Tweed Gibson GA30RV

capnjuan

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I bought this amp last summer ... just now getting it on the bench; 1960-1962 Gibson GA30RV 'Invader'; two channels, 3X6EU7 including reverb drive and recovery and 12AU7 phase inverter/driver, 2X6V6, and 5Y3 rectifier, 12" Jensen P12R and 8" C8S. On paper, with its 6V6s and cathode-bias, the amp is similar to a cross between a Fender 6V6 Super 5E4 with fixed-bias and a Fender 6L6 5E5A Pro w/ cathode bias but without Fender's better-grade transformers and cleaner phase inverter.

In the darker/later Gibson tweed:

GA30cluster.jpg



Predecessor model:

GA30early.jpg



Successor model; fitted w/ Jensen P10R and C12R speakers:

GA30late.jpg



Chassis view; re-capped and new power cord within the last few years but didn't bother with the preamp cathode bypass caps:

chassisfullb.jpg



Bonus! Speakers were recently reconed. Magnet on P12R measures 2.75" sq., period-correct if not necessarily original. The chatroom buzz on these is that the 8" speaker with it's smaller voice coil, breaks up before 12" speaker. Since it isn't reverb-specific like a Guild, chances are that I'd just leave it disconnected before I'd replace it:

speakersall.jpg


Reverb can w/ hold-down on left and split lower baffle batten in left-center. Cabinet also used for models like the relatively uncommon GA40LP/15". I'll do something like this to the GA20T to allow the speaker and bell to sit lower in the cabinet. Note footswitch; typical block of mahogany ... this one used as doggie chew-toy:

splitbatten.jpg




The amp presented with a dull, constipated tone, a 'presence/hum' thing in the 12" speaker ... as though it were going to jump out of the cabinet, and channel 2 has noticeably less volume than channel 1. Seller said that he switched wiring from former daisy-chain config back to design. It's also possible that since the reverb lives on channel 2, it would never have the same volume. I have the amp apart to confirm the un-do of the 'daisy-chain' - potential source of volume imbalance - and general maintenance.

In another forum, a poster suggested that the tone problem may have to do with the possibility of the voice coils having been replaced along with the cones but not put back in original polarity; that is, if/when speakers went back in cabinet and the vcs were reversed, then hooking it up as it had been would result in either/both speakers being out of phase. And, they look new enough that they may not have much play time on them to loosen up the cones. With the speakers out, I can confirm + and -, mark them, and they will be right on the go-back-in.

The several National (Yugoslavian) 6EU7s will be replaced by NOS GEs, a NOS Brimar 12AU7 for the phase inverter, a pair of NOS TungSol 7408 6V6-equivalents, and a mil-spec 6087/5Y3 for the rectifier. If a premium tube set, circuit checks, and confirmed speaker polarity don't do it, then I'll probably replace the output transformer. In one sense, I don't care what it sounds like ... so long as I think it sounds as good as it can.
 

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That's a nice project you have there capn. I'm looking forward to seeing your progress.
 

capnjuan

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Thanks JP; it's been sitting in part because the Univox is stuck on the bench (on a contingent basis, I have filed for a permit from the Florida Department of Environmental Protection to throw it in the canal) and in part because of the reasons stated; I was disappointed with it. Hitching up my LTG BVDs, gonna see if I can get it to rock and roll! CJ
 

capnjuan

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On another BB and referring to this GA30RV said:
Okay ... mighta been born that way; stuffy, veiled sound. Re-coned original Jensens, recent filter caps, couple of National/Yugo 6EU7s. Amp torn down; besides new tube set, anything I should be looking for?
Rob of Buffalo Amps said:
Just for the hell of it, since you wrote that this amp was torn down, verify that the grids on V3 [the connections where the music goes in ..CJ] aren't swapped. I've seen this done accidently and the amp sounded way under powered.
Amazing .... just amazing; mine are reversed. :shock: 8)

On the left in the pic below; that's the base of Tube V3; there's a green wire identied as Pin 5 terminated on pin #5. There's another wire, identified as Pin 8, terminated on pin #8. The pin #5 wire is coming from the reverb depth control on the chassis control panel. The pin #8 wire is coming from the circuit board; the junction of a 100K resistor and a small cap.

V3D.jpg


In the schematic on the right, the blue box up top is the reverb control; according to the schematic, the feed from the reverb control appears on Pin #8. At the bottom of the schematic, the feed to Pin 5 is shown coming from the reverb can (in red) and a 100K resistor and small cap; this is the reverse of the condition of my amp; as wired, Pin 5 is fed from the reverb control and Pin 8 is fed from the reverb can.

Default or Hatted Frau probably would have spotted this right away; chances are I would have given up. The amp looks like it was played at top volume to overcome this 45 year-old factory wiring error; waxy potting melted out of the power transformer, the choke, output, and reverb transformers the same way; the chocolate drop caps in the circuit board oozing gooey tonecheese ...

This is Rob's webpage; http://www.buffaloamps.com/ cool stuff; guy really loves his Gibsons and he saved me hours of torment. CJ
 

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Wow, that's an impressive diagnosis. It looks like you've found the zen master Gibson amp guru. Now if only he'd clone the GA-79RVT. . .
 

capnjuan

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Hi JP; downright startling. He may yet get to the GA79. On his Buffalo Amps MySpace Blog, he says he's going to produce a GA50. He lurks on Weber's Amp BB and handles most of the Gibson-specific 'help' posts. Otherwise it's pretty much Fenderistas there but a very bright bunch including at least two people producing boutique gear under their own names.

I posted the amp's problems and had expected the usual sort of disinterested suggestions: 'change the cathode caps' or 'what's the voltage on the V1 plate' ... my intention was to run down any ideas that people were kind enough to offer. It isn't just that maybe the most knowledgeable Gibson guy available nailed it through cyberspace, there is almost no chance that I ever would have found it on my own.

The Bjorn ??'s Gibson Site reports that the model was only in production in 1961 ... I guess sales were poor ... at least in part because everyone who bought one would have wound up crabbing about its output. It's appalling to think that whoever converted the schematic to shop drawings made one boo-boo that was repeated 700+ times, or maybe worse, one person kept making the same mistake over and over and over ... dooming the model.

It isn't too hard to imagine Gibson engineers sitting around bickering about the amp's performance but, instead of looking at the amps, they were looking at the schematic: "Waddya talking about, there's no reason why that amp won't work?" Who knows ... but it saved me several weeks of torment and ( I hope) keeps me from dumping it.
 

capnjuan

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Sometimes you get the bear, sometimes he gets you .... On the left, the magenta line indicates the signal supply to the reverb section of the amp taken from the junction of the .02 coupling cap and the input of the channel 2 volume control to the reverb 'depth' pot and then into V2B. Fine ... except that's not how the amp's wired. On the right, the V2B signal in is taken from the grid/input of V2A and the reverb depth control is on the output side of the reverb can as shown on the right.
V2V3cend.jpg


I switched the V2 pin 5 and 8 connections as discussed above and got a thin, thready signal with a lot of distortion, reverb worked. I replaced all the filter, coupling, and bypass caps and changed the pin 5 and 8 connections back; after all, the amp worked that way originally, just not very well. Anyway, with new caps and wired like it was as-delivered, I get monster hum in both channels; plenty of signal just lots of hum too.

The revisions could have been to fix a crappy design, to simplify manufacturing or somebody else fiddled with it. Assuming it's factory wiring, I have no way of knowing whether mine pre- or post-dates the revisions. I have about 8 hours in this so far and no real idea of exactly what's wrong with it; I could spend another 8 and still not figure it out or possibly make it worse ... so while I hate to act like a Martin owner, it's going to the local amp tech. :( :evil: :wink: CJ
 

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capnjuan said:
. . . so while I hate to act like a Martin owner, it's going to the local amp tech. :( :evil: :wink: CJ
Sorry to hear it capn. I don't like that feeling of "unfinished bizness" either. Think the tech will fare better than you?
 

capnjuan

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jp said:
Think the tech will fare better than you?
He'll do fine. Depending on what's going on, he may just give it back to me and say; 'do this, this, and that'.
 

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Some days you eat the bear and some days the bear eats you.


Lets hope there's no unplanned romance before dinner with this one.
 

capnjuan

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Yes ... whatever ... It's gotten to the point where no matter how long I stare at it, I won't find my mistake as in the the proverb; 'None are so blind as those who can't see....' Verse 9, Chapter 4, Book of Esteban
 

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CapJuan,
I came across your post looking for info on a GA-30. I inherited an invader GA-30 (1961 according to pots code) I want to build the cab as I only rec'd the amp . Would you be so kind as to give me the cab measurements HxWxD so I can build a replacement cab. I luckily have 2 vintage Jensens sitting around waitng for a new home...
Thanks
 

capnjuan

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Hi Japes and welcome to LTG! I will post the cabinet dimensions ... when I get the amp back from the local shop; it might be another 10 days. I got to working on it and, as I agonized above, the amp doesn't match the public domain schematic, had issues when I got it, and I may have screwed it up too. As I recall my algebra, I think this is what's called solving for three unknowns. Anyway, the amp isn't here but I'd be happy to post the dimensions when it gets back.

I also have another Gibson tweed cabinet; smaller than the tweed GA30 cabinet; the chassis will fit but it would mean a single 12" speaker and it is ... well ... cosmetically-challenged. I don't want to get in the way of you and your cabinet project but ... let me know if you have any interest. Dimensions when available; welcome! CJ
 

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Thanks CJ,
Those dimensions will come in handy when you get yours back form the shop. I still have to check out the OT it hums in standby and when I aligator clip to the centertap can play through.... I am guessing its about 21x24x7 or 8in.... Were you interested in letting your other tweed cab go? You can pm me if this subject is not allowed on the forum...
JP
 

capnjuan

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Japes said:
Were you interested in letting your other tweed cab go?
Hi JP; yes, I'm interested in letting it go; now that you've seen the pics, you're interest might be diminished ....

Back panel missing; fitted with Celestion 70w test speaker (not in the deal unless you're interested):

cab02.jpg



Right side:

cab03.jpg


Left side:

cab01.jpg


That is a 3/4" hole drilled in the top ahead of the handle; probably to allow the bad notes to escape. If you're interested, I have a good condition polycarbonate insert for the handle but without Gibson logo. If you're interested in the cabinet and want the insert, say so in this thread. There's a member here who has one; if he'd be willing to let it go, he can get in touch with you.

The baffle board is +/- 1/2" 9-ply baltic birch cut for the 12" speaker you see. If you're interested, I can try and fit a chassis similar to a GA30 to see if it clears the ceramic magnet; it will be close. More likely than not, an alnico speaker w/ bell with not clear the chassis. If you're interested, PM me, I'll tell you what I'm looking for ... if you don't want it, I'll understand ... my feelings will be hurt but I promise I won't say anything to the cabinet about it. :wink: Best Wishes, CJ
 

capnjuan

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Back from the amp doctor ...

front04.jpg



Out-of-tolerance resistor replaced ... as well as the Jensen P12R which apparently somebody hammered after it was reconed; replaced by a Bell & Howell-branded Jensen P12N now reconed and with an 8 ohm voice coil; P12R to local speaker shop soon. Brown Power ... Hytron 6087 military-grade 5Y3 far right and a pair of rare Bendix 5992s ... the reigning 'holy grail' of 6V6s; the ones in the amp made in 1953:

back06.jpg



I'm sorry, did you ask how long ago was 1953? ......................This is how long ago:

53chevy.jpg



Control panel in pretty good condition, correct insert in the handle, and original Gibson ox-blood knobs ... apparently the color formula is an industrial secret; nothing close in the aftermarket:

controlpanelc.jpg


Big, thick, rich tone; good but not splashy reverb; the 6V6s are 'after midnight' dark compared to JJs 6L6-style clean and tinkly. Lush and woody with humbuckers. A little more diddling with the filter caps; 2X40uf at the primary and screens; these were caps that were on hand ...will do over with 2X30uf and 2X10uf for the reverb and preamp. CJ
 

capnjuan

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The short answer is that is that it could still use something. The 'getting ready to jump through the grillcloth' hum is gone; a bad coupling cap that I caught in the replacement process. I haven't had a chance to test it with the 8" speaker disconnected and this was my first chance to listen to the B&H/Jensen whose cone is still pretty stiff; so stiff that during polarity tests, the cone hardly moved at all - I had to put my finger on it to distinguish direction of cone excursion.

The first impression is that it doesn't have the 'presence' (old stereo term, not sure of ampista comparable) of the GA18; despite two speakers, you don't really get the feeling of sound waves hitting the body ... the room being filled with sound. In fairness to the amp, it's full of new tubes ... they haven't had a chance to get acquainted, hug, or swap spit ... or break-in; add the fact they are trying to push a speaker whose cone is still stiff. I'll know more after more play time and after some experimenting without the 8" speaker.

In the pic below; on the left is the actual switched inputs at V3; the reverb output appears at the grid of V3A [pin 8] and the tone control at the grid of V3B [pin 5]. The center shows the reverb control, fed from the plate of V1B, via the coupling cap, feeding V2B [pin 5] ... except it isn't built that way. On the right, V2B is fed from the junction of the two 270K resistors at the grid of V2A and the reverb control is at the ouput of the reverb tank / grid of V3A; in short, the reverb control was moved, it displaced the tone control, and the tone control was moved to the other half of V3. Not illustrated; the NFB loop is tied back in at V3A ... not V2A as shown in the schematic.
changes02.jpg


The seller had said the channels had been daisy-chained and that he'd put it back. I think he, or the former owner, spent some time dealing with this above. I note that the as-built feed to V2B, now a 270K resistor not a variable resistor, puts the line voltage at both channels equal; suggesting that any gain imbalance between the channels (a complaint I had) has more to do with the presence of the reverb on channel 2 than anything else.

Lesser ampistas might have found all this daunting; since I am a lesser ampista, I found it daunting ... in-the-door problems, mis-match between as-drawn and as-built, and chance of me contributing to the mess by slinging solder at it ... and I would not have found the buzzy resistor on my own. The tone is thick, rich, and meaty but the reverb could be a little more pronounced and, subject to the tubes/speakers breaking in, it still needs something in the 'vitality' department ... Metamucil?
 

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capnjuan said:
. . . it still needs something in the 'vitality' department ... Metamucil?
Maybe it just needs to be Spinal Tapped a bit? Change the volume knob to one that goes to eleven? That should "loosen" things up a bit. A gift certificate for new shoes for the wife and ear plugs for Crimedog!
 

capnjuan

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jp said:
capnjuan said:
. . . it still needs something in the 'vitality' department ... Metamucil?
Maybe it just needs to be Spinal Tapped a bit? Change the volume knob to one that goes to eleven? That should "loosen" things up a bit. A gift certificate for new shoes for the wife and ear plugs for Crimedog!
Well ... I've changed the oil, lubed it, and done the plugs/points/condensor ... and even considered http://www.ToneDrano.com but I'm thinking dolly wheels or maybe a sticker or two. Best. J
 
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