1959 Gibson GA8 Gibsonette: Rare Class A Design

capnjuan

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New to me Gibson GA8 Gibsonette:

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Not rare in total produced; but a rare Class A, 'student'-grade commercial amplifier. Without wearing anyone out, Class A amps are typically single-ended amps; like a Fender Champ, Silvertone 1471, early Kalamazoos, and others. Single preamp tube and single output tube which means that the output tube handles the positive and negative halves of the signal - it never cuts off. But the distinguishing feature of Class A operation is that when it's overloaded to the point of distortion, it generates even-order harmonics as opposed to odd-order from transistors and, to a lesser extent, cathode-biased, Class A/B push/pull amplifiers which is nearly every twin output, non-Fender amp made before 1965.

Schematic below; most unusual parallel 6V6 output sections; essentially each tube sees and amplifies the same signal; for reasons of physics that it would take Neils Bohr to explain, the output of parallel 6V6s is closer to 8 watts while in A/B-Push-pull, twin 6V6s are good for closer to 15 watts. The amp is widely recognized for its lush, warm sound. It needs some work but already has the sweet, crunchy distortion thing going very well!

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Sure there's enough work in that to keep you busy? Clean, simple design and great tone without making you deaf. :D
 

teleharmonium

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Those are cool. I've got a Noble (made by Ampeg) combo which also has a single ended amp of about that wattage into a single 12", it's an excellent configuration. (The Noble uses a single 5881 instead of parallel single ended dual 6V6s.) I love the look of the white Gibsons from that time too. If I didn't have a pack of 6V6 amps already, one of those would be on my shopping list.
 

capnjuan

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Default said:
Sure there's enough work in that to keep you busy? Clean, simple design and great tone without making you deaf. :D
I can always put in and take out the same resistors over and over to make it seem more challenging. :wink:
 

capnjuan

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teleharmonium said:
Those are cool. I've got a Noble (made by Ampeg) combo which also has a single ended amp of about that wattage into a single 12", it's an excellent configuration. (The Noble uses a single 5881 instead of parallel single ended dual 6V6s.) I love the look of the white Gibsons from that time too. If I didn't have a pack of 6V6 amps already, one of those would be on my shopping list.
Hi TH: I appreciated your and Kap'n's comments in the 'Voicing Amps' thread; got me headed towards Dave Hunter's book and his discussion of this amp. Not sure yet but this amp likely to get a fresh Weber 12" speaker; currently has 10" RI Jensen alnico. Fired it up today, baffleboard buzz but the tone is there! cj
 

matsickma

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Actually the difference between power output of a pair of 6V6 in parallel verses a pair in Push Pull A/B is simple to understand if you know that Power = Voltage multiplied by Current or Voltage Squared multiplied by Resistance (or the more widlely known formula Current Squared divided by Resistance). For the pair of 6V6 tubes you can add the power of each tube together because they are in parallel and contribute equally. For the A/B design the peak-to-peak Voltage is 2x. So when you square the peak Voltage (2) The Power goes up by (2 x 2 =4). So for a AB design using a pair of 6V6's you get 4X the power of a single 6V6 and 2x the power of a pair of 6V6 in class A.

The key thing to know is that when Ohms Law applies Power is porportional to the Square of the Voltage or Current.

M

Maybe this response is useless but it seemed easy to clear up.
 

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Curious about the impedance of the output transformer though.
Standard Champ is 7000 ohms for a single 6v6.
3500 ohms for 2 tubes in parallel?
 

capnjuan

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Thanks Mike; that would be the arithmetic. Looking to go the other way; less power, more tone - this gets me going in that direction! cj
 

capnjuan

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Default said:
Curious about the impedance of the output transformer though. Standard Champ is 7000 ohms for a single 6v6. 3500 ohms for 2 tubes in parallel?
Not sure and the batteries in my Ouija Board are dead ... :wink:
 

capnjuan

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BluesDan said:
Nice!.........another amp to add to my wish list!!!
Thanks Dan; there are a lot of these around; two weeks ago, there were three of them marching by on eBay. This one was the 3rd of that group and for 'vintage' (code for old and tatted) amps, the prices aren't too bad compared to Fender prices for the same age amp. If you go looking, Gibson also made the GA8T, the successor to this amp, which is much different. John
 

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Congrats on the new toy capn. I'm always eyeballin' those on ebay. Good to hear some firsthand experience. I just ordered the Dave Hunter book! Hope to start in on it soon.
 

capnjuan

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jp said:
Congrats on the new toy capn. I'm always eyeballin' those on ebay. Good to hear some firsthand experience. I just ordered the Dave Hunter book! Hope to start in on it soon.
Hi JP; I thought the Hunter book was really well-done. Except for a buzz in the baffle board, the amp just plain oozes rich, thick chimey tone; compared to the GA18, like being 'drenched'. J
 

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capnjuan said:
BluesDan said:
Nice!.........another amp to add to my wish list!!!
Thanks Dan; there are a lot of these around; two weeks ago, there were three of them marching by on eBay. This one was the 3rd of that group and for 'vintage' (code for old and tatted) amps, the prices aren't too bad compared to Fender prices for the same age amp. If you go looking, Gibson also made the GA8T, the successor to this amp, which is much different. John

Thanks Cap......just put a GA8T on watch, see where it goes pricewise. I've had quite a few "vintage" instruments & amps, familiar with the code, "I aint no purist & I loves me a great amp or geetar with character".........its all about the tone..........I dont sweat little rips and tears in the tolex.........a dent or two.............had me an amp or two like that. My favorite blondes always seemed to have a stain on the top in the shape of a Jack Daniels bottle bottom, can't imagine why..... :wink:

I'm all over the place looking at tube amps right now......got me some AAS goin' on, Fenders, Marshalls, Ampegs, Standels, Gibbys...............so many to choose from, head spinnin'.....
 

capnjuan

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BluesDan said:
Thanks Cap......just put a GA8T on watch, see where it goes pricewise. ...............so many to choose from, head spinnin'.....
Hi Dan: if this is the amp ... Gibson 8T schematic, pretty interesting layout. Uses twin push/pull 6BM8s for outputs. The 6BM8 is a cousin of the 6GW8 used by Guild in the T1s and T1 RVTs and has triode and pentode sections. One of the triode sections is used as the phase inverter and the other triode section is the tremolo oscillator. The 6BM8 is being supported by Electro Harmonix; I use them in a Gibson GA1RT and I think they put out good tone. Good choice and good luck!

Edit: If this is the amp you are watching on deBay Gibson GA8T eBay Auction Link then it's a 6BQ5 amp, not 6BM8s like I was yacking about above and this is the closest schematic I could find for an Epiphone EA35 Devon EA35 schematic which, according to these guys Link matches the layout of the GA8T.

If so, the eBay seller bungled the ID of the tubes but you get the idea. J
 

BluesDan

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capnjuan said:
Edit: If this is the amp you are watching on deBay Gibson GA8T eBay Auction Link then it's a 6BQ5 amp, not 6BM8s like I was yacking about above and this is the closest schematic I could find for an Epiphone EA35 Devon EA35 schematic which, according to these guys Link matches the layout of the GA8T.

If so, the eBay seller bungled the ID of the tubes but you get the idea. J

Hadn't seen that one Cap. This is the one on watch:
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... :IT&ih=005

While I got ya Cap, any reason I'm not getting the e-mail notifications for a post reply from the forum. Used to get 'em, not any more........something I said??? :shock: :wink: I do check the box for a reply request.
 

capnjuan

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Doesn't look too bad; a little weird than the interior metal surfaces look a little worse than the faceplate. Email - thing is busted for now; thread running, Donnie knows about it. cj
 

BluesDan

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capnjuan said:
Doesn't look too bad; a little weird than the interior metal surfaces look a little worse than the faceplate. Email - thing is busted for mow; thread running, Donnie knows about it. cj

10-4 on the email. Thought about the corrosion issues mentioned, may or may not sway me depending on extent of upward swing of bid price. Looking at an old Fender Champ locally this weekend, and maybe a friends old Standel, lots of irons in the fire, wife on standby with extinguisher as usual, we'll see.
 
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