Half Of A Gibson GA40 Les Paul Amp Is ...

capnjuan

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A Gibson GA20T ... in the house at JuanzAmpz: In the smaller of the Gibson tweed cabinets. Bought from our friend Michael's Music in an eBay auction for about half of his former asking price ... deflation ... 12" replacement Qunicy alnico speaker, twin 6V6s, 5Y3 rectifier, and a Strange Brew of preamp tubes including a 6SQ7 and a 5879.

4views.jpg


GA20T Schematic here and below with the differences between the GA20 and GA40 as shown:

GA20Tschematiclandb.jpg


The 5879 is a pentode, more like an output tube than the twin-triode 12A_7 family widely used in preamps, making its way into boutique amps like Fungus' Winfield Cyclone. Our BBer Mad Dog had nice things to say about the GA40 Here If I get half of that ' ... dark, hot sound, swirly and alive' thing, that'll be fine. Except for the 5879, all the preamp tubes still have their Gibson painted logos pretty much intact. Tremolo controlled by switch in mahogany box. Cleaned up, they come back as:

ga18d.jpg



Fires up, passes a clean signal, lacking some volume, but that channel 2 ... that one has tone written all over it. 8)
 

Walter Broes

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Looks nice....

So if I can make a very crude comparison (to amps I do have a little experience with), this would be kind of like an AC15's front end into a tweed Deluxe's power amp? That can't sound anything but lardass fat. (and pretty gainy and distorted?)
 

capnjuan

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Walter Broes said:
So if I can make a very crude comparison (to amps I do have a little experience with), this would be kind of like an AC15's front end into a tweed Deluxe's power amp? That can't sound anything but lardass fat. (and pretty gainy and distorted?)
Yes; the 'top line' across the schematic is a Gibson GA18 / tweed deluxe. Yes, fatness supreme, distorted .. and all before getting it on the bench!
 

teleharmonium

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Looks great, what do you think of the speaker ? Seems like you might be able to get a significant improvement there unless the Quincy is a sleeper. The magnet size looks good, but usually when I run across a replacement speaker with an unknown brand, it's more of a hi fi speaker that can't handle the transients and low end of a guitar amp very well.
 

capnjuan

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teleharmonium said:
Looks great, what do you think of the speaker ? Seems like you might be able to get a significant improvement there unless the Quincy is a sleeper. The magnet size looks good, but usually when I run across a replacement speaker with an unknown brand, it's more of a hi fi speaker that can't handle the transients and low end of a guitar amp very well.
Hi TH; I think it's a hi-fi speaker too ... no bottom to speak of. Schematic indicates P12R; but this magnet is so large, it interferes with pulling the closest tube shield. Going to measure for either a rehabilitated Bell & Howell-branded P12N that I had reconed and a new coil to take it down to 8 ohms ... or a new Weber. I'm hoping that the cabinet allows enough space for a belled speaker. Unless there's noise in the baffle, I'd like to avoid a new one to gain 1.25" of clearance. Must say the 5879 puts a little twist in this one. NOS RCA on order from KCA.
 

Walter Broes

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John, you can take the bell cover off a speaker if it's in the way, right? It's mostly decorative anyway.
 

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I would advise yez to watch fer whatever cj kicks to the curb amp-wise....

Thank sweet Minnie Pearl that I don't have anymore hiding places left, otherwise I'd have another tweed in the house!
 

capnjuan

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Walter Broes said:
John, you can take the bell cover off a speaker if it's in the way, right? It's mostly decorative anyway.
Yes, there is no need for the bell but I jammed myself up on the GA8 ... do-over on baffle because I messed the geometry problem up. I really like the Weber speakers but they don't finish the area under the cover as nicely as they might.
 

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I guess I need to hear an amp with a 5879 preamp, seeing as I'm a big fan of the EF86.
By "need to hear", of course, I mean own...
 

capnjuan

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Hi TH: this is the ToneQuest Article threaded on LTG Here that discusses Victoria Amp's resurrection of the GA40 and the 5879 which Mark Baier of Victoria says is what distinguishes the GA40. The article also has some respectful, good-natured (well-deserved?) fun poked at Gibson engineering back in the day. Deserved? In the pic below, an eBay auction GA20T is on top, my GA20T on the bottom. In the amp on top, the circuit board is on the visual right; in my amp on the bottom, it's on the left (Gibson secret research into tone?)

I've added lines to show how much difference there is in the center gap. It isn't just the distance between the amp and camera lens; in the amp on top, the preamp tubes on the right are tightly bunched; on the bottom, nearly an inch or so gap which accounts for the interference between the speaker magnet and the phase inverter tube.

leftright.jpg



Baier also gives the GA20T an 'Honorable Mention' ... of course, since he's 're-issuing' the GA40, it isn't really in his interest to draw a lot of attention to the GA20T. Now that I have a NOS RCA 5879 headed my way, the article suggests that Sylvania made the preferable version :evil: :lol:
 

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Capn: Cool score man! That's the right one to get, that extremely narrow panel, all tweed look. Once you work your magic, that's what you'll get from this amp. Doesn't look like a whole lot of room for a speaker. Would a Weber 12A150A fit in there sans bell cover? That's the alnico I'd try (based on listening/drooling over my lo power tweed twin ...) Or a safe bet for max volume, either the 12F150 or cali paperdome ceramic. That is, if you want to swap out the speaker in there now.

On tone: It really is hard to compare these (talking GA20 and GA40) to fender tweeds. Verbal descriptions are not adequate. It's not only sound either. A different feel. So old school (to use another inadequate description.)

Good luck with it.
 

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That's a nice amp you got there capn. Surprising that it wasnt' the usual rip-off BIN auction from "that place."
 

capnjuan

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Hi Michael; yes ... I'm pumped! The speaker fix involves cutting the lower baffleboard batten ... notching it between the bolt holes ... (forgive me Orville) taking out the center third or so and positioning the center of the speaker hole on the new baffleboard off-center and down an inch to move and drop the bell. Pic of a Weber 12A125 with its bulky magnet; visually about the size as the one in the pic (like many geezers, this speaker looks better with its pants on).

Weber02.jpg



Pants on ..... you get my point ... besides, everyone knows a clean car runs better than a dirty one:

speakerb.jpg


The interference with a new speaker isn't the only driver here. For all of Gibson's 'quaint' (cheap?) manufacturing tricks, they used 1/4 - 5/16" plywood for their original speaker baffles which is to say that if this one doesn't buzz now, it will in the future and just rotating a new speaker with a similar sized magnet on the existing nails won't bring much relief. So, a new baffleboard it will be with probably another Weber on it. CJ
 

capnjuan

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jp said:
That's a nice amp you got there capn. Surprising that it wasnt' the usual rip-off BIN auction from "that place."
Hi JP; so was I. Despite his wholesale/retail practices, he was cordial and communicative ... and somebody other than I paid the freight. I don't know whether it was a consignment or not; if it had been, the owner might have just said let it go 'at the market'. The other GA20T linked above - with the better tube/speaker spacing - went for $100 less than what I paid for this one ... while Gibbie prices have been generally falling for the last three months. Whoever scored the linked GA20T, that person got the real deal. CJ
 

capnjuan

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teleharmonium said:
I guess I need to hear an amp with a 5879 preamp, seeing as I'm a big fan of the EF86. By "need to hear", of course, I mean own...
Hi TH: pretty sure you were only musing but .... http://www.webervst.com/vstbbs/bbs.html On the left, click Amps
Scroll down to refurbished 1957 Gibson GA20T for sale

From Rob a/k/a Buffalo Amps; well-priced and a thing of beauty!
 

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Listened to that Victoria youtube clip. If that's what the ga-20 sounds like, I'm jealous! :mrgreen:
 

teleharmonium

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capnjuan said:
teleharmonium said:
I guess I need to hear an amp with a 5879 preamp, seeing as I'm a big fan of the EF86. By "need to hear", of course, I mean own...
Hi TH: pretty sure you were only musing but .... http://www.webervst.com/vstbbs/bbs.html On the left, click Amps
Scroll down to refurbished 1957 Gibson GA20T for sale

From Rob a/k/a Buffalo Amps; well-priced and a thing of beauty!

Looks like a cool restoration job, but I've pretty much closed the door on music gear that has had significant repairs or non original parts, unless it's a crazy deal or I fall in love with it after a hands on experience. Especially for amps, because I have so many as it is. But thanks for pointing it out.

Recording gear is my #1 priority for now, and I also have been jonesing for a good acoustic archtop, or at least a full depth archtop with a pickup like an X-50 or such. Maybe a groovy looking vintage German guitar if I find the right one.
cheers, M
 

capnjuan

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Progress Pics: two of the power supply caps were shot. Previously strapped in with electrical tiewrap through two holes drilled in transformer deck. Using those two holes and two wiring strips, new caps and wiring strips dry-fitted in center top, and, top right, primary and screen filter cap leads attached. Bottom left, separate preamp and power amp grounds soldered on, bundle dropped/fastened in, and connected. Bottom right finished product:

ps07.jpg



Tube deck: before and after cleaning off the Tone Cheese with Flitz; insert is closeup of bases around preamp tube sockets:

tubedeck07.jpg


Haven't resolved interference between tubes and speaker bell. Will try fitting a smaller speaker.
 

capnjuan

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Progress pics: Upper left; Flitz metal polish used on the left side of the transformer deck (reflection of garage door confusing things) and untouched on the right. Upper right added a wiring strip adjacent to output transformer to terminate the original output transformer leads and add better grade wire to speaker. Lower left; just about ready; 'Brown Power Rules'; CBS (formerly Hytron) JAN 6V6s from the mid-'50s and a 6087, military grade 5Y3 ... the kind formerly used in aircraft radios ... Lower right; the going-dead filter caps. In the background, corrosion from electrolytic paste leaking out eating up the casing and, in the foreground, internal pressure forcing the plug out the top.

multi.jpg
 
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