of the ordinary. replacing all the caps and regrounding all seem a little extreme to me. You may acutally hurt the amp if you just through a ground on it. Remember that a "ground" is a zero voltage reference for the circuit, and not necessarily at the same potential as earth ground.

The amp was most likely designed for guitars in which the strings and pickups are not electically connected (which I find strange anyways). If this were the case, then only the pickups would be at the higher potential.
I believe the reason for the amp only having a 2-prong power cord is the fact that in 1968, most homes didn't have grounded outlets. So, the design was more out of necessity. I did find a schematic though.


If this isn't my exact amp, it's darn close. I'm at work right now, and don't have access to my amp, but it seems to be the right model. Also, the amp hasn't been on in about 4 months (I'm not allowed to have it at my college, and I do have a fear of killing myself or releasing my bowels on my bassist's couch because of the electrocution danger), so all electricity should be out of the circuit by now.

http://www.schematicheaven.com/ampegamps/j12b_jet.pdf
post #11 of 14
is the secondary of the power transformer tied to the cabinet? If so, then I don't see a problem grounding the input from the line cord. It's a basically rereferencing the secondary neutral (center leg) to ground.

BTW all the caps are usually drained within a couple of seconds or minutes at the longest. If it's UL, CSA, or ETL listed, it must be within 5 minutes. Not 48 hours like others have mentioned.
post #12 of 14
Just had a look at your schematic and I don't see a problem grounding the amp, I would still use a ground loop breaker and hopefully the amp uses a "star ground" which you could simply hook up to the mains ground via the G-L breaker. The transformer center tap goes to ground (a common practice). The power caps are not substantial and don't appear to be biased differentially as in some of these "multicaps". I do agree with bjornboy that the caps should drain near instantaneously but I have seen 200+V on a cap after 24hours without power. (a good reason to install drain resistors if this were the case). It looks as if you are all set, feel free to pm me when the time comes as I will have long forgotten about this thread. I may even have a set of NOS tubes for you, not the rectifier but I think I have some 6SL7 and 6V6 tubes in a box..dB
post #13 of 14
Thread Starter 
Thanks guys - I plan on getting around to this sometime this week. If I have any questions after I open up the amp, I'll be sure to come back here.
post #14 of 14
Quote:
Originally Posted by Eagleboy View Post
I believe the reason for the amp only having a 2-prong power cord is the fact that in 1968, most homes didn't have grounded outlets. So, the design was more out of necessity. I did find a schematic though.


If this isn't my exact amp, it's darn close. I'm at work right now, and don't have access to my amp, but it seems to be the right model. Also, the amp hasn't been on in about 4 months (I'm not allowed to have it at my college, and I do have a fear of killing myself or releasing my bowels on my bassist's couch because of the electrocution danger), so all electricity should be out of the circuit by now.

http://www.schematicheaven.com/ampegamps/j12b_jet.pdf
Guitar amplifier design is like black magic to me. Short of sitting down and actually working out the impedences of the various filters I don't think I will ever understand all of the wacky schematics I have seen of guitar amps.
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