grounding a guitar amp

[yardleyone]

i have a vox berkley II. Kind of a weird, solid state that they madein the seventies. It sounds real cool, but has no ground so it can give some really nasty shocks. Any advice for grounding a guitar amp. I really want to do it the right way, but I'd also like to be able to do it myself as i'm cash poor and every other kind of poor also.

[NewYorkDave]

I'm reluctant to advise any inexperienced person about messing around with AC power wiring. But since you're bound to get a bunch of misleading and ill-informed replies, I'll offer a few simple rules which should help to keep you out of trouble.

1) Remove the original two-wire AC cord and replace with a good-quality three-wire cord. If it's not a very high-powered amp, 16-gauge should be fine. You might have to punch out the chassis to accept the larger cord. You must use a strain relief bushing--don't even think about doing without it!

2) The ground wire (green) should be soldered to the chassis using a high-powered iron and lots of rosin flux. Clean and "rough up" the area of the chassis to which you plan to solder. Use lots of flux and consider taping the wire to hold it in place as you solder (to avoid burning your fingers). The flux will leave a big mess, but this can be cleaned up with isopropyl alcohol and a toothbrush after the solder joint cools.

Some manufacturers solder the ground wire to a ring lug and secure to the chassis using hardware including screws, nuts and star washers, but I prefer the more positive connection soldering provides. If the chassis is aluminum, soldering is not an option, however.

3) The remaining two wires will be either black (hot) and white (neutral), or brown (hot) and blue (neutral). The latter color scheme is used on most power cables manufactured outside the US. The hot MUST go to the fuseholder first, before the power transformer, and it should connect to the rear terminal of the fuseholder. In fact, it's best for the hot to connect to the fuse, then the on/off switch, then the power transformer. The neutral connects to the other lead from the transformer. Make sure the hot and neutral are not connected directly together (disastrous!), but instead that each goes to one of the two leads from the transformer primary winding.

4) Lastly, remove any capacitor(s) that may be connected from the AC circuits to the chassis. These may be connected directly, or via a switch. This cap is the path that the current is taking when it shocks your ass (in the amp's stock configuration), so remove it!

5) I strongly recommend powering up, for the first time, with a 50-watt lightbulb in series with the line to limit the current. If the bulb lights up, you've made a bad mistake somewhere. You can rig up this current limiter with parts from Home Depot. Follow safe wiring practices, of course.

The above advice is provided without any guarantee. I'm a Fender Gold tech and former full-time guitar amp repair guy, but you should be very careful about following any advice you receive on the internet, especially when it pertains to something that involves your own safety or that of others. Really, the best and safest thing to do is to forget about doing this yourself and take it to a qualified tech. It's about a one hour job.

[Rigsby]

I'm sure everyone has such a story, but just a word of warning:

About eleven years ago i was in a group as a vocalist/guitarist and after not using my guitar at the start of practice, asked the other guitarist to switch my amp on for me as he was closer. He was holding his guitar at the time, and after switching my amp on, just stood there leaning against a table. After a bit we started wondering why he was so quiet, just then he fell to the floor and started convulsing. Turns out my amp had electrocuted him as the voltages weren't matched (my amp and his, via his guitar) due to a loose earth on mine. He went to hospital and took a few days to properly recover. They said that if his strap hadn't come loose and made him drop the guitar things would've been a whole lot worse. This frightened all of us a lot.

I second paying someone who knows what they're doing.

[leigh]

I just read a back issue (within the last year I think) of Recording magazine that had a detailed article about how to do just this. Of course, if you have any doubts in your own understanding of electricity, I'd echo what others are saying: let a qualified tech make this quick mod.

Leigh

[Scodiddly]

New York Dave's stuff is 100% right. And I'd strongly recommend having a tech do the work, if you're not much on soldering.

Pretty much any old ungrounded amp I work on gets a proper grounded cord... these days I'd probably have some liability problems if I didn't.

[object88]

I would also heartily recommend paying a qualified tech to do this if you're not familiar with the amp construction, circuit, and/or grounding scheme. It's not that it's hard, it's that it's dangerous.

[yardleyone]

in this case what is a reasonable price for a good tech to charge for this mod? I live near Indianapolis and was thinking of using Uncle Alberts. Anybody have an experience with them?

[Milkmansound]

I have done this in the past - it is REAL simple. Do not attempt to solder a ground directly to the chassis, since it will act as a huge sink, and you will not get it hot enough to form a proper electrical connection. The proper way to do this is this:

1 - attatch the green ground wire to a ground lug with solder (you can use an iron for this, since the lug is not that large and will not sink the heat), which you can find at any hardware store

2 - Find a good place on the chassis to mount the lug - I usually do it to one of the screws on the transformer (any screw that goes directly to the chassis will do though). Use your continuity tester to confirm the electrical conection (check it to the prong on the plug end), and make sure the mechanical connection is also secure (aka, the screw is tight and not going to loosen)

Now your chassis is grounded - provided you are using a 3 hole outlet that is properly grounded to a safety ground you should be nice and safe. Some old and out of code clubs do not have proper grounding - I think this is where these electrocution problems happen - also, old houses and buildings in general. If you are just going to lift the ground anyway, and use a 2 prong outlet, you are still at risk. Also, GFCI boxes are not attatched to any safety ground either - however, they do trip when an abnormal amount of current is drawn through them.

Good luck - and may you never be the path of least resistance to ground! - Tim

[NewYorkDave]

So, what was that I was saying about taking what you read on the internet with a grain of salt?

1) You CAN solder to a chassis. You just need a very powerful iron with a wide tip. MCM electronics used to sell a 100-watt model that worked well. Notice that I didn't say "soldering gun"... guns have tips that are too narrow for this work.

2) GFCI outlets DO (or at least should) have a safety ground, like any other outlet.

3) GFCIs do NOT trip based on the magnitude of current. They trip if there is an imbalance--even a small one--between the current being pulled from the "hot" and returning on the "neutral."

[Scodiddly]

3) GFCIs do NOT trip based on the magnitude of current. They trip if there is an imbalance--even a small one--between the current being pulled from the "hot" and returning on the "neutral."

No kidding - I even had one trip when I used a voltmeter to measure hot-ground on a stage outlet.