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How To Ground An Old Guitar Amp?
Old 26th October 2006
  #1
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Groove Dog's Avatar
 
🎧 15 years
How To Ground An Old Guitar Amp?

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!
Attached Thumbnails
How To Ground An Old Guitar Amp?-banshee.jpg  
Old 26th October 2006
  #2
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DeepSpace's Avatar
 
🎧 15 years
Quote:
Originally Posted by Groove Dog ➑️
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.
Old 26th October 2006 | Show parent
  #3
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absrec's Avatar
 
🎧 15 years
Quote:
Originally Posted by DarkSky ➑️
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. heh

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
Old 26th October 2006 | Show parent
  #4
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Geoff_T's Avatar
 
🎧 15 years
Quote:
Originally Posted by absrec ➑️
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. heh

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!
Old 26th October 2006 | Show parent
  #5
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DeepSpace's Avatar
 
🎧 15 years
Quote:
Originally Posted by absrec ➑️
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.

Quote:
Originally Posted by absrec ➑️
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.
Old 27th October 2006 | Show parent
  #6
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Geoff_T's Avatar
 
🎧 15 years
Quote:
Originally Posted by DarkSky ➑️
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!

Old 27th October 2006 | Show parent
  #7
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DeepSpace's Avatar
 
🎧 15 years
Quote:
Originally Posted by Geoff_T ➑️
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...
Old 27th October 2006 | Show parent
  #8
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🎧 15 years
Quote:
Originally Posted by DarkSky ➑️
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!

Old 27th October 2006
  #9
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Groove Dog's Avatar
 
🎧 15 years
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!
Old 30th November 2011 | Show parent
  #10
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🎧 10 years
Quote:
Originally Posted by Geoff_T ➑️
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
Old 1st December 2011
  #11
Lives for gear
 
🎧 10 years
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.
Old 1st December 2011
  #12
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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.
Old 1st December 2011
  #13
Registered User
 
🎧 15 years
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
Old 21st June 2012
  #14
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🎧 10 years
OK...sorry to resurrect this older thread but better than making a new one for my 1st post, yeah :D

Rewiring my old 1966 Ace Tone Rockey and omitted the ground switch and death cap but I am blowing fuses.
It has a 100v and a 117v fuse switch in parallel but I have omitted the 100v, blew fuses, included the 100v, blew more fuses lol.

Here is how it is right now (orange is 117v, white 100v):


The thin white coiled wire on the left was direct to the switch where the new white is now and the other thicker one next to it was connected in parallel to the 2 upper fuse posts where the black now resides.
The two whites go to the ground switch and aux plug in.


I assume I would wire is as per Ofajen's suggestion or?
I saw a rewiring diagram on how to wire up a old Fender with a 3 prong but it shows to incorporate the ground switch still?

Appreciate the help!
Old 21st June 2012
  #15
Lives for gear
 
🎧 10 years
On the old Victor amp, it could be not only the cord, but also your filter caps that need to be changed. Do not plug the amp in again; with filter caps that old it doesn't take much for them to go and then you can lose transformers, tubes, etc. downstream. Filter caps and/or cap cans are cheap compared to trashing your amp. Take it to a good amp tech who knows older amps.

Whoever told someone a 3 prong grounded cord gives your amp a different sound was pulling your leg. Unless you like the sound when you're being electrocuted. No tonal difference. I've done this conversion on 3 Fenders and besides being reversible, it doesn't change the sound 1 bit. Neither does changing the filter caps. I consider both mandatory on a 30+ year old amp.

Ground switch = irrelevant on a 3 prong grounded amp. this becomes a cosmetic leftover or can be used for other things, but it's taken out of the Fender circuit as well as the "death cap."

Udonitron - I'd recommend taking it in. If it's blowing fuses, it may be this incorrect wiring, a bad rectifier, or something else. If you like the amp, worth having someone who knows electronics to fix it.

To reiterate, mucking around inside a guitar amp shouldn't be attempted by most; there are lethal voltages inside an amp that can seriously hurt you. While some advice on the web is great, anything dealing with electricity should be taken with care. If you don't know what you're doing, don't attempt to fix yourself. Start off with a broken guitar pedal and work your way up.
Old 21st June 2012
  #16
Registered User
 
🎧 10 years
Cheers for the reply.

Yeah I have some knowledge...just finished rebuilding my Elk Twin Amp 45 head via a total recap and it sounds amazing now!
Just tossed some new EL34B's in it yesterday and it sounds even better vs the old original Japanese tubes.

The Ace Tone Rockey was just recapped as well with original working tubes other than a new 12AX7 and a 6AV6 and was working but blew a fuse before swapping out the 2 prong.
Once I put the 3 prong in I have no difference...it is based off the original VOX AC15 circuit point to point.

I am no master amp tech but know enough to bleed my electrolytics and volt and capacitance meter all the stuff that I do and know what to replace and what not too but the 3 prong looks right to me but not sure why it is still blowing the fuse.
Old 21st June 2012
  #17
Lives for gear
 
🎧 10 years
Good to know you know your way around Japanese amps. Wondering if there isn't a user group that may have more insight on that specific amp's wiring layout.

As for me, I know only the numbers of my 2 amp techs. But having been shocked by an old Hilgen Basso Grande, I appreciate well-grounded 3-prong cords on my amps...
Old 21st June 2012
  #18
Registered User
 
🎧 10 years
Yeah this is the schematic for the Ace Tone

No note of a fuse so I assume it is a 2A but...maybe 2.5 or 3?


http://rkerkhof.ruhosting.nl/Posts/Acetone.jpg
Old 21st June 2012 | Show parent
  #19
Registered User
 
🎧 15 years
Quote:
Originally Posted by Udonitron ➑️
Yeah this is the schematic for the Ace Tone

No note of a fuse so I assume it is a 2A but...maybe 2.5 or 3?


http://rkerkhof.ruhosting.nl/Posts/Acetone.jpg
A general starting place for Most power amps is:
Double output wattage and then Ohms law; example 50 watts output *2= 100 watts, (100 watts total draw) 100W/120V= .83amps or 1 amp..
A solid state amp MAY need a slightly higher fuse OR a slow blow for the surge current is very different (Higher) from a ALL Tube amp..
Old 21st June 2012
  #20
Registered User
 
🎧 10 years
Gotcha!
So 2 x EL84 or about 20 watts divided by 117v would give me 0.17 so a 0.5A?
Seems low but I think my wiring needs to go: ground to PT, black to fuse, other side of fuse to switch, switch to tranny yeah?

However regarding the 100v lug, should I omit that completely then and just leave the power light wired with that yellow wire or wire that off the 117v lug as well?

Cheers
Old 12th August 2012
  #21
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MarkF48's Avatar
 
🎧 10 years
Kind of late to this post, but I would suggest to Udonitron that he sketch up a schematic of the circuit as he has it wired. It may look OK physically, but an accurate schematic may show some not so obvious errors. One in particular that sticks out is where he has the Black/Hot lead from the power cord. This should be to one side of the power switch and then from the other side of the power switch to the fuse. You want the switch to kill the hot side feed. The White/Neutral lead of the power cord goes to the "low" side of the transformer. If the pilot lamp is desired, connect the resistor to the "hot" side of the circuit after the switch where convenient and the other side of the resistor to the lamp and then other side of lamp to a neutral point. Hope he didn't mix up the transformer leads for the voltage taps and low side transformerconnection heh

The physical install of the power cord is disturbing as there is no strain relief or protection to the cord chafing against the metal chassis. At the very least use a rubber grommet in the feed through hole and put a tie wrap around the cord to prevent it from pulling out.

Below is an edited/simplified schematic of a Fender Blues Jr power circuit for reference...
Attached Thumbnails
How To Ground An Old Guitar Amp?-ac-amp-convert.jpg  
Old 12th August 2012
  #22
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🎧 10 years
Cheers Mark, I will try that out and report back!
There is a relief on it now, was just wiring to test in that photo.
Old 14th August 2012
  #23
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🎧 10 years
Heads up Mark, haven't had a chance to wire this up as you stated but I wanted to clarify for you that all the leads from the tranny are in their original untouched positions.
Only soldered in the new cord points in place of the old cord points minus the ground of course.

Schematic here shows what's up:

http://rkerkhof.ruhosting.nl/Posts/Acetone.jpg

Cheers
Old 14th October 2012
  #24
Registered User
 
🎧 10 years
Hey Mark

Finally got around to trying your suggestion and it is still blowing fuses...typical building hum in the power tranny and then fuse blows within 2 seconds of power being on.
However your method is still supplying power to the light when turned on even after fuse blow .
Old 15th October 2012 | Show parent
  #25
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🎧 10 years
Quote:
Originally Posted by Udonitron ➑️
Hey Mark

Finally got around to trying your suggestion and it is still blowing fuses...typical building hum in the power tranny and then fuse blows within 2 seconds of power being on.
However your method is still supplying power to the light when turned on even after fuse blow .
Can you identify and lift one of the transformer secondary leads (tape it so it won't short to anything and then try to power it up? This at least may show that it's not the transformer is not blowing the fuse. My guess might be a diode or cap in the power section.

Looking back at my post I should have stated to connect the lamp after the fuse rather than after the switch.
Old 4th March 2013
  #26
Registered User
 
🎧 5 years
Hi there,

I've recently acquired a 1975 Gibson Medalist 4x10 and I opted to swap from a 2-prong to 3-prong power cord. As observed in the schematic, the fuse was on the line side and was connected as such. The same followed for the common. The ground was connected to the chassis.

Anyway, the exchange was successful, but my concern is the fact that it has a 3-way rotary switch (polarity switching) that had the "death cap" (C41, lower left hand corner of schematic) connected to it. I've attached the photo and schematic. For the most part, I just wanted to make sure that it was only necessary to remove the capacitor and nothing else.

So to reiterate, I swapped the cord as described above and removed the death cap.

Any help on this would be great.
Thanks.
Attached Thumbnails
How To Ground An Old Guitar Amp?-img_2196a.jpg   How To Ground An Old Guitar Amp?-medalisty-schematic-small-.jpg  
Old 4th March 2013
  #27
Registered User
 
🎧 5 years
Here's a better version of the schematic.
Attached Thumbnails
How To Ground An Old Guitar Amp?-medalist-power-small-.jpg   How To Ground An Old Guitar Amp?-medalist-preamp-small-.jpg  
Old 7th May 2015
  #28
Registered User
 
🎧 5 years
I recently installed a 3-prong onto my Ampeg GU-12 because it was shocking me when the 2 prong was in upside down. I installed the white where the old white was, the black where the old black was and mounded the green to the chassis. I also got rid of the death cap. I'm not getting shocked anymore, however my amp is now acting in ways that it didn't before.

1.) buzzing sound. When I put my hand on the strings the buzzing gets quieter.
2.) the "correct" volume level is intermittent. First it was GREAT and performing awesome, then it diminished a good deal, then corrected it self, and now its stuck in the puny territory.

All of the filter caps tested good, and I just did a test on the resistors and those seem good too. However, I am out of ideas on what it could be and how I can fix the problem. Does anyone have any ideas?
Attached Thumbnails
How To Ground An Old Guitar Amp?-photo-3-9.48.45-am.jpg  
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