1956 Gibson GA 20 rehab questions

stupidhead

aka Ratdogheads
Subscriber
Hello folks! I am not new to Audiokarma but have never visited or posted in this particular forum. My focus is generally vintage audio and Acoustic Research speakers are my favorite. Am also a Marantz fan with a number of models and have an HK Citation II as main amp. Recently I stumbled across an old guitar amp that I decided to snag. There are no musicians in our household, but do have many friends who are and it will be nice to have for someone to plug in during a visit. Also daughter Grace is getting married this Fall and we can use this as a PA for the reception!

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As you can see the applique is off, but intact! I do have some knowledge and hobbyist skills for speaker rehab in general but this seems a bit more involved. I have done a bit of research and I do have the original schematic. The questions I have are pretty simple but didn't want to place any parts orders before getting guidance/confirmation from others more savvy on these matters. For example in the "parts list" on the schematic it calls out mica and ceramic caps for a couple of the components. I am by no means an engineer and do not possess the knowledge of actual differences in these things. I assume also that all caps likely should be replaced, but what else? Also in this case does brand or type effect the end result sound? I know this is a hotly debated topic in speaker circles but with this application differences may be part of the equation when replacing parts. I assume all dials and switches need a deoxit treatment but again looking for some confirmation before doing anything. I also have not plugged it in and am told it has sat in a closet for some 30 years prior to my ownership. The "seller" is a descendant of the original owner and has very little knowledge of it's history. Here are a couple more angles

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All of my impressions are that this is a worthy candidate for a rehab. My mentality is usually bringing projects back to oem build as opposed to making mods. This unit sure seems to be all original which is always a great place to start. The driver in this cab appears to be well regarded in the research I have done.

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and here is a photo of schematic. I will have this scanned, as well as the rest of the "Instructions" fold out that contains it for any that may be interested.

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I have given a cursory look at the wiring by removing the back panel to expose. Looks like RCA tubes and I would suspect originals. The layout seems pretty easy to work on and does not look intimidating to me.

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Look forward to any feedback from the community.

Geoff
 
thanx folks. Celt, talk to me about cap options. I see a parts list of eight .05 mfd 600 caps as well as others. Replace them all? Doesn't seem to be a typical value, at least at PE or Madisound. Then I see a bunch of NOS vintage caps on the bay? Wouldn't they be suspect as well. Any ideas on vendors? Then there is a ceramic cap needed? And a mica one? I scanned the manual for a clear look at this parts list, see page 2. Looks like they did a two caps in one can thing as well, reminds me of when I did my KLH 5s. What about resistors?

louisjames, I appreciate your thoughts. is there something I should know about using an amp like this as a PA? Page three of the manual actually speaks to the proper use as a PA and seems to be presented as being suited for it. if you have followed any of my speaker rehabs you know that I am usually an oem kind of guy. at least with this vintage stuff.

Geoff
 

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Agree with Farmhand. But if you keep it please reconsider using it as a PA. That speaker alone is worth a nice bit of change and the value of the amp, other than bringing it up to spec, is in keeping as much original as possible. Just my 2c's.

Seconded. Strongly.
 
thanx folks. Celt, talk to me about cap options. I see a parts list of eight .05 mfd 600 caps as well as others. Replace them all? Doesn't seem to be a typical value, at least at PE or Madisound. Then I see a bunch of NOS vintage caps on the bay? Wouldn't they be suspect as well. Any ideas on vendors? Then there is a ceramic cap needed? And a mica one? I scanned the manual for a clear look at this parts list, see page 2. Looks like they did a two caps in one can thing as well, reminds me of when I did my KLH 5s. What about resistors?

louisjames, I appreciate your thoughts. is there something I should know about using an amp like this as a PA? Page three of the manual actually speaks to the proper use as a PA and seems to be presented as being suited for it. if you have followed any of my speaker rehabs you know that I am usually an oem kind of guy. at least with this vintage stuff.

Geoff

Celt's the Man, I suggest that you heed his advice and recommendations re: tech work @ the resto/updating of this geetar amplifier.
 
Celt's the Man, I suggest that you heed his advice and recommendations re: tech work @ the resto/updating of this geetar amplifier.

I do look forward to more insights from Celt, and others regarding this rehab. This community is a wealth of knowledge and info, and folks who are happy to provide guidance to those looking.

Seconded. Strongly.

Please understand I am not trying to be contrary here but I am confused by this. The only reason I am considering the PA angle is because the owner's manual indicates that is one of it's functions. The amp has three instrument inputs and a dedicated mic input. A direct excerpt from page three is "Because of the high power output, high gain and high fidelity characteristics of the Gibson GA-20 amplifier it makes an exceptionally fine public address system when used with either the crystal, dynamic or velocity types of microphones."
Again I am not trying to be contrary or question anyone's knowledge but I am curious why there seems to be this apparent difference in information. Possibly 60 years later there is a better understanding of this topic and what used to be acceptable has been debunked?

Geoff
 
thanx Louis for clarifying and your thoughts are duly noted.

where does one find .05 mfd 600v caps. I have searched a good bit for them and am coming up empty. I do find .047 600v (actually 630) and wonder if that is "close enough".
 
15-20w of 6v6 power into one 12. That's awesome.

Take care of that amp.

My favorite amp these days is my 62-ish Gibson Falcon- kind of the same amp with reverb and tremolo.

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I had a 1951 GA-20, looked nothing like yours. Mine had the tan colored 15" Jensen red label coaxial in it. They really changed the style it appears. Great amp worth restoring
 
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where does one find .05 mfd 600v caps. I have searched a good bit for them and am coming up empty. I do find .047 600v (actually 630) and wonder if that is "close enough".

Parts Express and Madisound are generally quite limited in their selection. I'd source these through Mouser or Digikey.
 
So,you guys have made me drag out the camera and some amps.Apparently,you're not alone with your Gibson Geoff!! At the risk of hijacking your thread,I'll see your GA-20 and raise you a GA-6,and I'll see TheGoldenBoy's Falcon and raise him a Scout:



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Those old amps are fine for PA use if your just putting vocal, speech or a little prerecorded music. But try to use for hip hop, Dance big subwoofer stuff it wont sound very good.
.047uf is the modern equivalent of .05uf. Your ceramics and mica caps are probably good along with those white film caps. Black beauty's with the nipple on one end are prone to leakage. the later ones are pretty good.
I just rebuilt a 57' ga40t. most of the film caps were still good but a few did leak at 50v. Replace the stick o dynamite electrolytic's and see how she sounds. I used all 20uf even for the 10uf and 16uf. But a purist might have issues with that.

One more thing, Gibson on some models used a floating ground, the chassis not ground. But I found no problems grounding everything to the chassis and installing a grounded AC cord.
 
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This is a very nice amp. You find many Gibson's deviate from the schematic. Some of them sound amazing. After the electrolytics, you probably should replace all the plates resistors. I would leave the ceramic capacitors unless they've drifted but they rarely leak. Personally I usually only replace coupling capacitors if they are leaking, others folks replace all of them. Either way put all the replaced parts in a bag and pass it along to the next owner. Antique Electronic Supply sells polyester Orange Drop capacitors in the the values specified. Or if you want to install vintage capacitors, I would find the green Cornell Dubilier. These capacitors are relatively robust for vintage capacitors and Gibson periodically used them in their amps during that era.
I have a GA20T that is my favorite amp. You're in for a treat when you hear it overdrive for the first time!
 
I restored a Gibson GA 40T, here's a few picks. I changed all the electrolytic's and two of the coupling
caps from the Disc capacitor to two .1uf Sprague Black Beauties caps. You could get .05uf on ebay
or get .047uf like I used in my Fender Deluxe restore, just check with the seller's listing to see if there
tested to voltage.

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thanx Louis for clarifying and your thoughts are duly noted.

where does one find .05 mfd 600v caps. I have searched a good bit for them and am coming up empty. I do find .047 600v (actually 630) and wonder if that is "close enough".

Ebay, but check if there tested, in my GA40T, I used a pair of black Beauties that were tested by the seller on
ebay, for the Deluxe I used a set of blue Mallory's that Fender used in their early sixties blackface amps they seem
to hold up. Or go the new route the Jupiter's or Sozo's are good, but when it come to guitar amp's changing anything
is bad for the value but you will have to decide to have a all original amp that could go pop! or a good working one. Just
remember to save all the original caps in a plastic bag incase you want to sell it later, and take care of that original
speaker.
 
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