How To Ground An Old Guitar Amp?

[Groove Dog]

Hi all -

I just got a hold of an old amp... it's a 60's Garnet Banshee in stuffed in a slutty old Victor amp case. It's an absolutely amazing amp, but it's got the old 2 prong power.

I literally get shocked if it touch anything else electric while holding my guitar.

I'm pretty handy with a soldering iron and fixing minor gear problems - can anyone point me to someplace where I could learn how to install a ground on this baby?

Thanks in advance!

[DeepSpace]

Originally Posted by Groove Dog

I'm pretty handy with a soldering iron and fixing minor gear problems - can anyone point me to someplace where I could learn how to install a ground on this baby?

Take it from me, this is not a project to learn on. The stakes are way too high if you get it wrong.

Being handy with a soldering iron and minor gear problems is not enough. Unless you're a fully qualified tech with a long history of working with this type of equipment (ie tube amps, which require high voltages and high current on the plates), then take it to someone who is.

[absrec]

Originally Posted by DarkSky

Take it from me, this is not a project to learn on. The stakes are way to high if you get it wrong.

Being handy with a soldering iron and minor gear problems is not enough. Unless you're a fully qualified tech with a long history of working with this type of equipment (ie tube amps, which require high voltages and high current on the plates), then take it to someone who is.

Really? I've watched my amp tech do it many times.

I think all you do is disconnect the old cord from the power transformer and install the new one the same way, keeping in mind that gold is hot (black) and silver is neutral (white). Except with the grounded cord, you take the green (ground) wire and affix it to chassis. Then you just turn it on and hope for the best.

Seriously though, if you never do this kind of stuff you might want to comfirm this with someone who is a qualified tech. But I think it really IS that simple.

-Aaron

[Geoff_T]

Originally Posted by absrec

Really? I've watched my amp tech do it many times.

I think all you do is disconnect the old cord from the power transformer and install the new one the same way, keeping in mind that gold is hot (black) and silver is neutral (white). Except with the grounded cord, you take the green (ground) wire and affix it to chassis. Then you just turn it on and hope for the best.

Seriously though, if you never do this kind of stuff you might want to comfirm this with someone who is a qualified tech. But I think it really IS that simple.

-Aaron

Hi

I second that. The power transformer is isolating the ac from the chassis but you have high voltages with nowhere to get a ground from except your body or those crazy switches I've seen on some old amps that connect chassis to the ac (hopefully neutral side but it's a lottery with a 2 prong plug) via a capacitor or even high value resistor.

Scary...

Yup, a 3 core ac cord and ground the chassis. If it doesn't already have one, a fuse would be a nice touch!

[DeepSpace]

Originally Posted by absrec

Seriously though, if you never do this kind of stuff you might want to comfirm this with someone who is a qualified tech.

That was pretty much the point I was attempting to make.

Originally Posted by absrec

But I think it really IS that simple.

Yes, sometimes it is.

When I was working as an electronics technician in the 1970s, three quarters or more of the old tube amps I worked on would have been fine if wired up the way you have described and put back into service. But some would not.

Groove Dog is talking about an amp that already has a significant voltage leak to the chassis which is giving him shocks when he touches anything metal. Simply grounding the chassis without checking what else is going on would not be my recommendation.

But then I guess some folk like to live dangerously, so YMMV.

[Geoff_T]

Originally Posted by DarkSky

That was pretty much the point I was attempting to make.



Yes, sometimes it is.

When I was working as an electronics technician in the 1970s, three quarters or more of the old tube amps I worked on would have been fine if wired up the way you have described and put back into service. But some would not.

Groove Dog is talking about an amp that already has a significant voltage leak to the chassis which is giving him shocks when he touches anything metal. Simply grounding the chassis without checking what else is going on would not be my recommendation.

But then I guess some folk like to live dangerously, so YMMV.

Hi

Yes I agree that tech help would be wise and I suspect that the shocks he gets are related to those evil hum killing switches I mentioned in the last post. If his amp has one of those I recommend removing it and the associated wiring.

I come from an era where TV's did not have isolating transformers and just rectified the ac so, whichever way around you plugged the two pin plug in, the chassis was either live to ac line or ac neutral.

Compared to that technology, the shocky little guitar amp is a walk in the park!

[DeepSpace]

Originally Posted by Geoff_T

Compared to that technology, the shocky little guitar amp is a walk in the park!

LOL. Yeah I've seen a few of those.

I have also seen a guitar amp that worked on a similar principle - ie rectified the AC with no isolation. Now *that* was a shocker!

I wouldn't expect that to be the case with Groove Dog's Garnet Banshee - unless it has some very shonky mods. But then it has been stuffed into an old Victor case, so anything is possible...

[Geoff_T]

Originally Posted by DarkSky

LOL. Yeah I've seen a few of those.

I have also seen a guitar amp that worked on a similar principle - ie rectified the AC with no isolation. Now *that* was a shocker!

I wouldn't expect that to be the case with the Groove Dog's Garnet Banshee - unless it has some very shonky mods. But then it has been stuffed into an old Victor case, so anything is possible...

Hi

Actually, thinking back, a lot of equipment was sold to the public without any isolation from the ac current... especially record players with wooden cases and plastic knobs.

I recall a teenage girl dying because she took her record player into the bathroom with her and reached out from the bath to turn the record over.. zappp!

Now you can't have a power socket in a UK bathroom other than an isolated shaver socket.

Thinking back, it's a miracle I survived!

[Groove Dog]

Wow, great info. Thanks everyone.

I think my plan of action will be to take it to a tech, have it looked through, and we'll decide from there what needs to be done.

It's a pretty simple amp - just one on/off switch, bass treble and vol. I think the idea behind it was based on a bigger, sweeter version of a Champ.

You should hear this thing break up on the high end though... it's one of those sounds that has either been overlooked by modernity or just isn't available outside of "the shocky little guitar amp"!

Thanks again!

[Ergonomic]

Originally Posted by Geoff_T

Hi

I second that. The power transformer is isolating the ac from the chassis but you have high voltages with nowhere to get a ground from except your body or those crazy switches I've seen on some old amps that connect chassis to the ac (hopefully neutral side but it's a lottery with a 2 prong plug) via a capacitor or even high value resistor.

Scary...

Yup, a 3 core ac cord and ground the chassis. If it doesn't already have one, a fuse would be a nice touch!

Wouldn't connecting a light via receptacle, with electrician gloves of course, effectively drain the capacitors of their stored energy? Just hook it up until the light bulb goes dim and finally out. That should expel all remaining current from the circuit. *REMEMBER* though, it is not a good idea if you are not wearing the proper safety equipment and/or if you are accident prone, or "clumsy" in laymen terms. A bit of advice to whoever may or may not attempt this, if you are the LEAST bit hesitant, DO NOT PROCEED

[Pchicago]

On this vein..I've heard from a few folks that doing the 3 wire grounded upgrade totally changes the sound of many amps?

Is this a myth or is it true?

Never had to do this so, no clue.

[mdme_sadie]

It's possible, though it does sound like a bit of a myth to me. It's just shifting where earthing is happening in the circuit from you and your guitar to the chasis, I guess that could change the draw and impedance a little throughout the circuit. More likely though is that it'll just clean up some of the hum, which could in turn be masking the amps actual sound a little.

[ofajen]

Actually pretty simple. Remove the old cord. Install a three wire cord with proper strain relief. Green ground wire firmly grounded to the chassis, say with an eyelet connector fastened down to a bolt on the power transformer. Black (hot) wire to the inner connector on the fuse holder and from there (outer connector on fuse holder) to the power switch and then on to the transformer. Remove any "ground" switch and associated wiring. White (neutral) wire directly to transformer. Done.

Cheers,

Otto