Gibson GA18

capnjuan

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This was on the bench this morning and I took some beauty pics. This amp bears a fair electronic resemblance to the Fender Princeton (AA964 model) twin 12AX7s, twin 6V6s, 5Y3 rectifier, Class A/B push-pull, cathode-biased, rated at 15 watts. This amp is fitted with a 10" Weber 10A125 on a new 9-ply Baltic birch baffleboard; formerly had a Jensen P10R that now has a happy home with one of our BB broz. Logo is aftermarket plastic held in place with brads.

These amps were sold with slightly lighter and slightly darker tolex; this is the lighter tolex. Unlike Fender's cotton tweed covering, this covering is sheet vinyl with patterned cotton fabric bonded to the back; more like the old sticky tablecloths. These amps commonly have stains caused by the interaction of the covering and the water in the glue used to adhere it; this amp has almost none of the characteristic orange staining.

ga18e.jpg



View of the control panel, right to left; volume, tone, tremolo speed and tremolo intensity:

ga18c.jpg



Footswitch made of a block of solid mahogany; paint removed and FS refinished:

ga18d.jpg



View of the chassis:

ga18a.jpg



Pre-amp and tremolo sections, pretty crowded for pulling/replacing resistors and capacitors. Today's work included change out three Orange Drop coupling caps for Sprague 150s (the three little yellow guys), refreshing the preamp cathode caps and resistors, and re-tensioning the rectifier socket:

ga18f.jpg



Power supply and output tubes. The configuration of the blue caps on the left is the work-around in this amp for replacing 'cigar-style' paper capacitors; the 'tweed' generation Gibsons did not use multi-section can caps - 30uf for the B+, 30uf for the screens, and 22uf for the preamp. The blue/dark blue cap in the foreground is the cathode cap in parallel with the cathode resistor:

ga18g.jpg



The GA18 has been completely refurbished; it was in good condition when i got it with stock parts and it sounded pretty good but I was interested in hearing what the original design would sound like with fresh and/or better-grade parts, fresh tubes, and a new speaker. The sound is quintessential 'tweed-era'; throaty, meaty, thickish, compressed, chimey, and syrupy; the amp starts to break up at 4/5 on the volume into the gravelly, crunchy thing. It isn't Fender sound, it's Gibson sound ... no matter what the Fenderistas say, an amp doesn't have to sound like a Fender to have great sound. CJ
 

Walter Broes

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capnjuan said:
The GA18 has been completely refurbished; it was in good condition when i got it with stock parts and it sounded pretty good but I was interested in hearing what the original design would sound like with fresh and/or better-grade parts, fresh tubes, and a new speaker. The sound is quintessential 'tweed-era'; throaty, meaty, thickish, compressed, chimey, and syrupy; the amp starts to break up at 4/5 on the volume into the gravelly, crunchy thing. It isn't Fender sound, it's Gibson sound ... no matter what the Fenderistas say, an amp doesn't have to sound like a Fender to have great sound. CJ
How would you say it compares to a tweed deluxe Gain/volume/headroom-wise though?

It looks great and sounds very interesting in any case.
 

krysh

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great work, john.
...you know my adress..... 8)
:mrgreen: :mrgreen: :mrgreen:
 

capnjuan

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Walter Broes said:
How would you say it compares to a tweed deluxe Gain/volume/headroom-wise though?
Hi Walter, can't say directly but going by the 'lore' of the Deluxe, I'd say the volume would be similar, the Deluxe might have more headroom, and I'd guess the Deluxe - with its superior output transformer, would stay cleaner longer ... it will break up eventually but the GA18 with its cheesey OT would probably break up sooner. John
 

jp

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capnjuan said:
. . . no matter what the Fenderistas say, an amp doesn't have to sound like a Fender to have great sound. CJ
You betcha! Fine handiwork capn.
 

capnjuan

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Thank you Gents; CJ
 

Walter Broes

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I totally forgot to mention that in my post : your handiwork looks great indeed, I wish you were my tech!
 

capnjuan

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Hi Walter; if your musical career brings you to the US, I'd be happy to serve as your Road Tech! John
 

slidincharlie

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Hi CJ,
I have an amp like this one. Labelled GA18, although it fits the GA16 schematic.
I did a similar job to mine (congrats for the neat job in the filter/power section!).
Yet mine stays cleaner than yours apparently. It starts to break up around 5 with humbuckers.
I bought a Hammond tweed Deluxe type OT that haven't still installed, hoping to have some more volume and balls.
I love this amp anyway. It's my main amp in small bar gigs.
 

capnjuan

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Hi Charlie; you're right, they are terrific amps and yes, the GA16 is the only relevant schematic. I run mine with a GE 6087 rectifier that bumps up the B+; very punchy and brisk. I look forward to your comments on the Hammond OT; if there is a criticism common to all Gibson amps from the period, it was their (cost-driven?) insistence on cheesey output transformers; I put a Weber on my GA8; just makes a world of difference. Regards, John
 
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