Members amp_surgeon Posted January 29, 2012 Members Share Posted January 29, 2012 Originally Posted by H.R. Shove and Stuff This one?I'd love to see how it turns out! Nope, that's a later model. Before 1955 they were using pentodes in the pre-amp stages. 1955 was the first year they used triodes, and the design can only be described as bizarre. Dual 12AY7's in the pre-amp, with each of the four inputs (3 instrument and 1 mic) getting it's own triode pre-amp. One 12AX7 split as separate drivers for two 6V6's. A "tone expander" switch that bypasses a 3000pF cap in a degenerative feedback loop that, as far as I can tell, does absolutely nothing.Some other oddities:Usually, when you want to control the signal level going from one stage to the next you dump the signal into the top of a pot, and pick your signal for the next stage off the wiper - a traditional voltage divider that gives you a constant load on the output of the prior stage. This circuit actually dumps the output into the wiper, and picks the signal for the next stage off the top of the pot. This means the load on the preamp stage changes depending on where the volume is set.The three triode sections used for the three instrument inputs have their plates all tied together to one plate resistor. Weird stuff happens when you try to use more than one instrument input at the same time - when the current through one triode increases then the voltage drops on all of the plates.Also, the quiescent (no signal) plate voltage is lower on the instrument triodes than it is on the one mic triode because both sides use the same size plate resistor, but the instrument side has three tubes sucking current through the same resistor. Because of this, there is far more headroom through the mic input than there is through any of the instrument inputs. I've read comments from two other people who own this particular year and model that confirm that this is the case with their amps, as well, so I know mine isn't unique.I drafted a schematic of this thing years ago, but I couldn't believe what I was seeing. I was sure somebody must have severely modded it until I found pics on the internet of another 1955 model that appeared to be identical inside, and comments from other owners who described the same anomalies I was hearing. I sent an email to Gibson asking if they had a schematic for this model. I sent them my schematic for comparison. They said they no longer had any records whatever from that year, but thanked me for providing a schematic because they'd been trying to find one for years. Anyway, it seems that 1955 was the only year they used this particular circuit design, and only about 365 were made that year. Link to comment Share on other sites More sharing options...
Members erksin Posted January 29, 2012 Members Share Posted January 29, 2012 So do you use the Mic input or one of the guitar inputs? That thing is beautiful - my favorite era of Gibson's cab cosmetics. Link to comment Share on other sites More sharing options...
Members amp_surgeon Posted January 29, 2012 Members Share Posted January 29, 2012 Originally Posted by erksin So do you use the Mic input or one of the guitar inputs? That thing is beautiful - my favorite era of Gibson's cab cosmetics. Always the mic input. Nobody who owns one of these uses the instrument inputs, as far as I've been able to tell. The outputs of the preamps go to the same place on the tone stack, so there's no difference other than the lack of headroom on the instrument inputs. Link to comment Share on other sites More sharing options...
Members erksin Posted January 29, 2012 Members Share Posted January 29, 2012 Yeah its funny how the non-guitar inputs usually sound a lot better on a lot of those old amps. I like the Accordion and Mic inputs way better than the Guitar input on my old Reverberocket. I'd love to try that thing sometime! Link to comment Share on other sites More sharing options...
Members amp_surgeon Posted January 29, 2012 Members Share Posted January 29, 2012 Originally Posted by erksin Yeah its funny how the non-guitar inputs usually sound a lot better on a lot of those old amps. I like the Accordion and Mic inputs way better than the Guitar input on my old Reverberocket.I'd love to try that thing sometime! Funny indeed! The old GA-30's got a reputation for being "harp amps" because they were apparently popular with harmonica players. Could be that guitar players plugged into the instrument input, didn't like what they heard, and moved on to something else. If you're ever in the bay area give a holler. Link to comment Share on other sites More sharing options...
Members erksin Posted January 29, 2012 Members Share Posted January 29, 2012 I come down to Sunnyvale/San Jose area a few times a year - I'll definitely take you up on that! Link to comment Share on other sites More sharing options...
Members lefort_1 Posted January 29, 2012 Members Share Posted January 29, 2012 Huh....late to this discussion/amp-disection, but I wonder if that common-late-resistor design was an effort to keep the amp from "distorting" (in the 50's sense) when multiple players plugged into the "only amp the band owned"? Link to comment Share on other sites More sharing options...
Members erksin Posted January 29, 2012 Members Share Posted January 29, 2012 Originally Posted by lefort_1 Huh....late to this discussion/amp-disection, but I wonder if that common-late-resistor design was an effort to keep the amp from "distorting" (in the 50's sense) when multiple players plugged into the "only amp the band owned"? I bet that factored into their thinking on some level - PAs were awful back then. Link to comment Share on other sites More sharing options...
Members lefort_1 Posted January 29, 2012 Members Share Posted January 29, 2012 It does limit the fun of bridging the inputs on that design... I love doing that on my old 1482 Silvertone... brings the rawk...and the sag. Link to comment Share on other sites More sharing options...
Members amp_surgeon Posted January 29, 2012 Members Share Posted January 29, 2012 Originally Posted by lefort_1 Huh....late to this discussion/amp-disection, but I wonder if that common-late-resistor design was an effort to keep the amp from "distorting" (in the 50's sense) when multiple players plugged into the "only amp the band owned"? That would make sense as a design objective, but if that's what the design engineer was trying to accomplish then he missed the entire broad side of the barn. The current going through any of those triodes has a direct effect on the plate voltage through the other two triodes. If one guy plays a chord then it sounds ok - weaker than the mic input because of the lower quiescent plate voltage, but ok. If a second guy plays a chord then the first guy's volume takes a dive because of the drop in plate voltage.On the upside, one of the 12AY7's is used for two of the instrument inputs, and the other is shared between the third instrument input and the mic input. If the mic pre goes out on stage then there's a built-in spare you can swap with. Link to comment Share on other sites More sharing options...
Members lefort_1 Posted January 29, 2012 Members Share Posted January 29, 2012 Originally Posted by amp_surgeon That would make sense as a design objective, but if that's what the design engineer was trying to accomplish then he missed the entire broad side of the barn. The current going through any of those triodes has a direct effect on the plate voltage through the other two triodes. If one guy plays a chord then it sounds ok - weaker than the mic input because of the lower quiescent plate voltage, but ok. If a second guy plays a chord then the first guy's volume takes a dive because of the drop in plate voltage. That sounds exactly like 50's Logic at work: "If two are playing chords, then left the whole thing drop in volume to keep our 'clean' amp from (shudder) distorting." Link to comment Share on other sites More sharing options...
Members echodeluxe Posted January 29, 2012 Author Members Share Posted January 29, 2012 Mmmmmmmmm, the 50's....... Link to comment Share on other sites More sharing options...
Members butterknucket Posted January 29, 2012 Members Share Posted January 29, 2012 I know quite a few people who would take a 50's Gibson amp over a Tweed Fender anyday. Link to comment Share on other sites More sharing options...
Members lefort_1 Posted January 29, 2012 Members Share Posted January 29, 2012 I just got back in from feeding the sheep. While working in the barn today, I had this interesting 50's-design-theology/monologue/design-review running thru my brain: Set in 1954: The musical deacons of the days are Perry Como, Frank Sinatra and that kid, Dean Martin. The Approved Trend (as seen by the Company) is Las Vegas Cool. The Nod-and-a-Wink Undercover Trends are Betty Paige, Martini Burlesque and The Wife Down The Block That Gets Drunk and Loose at the Neighborhood Picnic. Junior Barnard is as dangerous as music can be allowed to get, and Danny Cedrone had just stepped over the line with tha take-off on Rock Around The Clock. The UnApproveable Notions include: Negro Music, yelling at the microphone and that Elvis Whoever. Scene: Three white-shirt/black-tie, thick-eye-glassed engineers sit around a war-surplus card-table in the back room at Gibson. They are discussing the design of their new amp, the GA-30, with a guy from Marketing... Mktg: so, what's so great about this new amp of ours? Eng1: Well, to start it is as pure as Doris Day's butt. Even when it's all the way up, it stays clean. Eng2: And it has 4 inputs. FOUR. Finally, everyone in the band can plug into an amp. Mktg: Wait a minute.... WAIT A MINUTE guys, we WANT to be able to sell LOTS of these amps, right? Why allow everyone to plug into ONE? Eng2: Well, we need something to sell to those kids....the ones who are doing all that hillBillHaley stuff. Mktg: no No NO! Guys, I don't care what McCarty thinks, those kids will never by single amp. They could put together all their nickles and dimes from delivering newspapers, mowing lawns and jerking the soda fountain...they still don't have the $38 bucks it takes to buy one of our amplifiers. SCREW THOSE KIDS, ignore them. Eng1: Well, Ted told us to make an attempt to tap some new markets. And they won't buy a PA (all laugh), so we've got to give them something. Eng2: Yeah, I don't like that new noisey stuff any better than you....the stuff that Carl Perkins is playing for that Presley kid.... it damn near belongs in a gospel tent, with all that yelling and yodeling and carrying on. They don't cotton to that stuff down at Stone Mountain, I tell you what... Mktg: Settle down, Deewayne... Your white hat is showing. And it's not like anyone is marching on the state capitol. It IS just music afterall...it's not like it's going to change the world or anything. ....and I guess a sale IS a sale, so if McCarty is going to make us Market to these kids, well, is there any way we could AT LEAST REIGN THEM IN A BIT??? (all scratch their crewcut heads) Engr3: Well, remember when Crosseyed Ethel down in manufacturing wired up the dual BR-1 prototype and gave them all that common plate resistor. Well THAT sure knocked down the collective volume a load. (all rub their chins, then begin to grin....) Link to comment Share on other sites More sharing options...
Members erksin Posted January 29, 2012 Members Share Posted January 29, 2012 lol I don't think marketing to kids was even a consideration until Fender started doing it in the early '60s, but hells yes to the rest. It's amazing that the guy most associated with Gibson up until Les Paul came on board in '52 - Charlie Christian - doesn't even get mentioned by them. Ever. Link to comment Share on other sites More sharing options...
Members erksin Posted January 29, 2012 Members Share Posted January 29, 2012 Originally Posted by butterknucket I know quite a few people who would take a 50's Gibson amp over a Tweed Fender anyday. Ry Cooder knows tone and I value his opinion. Link to comment Share on other sites More sharing options...
Members echodeluxe Posted January 29, 2012 Author Members Share Posted January 29, 2012 Wait. Erskin in the wattson shop?! No pants ever made could contain my excitement. Link to comment Share on other sites More sharing options...
Members lefort_1 Posted January 29, 2012 Members Share Posted January 29, 2012 Originally Posted by erksin It's amazing that the guy most associated with Gibson up until Les Paul came on board in '52 - Charlie Christian - doesn't even get mentioned by them. Ever. Hi-Fi ! It's where it's at !No more crystal sets, and fire up the cigarettes,In the Rocket Age we don't tolerate no rage,Smooth bourbon on my ears and tongue, can see my girl so purt and young,In the silk dress we stole from old TokyoCool Saxaphone drones, our beatnik tones, musical heathens we are, not a Christian guitarand Perry croons it down low. Link to comment Share on other sites More sharing options...
Members erksin Posted February 1, 2012 Members Share Posted February 1, 2012 OMFG Nic just bought'd a '51 GA-30 and it sounds incredible. That Mic input is bonariffic - the other three completely suck balls. But damn - that warm crunch with the top end clarity is really fantastic through the Mic channel. I'm surprised at how tight the low end is on this thing. Sounds big - nice freq spread, lots of string detail within the chords. I was shocked at how much gain and punch it had. Only complaint is lack of volume at 12 watts - with an amp like this you just want MORE. Easily one of the best 'tweeds' I've ever played. Link to comment Share on other sites More sharing options...
Members echodeluxe Posted February 1, 2012 Author Members Share Posted February 1, 2012 mother {censored}ing PICS man. you know the score. how much did he pay? Link to comment Share on other sites More sharing options...
Members erksin Posted February 1, 2012 Members Share Posted February 1, 2012 Originally Posted by echodeluxe mother fucking PICS man. you know the score.how much did he pay? He'll have to post them, and it's not for me to say but I thought it was a bit of a steal given the condition. Pretty damn clean amp.It's a pre-'55 so it's the old '40s-style looking cab, not a two-tone like yours. Link to comment Share on other sites More sharing options...
Members echodeluxe Posted February 1, 2012 Author Members Share Posted February 1, 2012 yeah those earlier models looked real classy as well. Link to comment Share on other sites More sharing options...
Members DRGUN Posted February 1, 2012 Members Share Posted February 1, 2012 Originally Posted by erksin Nobody had onboard reverb in the '50s - Gibson debuted reverb amps in the early '60s. I think the one you're referring to is the GA-30RVT Invader.I love amps with different sized speakers in their cabs - really want one with a 10 and 12 in it someday. For now I run one combo with a 12" and one with a 10" in stereo and the freq spread sounds pretty fricking massive. what amps are you using in this setup? I use a super champ xd (10" speaker) and a blues junior (12" speaker) and its super rad.also I wanted your opinion on Nouriel Roubini's recent prediction about the ecomony, but I saw in your sig you didnt want business related communication. Link to comment Share on other sites More sharing options...
Members damacy Posted February 1, 2012 Members Share Posted February 1, 2012 I'll say it again. Originally Posted by damacy Please build us a "GA-30/whatever in a box" thing - an overdrive pedal of sorts. Link to comment Share on other sites More sharing options...
Members echodeluxe Posted February 1, 2012 Author Members Share Posted February 1, 2012 yeah i saw that, but i am not a fan of "amp in a box" pedals. Link to comment Share on other sites More sharing options...
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