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GA-25.5

#1 User is offline   dkevin 

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Posted 31 August 2016 - 07:36 PM

I have an amp that seems to be a "tweener". It is labelled a GA-30 but it only has 2 knobs on the control panel and the tone expander circuit slide switch is located below and between the knobs. I refer to it as my "GA-25.5" amp. I bought it as a chassis only. When I was sketching out the layout, I discovered that it differed from most other GA-30's. The preamp tube is a 6SJ7 and there is only one preamp tube like a 25. I was fortunate to find one other person who owned a similar amp. I have heard lots of opinions about Gibson's quality control and that's not really why I am posting. I guess that Gibson was trying to use up overstocked GA-25's and issue the new model (GA-30) at the same time. All in all, I feel fortunate to have one of these rarities.[attachment=21799:GA 30 back.jpg]
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#2 User is offline   badbluesplayer 

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Posted 05 September 2016 - 09:51 AM

Pretty cool! [thumbup] [thumbup]
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TENNESSEE AMPLIFIER COMPANY
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#3 User is offline   dkevin 

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Posted 06 September 2016 - 01:21 PM

The tube lineup is 6SJ7 preamp
6SQ7 PI
6SQ7 PI
6V6 output
6V6 output
5Y3 rectifier
I am attaching a GA-25 and a GA-30 schematic...This amp is like a single channel GA-30 with one volume and one tone control. One of these days I am gonna build a replica cabinet and crank it![attachment=21833:Gibson_GA-25.pdf]
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#4 User is offline   dkevin 

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Posted 22 November 2018 - 12:40 PM

Long time no post...recently I found several of these Gibson amps for sale. I will post a link to them....

https://www.ebay.com...T4AAOSwG-xb5I83

https://reverb.com/i...50-gibson-ga-30

This post has been edited by dkevin: 22 November 2018 - 12:44 PM

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#5 User is offline   dkevin 

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Posted 22 November 2018 - 12:58 PM

"GA-30

Gibson’s midline amp was changed for the third time in three years, as the short-lived GA-25 (which replaced the short-lived BR-4) was superseded by the identically priced GA-30. Cosmetically, the porthole speaker openings were replaced with a large, rectangular picture frame-style with a Gibson logo across the middle.

The 1949 Electric Guitars catalog showed the GA-30 with a two-knobbed control panel, as on the 25 (this was a new photo, not a reprint of the 25 from the ’47-’48 catalog), although most GA-30s had separate volume controls for the mic and three instrument inputs. An extra 6SJ7 preamp tube was assigned to the mic input to accommodate the extra volume control, in turn requiring the twin 6J5 phase inverter of the 25 to be replaced with a single 6SC7 to maintain a six-tube chassis. The two 6V6 power tubes and 5Y3 rectifier were basically unchanged.

A bass “Tone Expander” switch inserted/bypassed a low-frequency blocking capacitor in the negative feedback loop of the Instrument channel circuit; on the two-knob version, the switch was located on the control panel between the volume and tone controls, on the three-knob version, the mic input was moved into the row of instrument inputs and the Tone Expander was installed directly below, in its place.

The deeply textured “dark brown leatherette” covering of the 25 and early GA-30s was short-lived, replaced after about a year by a smooth, light brown material. This model would run until the change to the more powerful top-mounted chassis two-tone model in 1954.


From Vintage Guitar Magazine
Gibson Post WWII Amps
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