1959 Gibson GA8 Gibsonette: Rare Class A Design

[capnjuan]

Hi Dan: This is (yet another) Gibson GA8T - guessing - the last of the 'Gibsonette' series. I confused this model for the one you were/are spying on. This is the 2X6BM8 push/pull model mentioned above; single 12AX7 preamp, and 5Y3 rectifier. On eBay, auction link Here Has foot-switched tremolo operated by the 'front' half of one of the 6BM8s, the other 'front' functioning as the phase inverter/driver.

Not the best pic but in very original condition - the tan thing (power supply resistor) on the far right just above the rectifier in same condition as mine; ready to crumble. The 'brown foot pedal' mentioned in the auction text is actually a solid mahogany block drilled out for the switch; the 'web-lore' is that these wood switch blocks were the 'waste' from the cutting the cutaway out of LP bodies and they refinish to perfection.

 

[BluesDan]

Thanks for link Cap. I'm letting the other go, it has a few minutes left on auction as I write this, price in range but re-looking at pics and write-up it just doesn't "feel" right. Might be crazy but I have learned to trust instincts. Now watching Gibsonette, better feel on that one.
Looked at a Fender Champ locally today, pathetic shape, high asking price, waste of my gas money to look. Friend re-thinking selling Standel amp after I plugged my x700 into it for about an hour. Had fun though, many drinks, many laughs, good fun, well worth gas money........

 

[capnjuan]

Fresh speaker for the GA8; Bought off eBay a Jensen Alnico 5 dating to 1953 branded Bell & Howell, 12", 16 ohms.

Metal Tag:

Original condition of terminals: those are Bulgin connectors left and right; air and water-tight harware. Guild used Bulgin connectors on its mid-60s amps to connect the speakers to the chassis.

In this pic, a terminal strip has been bolted to the frame; to the left and right, you can see the 'solder pots'; where short wires will be soldered and connected to the terminal strips.

Although the bell interferes with the inverted 6V6 tubes on the chassis, a new Baltic birch baffle will be cut and the speaker center lowered to allow clearance. Gibson baffles are 1/4" plywood with edge battens; about as cheesey as possible; the amp was always going to get a new baffle and grill cloth. Finally, the impedance mismatch will be corrected with a 16 ohm / 15 watt non-inductive wire-wound resistor across the + and - speaker terminals creating two parallel 16 ohm loads for a net 8 ohm load on the output transformer.

Bell & Howell bought these speakers from Jensen who was required to meet the JAN - Joint Army Navy - standards. Today, there are still NOS JAN 6V6 and other vacuum tubes that were produced to meet military requirements. B&H, best known for its projectors, sold projectors and other equipment to the Defense Department. To acquire identical products from competing sources, standardized specifications were developed; the JAN standards.

B&H-badged Jensen speakers showed up in other B&H products. Link to a vintage audio gear site Here with discussion of this exact speaker. As of this post, there's another identical, B&H-badged speaker on eBay Here and an another Ampro-badged Alnico 5 speaker on eBay Here.

cj

 

[capnjuan]

GA8 Progress Pics:

On the left is the multi-section cap that was in the amp; don't believe it's original - would expect to have seen a pair of 'twin-cigar' style capacitors in cardboard wrappers. Can also see typical little cheesey (spliced) Gibson speaker wires coming out of output transformer (new shoes too). On the right replacement individual caps on wiring strips for ease of installation and maintenance (one more for the preamp yet to go).

The caps were always in the deal; replacing the rectifier socket wasn't. The power and output transformers are stuck facing forward in the cabinet in such a way that there's no chance of any air circulating. Over years and running at high volume and without heat dissipation, the potting in the power transformer can/did melt out. Add some DC leakage to the chassis and you get corroded mess. Pic on left shows corroded rectifier socket and rusted chassis. Considerable scraping and cleaning with solvents to get the waxy potting out of there. Sanding/Dremel/coat of phosphoric acid to abate further rot and new rectifier socket on the right:

Still calling it fun ... :wink: cj

 

[capnjuan]

More progress pics; chassis/electronics all done:

Control Panel; except for the lettering, in good condition - free from rust and corrosion:

Tube deck view: can see the pitting around the lip of the new rectifier socket left over from the damage done while the old one was in place:

Transformer deck view: new Weber output transformer and wiring strip to tie transformer output and speaker drops together. The coiled wire is the 4-ohm tap. Oversight on my part not to have put in wiring strip with one more lug; would've allowed easier switch to 4 ohm speaker or parallel 8 ohm / net 4 ohm load:

Chassis view: in the upper pic, the 3 yellow arrows are the power supply resistors in 'flying' formation. Instead of terminal boards or strips, Gibson perferred to mount resistors wherever they could including 'flying' over the tube sockets. All the coily nonsense in the upper pic to the right of the brown multi-section capacitor is replaced by the red wiring in the bottom pic.

'Cooking the Capacitors'; whether the amp has a tube or solid state rectifier, it's good practice to bring the power up slowly on new capacitors. In pic below, the blue box on the left is a Variac - 'variable AC'. It's an isolation or autotransformer with 117 volt wall voltage in and the ability to vary the output voltage from 0 to 120 volts. Allows the rectifier output to be slowly increased over the span of several hours. Note the dummy output transformer load in the lower right; an 8ohm/25W resistor. Never good practice to have the amp on with no speaker or load of some kind on the output transformer.

Left to do: the new speaker baffleboard. I have the 9-ply birch plywood on hand, new-to-amp speaker shown above in this thread. Once caps done, can hold speaker in the opening and gauge where to center it on the baffleboard so it will miss the inverted tubes hanging from the chassis.

cj

 

[Default]

Nice work Cap!
(as usual)

 

[capnjuan]

Thanks Steve; if I had one more do-over, I'd probably have put a longer terminal strip to catch the capacitor grounds in addition to tie point shown. That tie point collects all the preamp, control, and capacitor grounds and ties them to a 'star' ground point under a transformer bolt/nut ... sigh ... next time ....

 

[capnjuan]

Progress pics:

New 9-ply birch baffleboard, new grillcloth, tolex scrubbed with TSP, and coated with a light furniture wax - not quite as yellow as pic suggests. I have the original logo amp badge, will go on after I get happy with speaker.

Side view:

Top view:

Back view: close call with oversized speaker magnet. Just above and to the left and right of the magnet are the 6V6s. They are closer than I wanted but the cabinet geometry, the fact that the baffleboard lays back about 20 degrees, and the available thickness of the bottom batten meant that this configuration is the best I was going to be able to do.

Out of concern for tube/speaker proximity and cone condition, am going to a Weber 12A125-O without a bell and with a smaller magnet. Doing so will take tube/speaker proximity out of the equation and allow me to get the Bell & Howell / Jensen P12N re-coned for use in another project. Have been consulting w/ default over slightly elevated AC hum from the heater circuit. As ever, there are several ways to correct the problem including a hum balance pot which will go in next time it's on the bench.

This was a very successful Gibson model and intended to offer the potential Fender Champ buyer more output - parallel 6V6s v. single-ended 6V6 - and a larger speaker; 10" v. 8". Like the Champ, it was sold as a student / practice amp. This one produces the classic tube tone; round, warm, chimey, compressed and with rich harmonic distortion when pushed. Plenty of volume, bark, and with a 12" speaker, terrific frequency response.

 

[guildzilla]

Looks terrific, Obi Juan. Thanks for the step-by-step restoration.

Q's. Why are you concerned about the bell being too close to the tubes? How much space is enough?

 

[Default]

Vibration, for one. I remember reading on another forum how the speakers were so close to the tubes on one particular Peavey model, that tube life was atrocious.

 

[capnjuan]

Q's. Why are you concerned about the bell being too close to the tubes? How much space is enough?

Thanks 'Zilla; default is right - it's the vibration, even with the bell off I wound up with +/- 1/8th inch gap between the magnet and one of the 6V6s when 1/2" - 3/4" to me would be the acceptable minimum. The amp was originally sold with a 10" speaker which is proportionally shallower getting around any tube clearance issues. John

 

[teleharmonium]

I just lucked into a really clean one of these at GC, with the cover and info sheet with schematic. It's still all original including the power cord, so it might need some caps, but it's working fine for now and isn't especially noisy. It goes into the (long) queue of amps to go to my repair guy to get a checkup and new power cord, but anyway, I'm pretty happy with the sound of it.

I think it has what I am starting to think of as a very desirable and hard to find attribute, which is that the speaker breakup seems to come in right around the same volume where the power amp really gets into serious crunch, so that with the power amp distortion and the speaker distortion kind of lining up in that order, you get a really wide range of sounds controllable with your playing style without touching any controls. I mean, any good tube amp has that to a degree, but it's even more so than usual on this amp. I had the volume up about about 2/3 and was using the neck mickey mouse pickup on a T100; single notes played lightly had a nice, just slightly hairy/chimey jazz sound; more than one note at a time played lightly produced a good vintage mild distortion and is perceived as brighter than the single note sound; multiple notes played with some force caused heavy, snotty crunch with a tortured decay and a little bit of guitar body feedback on some notes, all while just sitting there playing 6 feet away from the amp without even working the guitar volume knob. Fun !

Tonight I'll probably put in some NOS brown base 6V6s and an old Tung Sol or Raytheon or Sylvania 12AX7. It's the white version with a 10" speaker like yours, cap'n. It's almost too cute, next to my blonde T100. Overall it reminds me a lot of the Noble I was talking about earlier, but with no trem, plus the differences you'd expect from a 10" rather than a 12" .

So to sum up - yeah !

 

[capnjuan]

Hi TH: your ear and style many more sophisticated than mine but, using fresh Groove Tubes, mine generates the sweetest of tones and when called on, growls nicely. I have a good-running copy of one of its cousins, a GA18 that, like many PP/6V6 amps, has some of Class A attributes ... but not all of them.

I have a few more kinks to work out in the GA8; I terminated all the filter cap and preamp grounds in a 'star' grounding config; turns out this works ok in some amps and not others; doing some hum-fighting and I think the preamp grounds have to be re-done. Heater winding is center-tapped but I see the twin 100 ohm resistors as more of safety feature than as a noise fix. I also have a 100 ohm pot that I will put in between the pilot light and ground; see if that helps. Gibson doesn't help with several variants of heater grounding shown in as many different schematics.

If I had it to do over again, I'd have replaced the baffle but left the speaker 10"; mostly due to the headaches the tube/bell interference is causing v. maybe not enough appreciable difference between the 10" and 12" to justify the trouble. I have a Weber 12A125 in the house but am now disappointed find that, with the bell off, the magnet on it is as big as the magnet on the mid-50s Bell & Howell-branded Jensen alnico 5 that's been giving me the interference problems. Should have realized that what Weber means by reproducing vintage Jensen speakers is actually reproducing them ... dimension-by-dimension.

Despite the crabbing, I couldn't be happier with the sound of the amp; rich, harmonic, a tad dark, warm, and chimey. Were I a better player, I might be more interested in a cleaner sound at louder volumes ... but I get all the sound I want as you say ... 6' from the amp. John

 

[teleharmonium]

Hey capnjuan

The tubes in the amp now work fine, but the 6V6s are two different makes (Syl and GE), and I haven't tested them yet so they may diverge in terms of output as well. I don't really know what the ramifications of that are in a parallel single ended circuit, but I figure I'll try a closely matched pair - and I know I have a real nice pair of NOS Hytrons that fit the bill, so that will be on the menu for tonight. I just like to swap tubes around and see if I can find a particularly sweet combination. I don't work on cars, so this tinkering fulfills my needs for man activities. (Mostly.) I've been a bargain hunter for vintage tubes, I have a fair number that I got for about the same $ that new production tubes cost, and I figure I'm probably 1/2 through my life already, may as well use them. I might end up back on the original pair, but it's pretty amazing what you can do sometimes just with a variety of good tubes and a lot of patience. As for the 12AX7, it's a GE, nothing wrong with it but they're not my favorite. Based on how one sounds in my similar Noble amp, I'm guessing I will end up with a used vintage Raytheon tube in the GA8 by the end of the night.

I hear you about the clean volume, this amp would not work at all for me when I play out, but I figure any amp that can do two great sounds is a winner, and when they come in a light, cute, affordable package (I got this amp out the door for just under what a new Blues Junior goes for), it's hard to resist. This sucker will work out fine if I'm going for a weekend trip and should be great for recording.

 

[capnjuan]

Hi TH: not sure I understand either how much matching counts but my guess would be not so much ... but it's just a guess. The new Weber 12A125 went in today; much better clearance from the tubes and standard can't-beat-it Weber tone, depth, and clarity. Hope your tube-rolling turns out well. CJ

 

[capnjuan]

Tube / Speaker bell conflict

The Bell & Howell-badged Jensen P12N was just too big; replaced with a Weber 12A125, as close as can be had new to a vintage P12R:

Fits well enough to end concerns over vibration and microphonics:

Hum-busting

When the speaker was switched out (and sonic mud from B&H/Jensen removed), there was a residual hum ... the demon of tube ampdom. Some of it coming from the 6.3V heater circuit fixed with an additional 100 ohm resistor from one side to ground and some from the preamp ... pic below of the preamp showing tedious bending/shaping of parts to fit in/around each other and demonstrating Gibson's desire to spare every expense in getting its products to market including 'flying leads' - passive parts jumping point to point .... :

... Instead of like Fender products from the same time frame using tag board construction ('63 Princeton below):

default bought my GA1RT but, just before it was to ship, several vintage Gibson knobs showed up on eBay and shipping stalled until the knobs were in the house, cleaned up, and new paint-fill in the indicator slot - Dakaware ... I guess made of cellulose before the widespread substitution of plastic. Matching authentic Gibson dakaware knobs now on both the GA8 and his GA1 making us matching knob dudes :shock:

It's now a very fine amp; quiet at idle, growly and punchy, and the degree of distortion very pick-sensitive. A Champ/Gibson GA5 on steroids; originally w/ a 10", not 8", speaker and the extra/parallel 6V6 to give it more marketing and electronic power than its Fender student-targeted rivals. This was one of Gibson's most successful products ... and for all my carping about build quality, good enough to last 50 years and enough amp there to justify refurbing it for another 50 years' service.

 

[Default]

Hey, leave my knobs out of this! I take it the hum reduction process went well?

I <3 my new-to-me GA1RT. One fine amp, sir!

 

[capnjuan]

Hey, leave my knobs out of this! I take it the hum reduction process went well? I <3 my new-to-me GA1RT. One fine amp, sir!

:mrgreen: Yes; have satisified my obsessive/compulsive thing for a few months .... a little zzz up around 7 with nothing plugged in; fine for now until a get a clearer idea of another move. As projects go, the GA1 could not have turned out better; glad it has a good home!

 

[Walter Broes]

Very interesting thread that I only just discovered for some reason. You do great work Cap'n.

Very interesting about the Bell and Howell speaker too - a P12N, no less!!
I'm still not sure whether I really like Weber speakers - I have those C10Q's that I found too loud and a little too "jbl-ish", "hi-fi".

The "new" amp I have, the Headstrong 5E7, came to me with two Weber P10Q's (10A125's, the 20 watt version) and one Eminence/Fender blue alnico speaker. (15watts, kind of like a P10R with a 1.25" voice coil), and again, I have the impression they're very efficient (loud!) speakers, and there's an edge to them I can only describe as "hi-fi".
But then I'm weird with speakers I guess - In general, I tend to favor cheap ceramics for stage, and I guess my ears and hands got too used to them.

 

[capnjuan]

Thank you Walter; speakers ... they can drive all of us nuts: "Gee what would this sound like with a different speaker in it?" Giving in to the curiosity can result in a pile of unwanted speakers! In low power amps, particularly 6V6 amps, my brain tells me there is some kind of affinity between 6V6s and alnico speakers. It may have something to do with the fact that 6V6 amps started to give way to louder cleaner 7591/6L6 amps at about the same time that alnico speakers vanished...who knows. Regards, John