Tweed Gibson GA30RV

Default

Super Moderator
Platinum Supporting
Joined
Jul 30, 2007
Messages
13,595
Reaction score
3,008
Location
Philly, or thereabouts
Guild Total
11
I don't see any wheels <shrug> but you're the boss...

dolly_parton.jpg
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
I spent an hour playing with her ..... uh, make that ... the amp; getting used to the tone and reverb controls; dark as a mine shaft with the treble fully rolled off. The 8" speaker needs replaced; either the voice coil is rubbing or it just breaks up too quickly ... and this from someone who likes amps that break up as soon as they're turned on. I think I see a Weber 8" sig alnico in the future ....
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
Japes said:
Would you be so kind as to give me the cab measurements HxWxD so I can build a replacement cab.
Hi Japes: Here you go, all dimensions include thickness of tolex:

Cabinet: H = 22", D = 9 3/4", W = 22"
Control Panel Cutout: W = 15", D = 3 1/2"
Back Panel: A = 20 7/16", B = 7 3/4", C = 2 3/4", D = 1 1/4", E =2 3/8", F =7", G = 1 5/8"

Notes:
Picture is not in proportion; rely on dimensions
Back dimensions are to the inside edges (back) of the radiused openings

GA30dims.jpg


Good luck with your project! CJ
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
Gibson GA30RV on eBay Here. A little cleaner than mine but ... seller says: "... Another neat feature is the tone expander from lows to highs that is very effective."

In the red circle upper right; that's the 'tone expander'; a switch that switches a small capacitor and resistor in and out of the negative feedback loop. Whether you think that's a good idea or not, the one on the auction amp has been disconnected; no yellowish wire leading away to the circuit board on the left from the switch lug/little bright thing in the dead middle of the circle. Easy enough to fix ... just sayin' ..... expand this MF :evil: The lower circle is an electrolytic capacitor whose function is to block low frequencies from hitting the 8" speaker on the left. Not original, not necessarily a bad idea (see my belly-aching about my 8" speaker), easily reversed ...
eBayInvader01.jpg



The BIN is $1,300 ... either I got a great deal on mine or they are asking to be mooned ... I'm sorry ... I meant asking for the moon ... or is that the same thing?
 

Walter Broes

Enlightened Member
Joined
Jun 13, 2005
Messages
5,892
Reaction score
1,957
Location
Antwerp, Belgium
I was thinking that the effect of that 8" speaker might be pretty neat - just goes to show how much I know and how things are never quite what you imagine them to be.

Think a Weber sig would be a good match for a P12N? P12N is a pretty macho speaker, as far as Alnico Jensens go..
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
Walter Broes said:
I was thinking that the effect of that 8" speaker might be pretty neat - just goes to show how much I know and how things are never quite what you imagine them to be. Think a Weber sig would be a good match for a P12N? P12N is a pretty macho speaker, as far as Alnico Jensens go..
I think I'll try the cap in the 8" feed. Both my speakers were re-coned and reason says nobody re-cones a speaker without repairing a bad vc unless, of course, the vc was damaged after the re-cone or they didn't know the vc was bad.

I've already switched out the P12R for the 12N which is a dark, rich, meaty thing; one day it will be in a Deluxe where it belongs. I still have some ffzzt on the leading edge of the tone that I don't think is either distortion, chassis rattle, or baffle buzz. Without a filter, that 8" ceramic sees the full frequency. I've had an 8" alnico Weber that gave it up but did so more softly and, yes, I think they'd be a good match particularly if the Weber didn't have to work so hard.
 

Japes

Junior Member
Joined
Nov 25, 2008
Messages
13
Reaction score
0
1961GA30.jpg

Well CJ thanks so much for the help and info. Here is a pic of my amp. GA-30 no reverb and no cab...sigh.
As I mentioned I get sound from the OT centertap when in standby mode. I will try the things you mentioned hopefully a little later on today.
Thanks,
Japes
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
Hi Japes; interesting pic; when you get a chance, will you also post a pic of the controls side of the chassis; does this have the maroon/brown finish on the control panel? What's in the preamp; are those 12A_7s? Whatever it is on the far left is a Telefunken; if you have a spare whatever that is, you should pull that tube for safe-keeping. J
 

Japes

Junior Member
Joined
Nov 25, 2008
Messages
13
Reaction score
0
front-1.jpg

I'll try and get one with less reflection. The stenciling is used in places. The front is all chrome like yours. The tubes are a 12au7 2 telefunken 12ax7's the original 6v6 and a vintage 6v6 and RCA 5y3... as per the schematic... it had only 1 tube when I got it.... I have tested all the tubes...
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
Hi Japes: are you reading this as the applicable schematic? No reverb? Tube lineup matches although it's reversed; 3 X 12AX7, 2 X 6V6, 5Y3 - it looks like it ought to be. Am I missing something; where is the other speaker lead wire away from the OT?

Don't know if this will help but here goes; pic on left, schematic right, block diagram of terminals on OT at bottom. The black wire is the (-) speaker feed regardless of how many speakers you have. The blue wire is the (+) speaker feed which looks missing in the pic. Either the center or left lug on the OT is just a tie point; no signal, just a place to join the negative feedback resistor and the return wire to the mind ... sorry ... tone expander. If you connect there and nothing happens, move the wire to the other lug; one of them is 'live', the other is a dummy. The magenta line is the feedback loop to the tone expander. Is that green/white laying on the chassis; is that connected to anything? The amp will work without the feedback loop connected; if you get signal out with the blue wire at either of the left or middle terminals, the other lug is where you tied the feedback/tone expander loop.

GA30Bschematica.jpg


The 50's black version of the GA30 was bottom-sitter using octals in the preamp (6SL7s and similar) which are triodes. Yours must be an early 'Fender'-style version and predecessor to mine; suspended chassis, rear-facing controls but using 12AX7s. My serial # is 42511 - I don't know of any organized information on Gibson serial #s. The hardcore Gibsonites swear the octals are better but ... that's just talk. Gibson also started using the 6EU7, another twin triode like the 12AX7 except with a different wiring scheme for the heaters and different pinout; my amp is loaded with 3-6EU7s and a 12AU7 driver. Hope this helps; mine is starting to cook, speaker getting broken in ... tubes swapping spit ... going good! CJ
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
I really don't know. I don't have a collection of catalogs or color pics and not sure the stuff dancing by on eBay is reliable. My GA18 from '60-'62 has smooth brown plastic knobs but, as Japes' amp proves, generalities about Gibsonia are pretty iffy. The knobs on my GA30 are dark brown almost chocolate-colored plastic; not the Bake-lite material that the knobs on your GA1 and my GA8 are made of like this one which has 'grain' or marbling':

knobs02.jpg


Some of the brown bases on tubes are the 'Bake-Lite' material like the knob above and other bases are the smooth, grain-less stuff that looks like generic plastic. Weblore is that Davies Molding of Chicago owned the Daka-ware name and made the knobs including both the 'grainy' and 'smooth' ox-blood/brown chickenheads that Gibson used; Fender and nearly everybody else used Daka-ware knobs too.
 

Japes

Junior Member
Joined
Nov 25, 2008
Messages
13
Reaction score
0
A lot to answer and trust me I appreciate all input. The OT in the picture was still in my OT test mode I replaced the resistor as it had drifted and the green and yellow wire connect to the center tap. The right side black wire is for the speaker and connected to chassis ground the left terminal has a yellow wire for the speaker. I only get sound from the center tap and black terminal when in standby mode (I havent tried cleaning the switch yet as you suggested) when the green and yellow wire is where it should be If I aligator clip to the yellow wire (left terminal) I get no sound hence why I checked the OT for shorts I get continuity from Center tap to black terminal on OT...
I am using that same schematic though I have now found a great copy from http://www.gibson.com/Service/Tech/Schematics/ they must've updated there site with new scans hi-res and I found the instructions manual here: http://www.harpamps.com/gibson/index.html
They have your GA-30RV manual...
I currently use knobs that are aftermarket...this is how I got the amp...
AsFound.jpg

CapnJuan can you upload an mp3 of how your GA-30 sounds? And why did you suggest I put the 12ax7 in safekeeping? I do have some new sovtek I could use...
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
Hi Japes; we're doing good now. First off, the term 'center tap' usually refers to the primary side of the output transformer (confusingly, to the secondary side of the power transformer) but the OT center tap is shown in red below. It's the source of the B+ for the 6V6 plates and screens from the choke and ultimately the rectifier. I now understand what you meant; the 'center' or 'middle' tap on the secondary side; where the speakers are connected. For discussion purposes, we'll call that the middle or (+) tap.

japes02.jpg



Okay; you are getting sound from the middle and black taps because the standby switch is shunting or grounding the weak signal to the chassis and the black (-) wire that you say is connected to the chassis is picking it up.

The standby function is shown in green; in either of the first two switch positions - Off and On/Standby, the signal, which sees that .05 cap as a short, bypasses the resistor and goes to ground ... where your grounded black wire is picking it up again. In the second switch position - On/Standby - the inbound 110V AC power is connected to the primary side of the power transformer and everybody fires up; 5 volt rectifier heater (at the top of the power transformer drawing), 6.3 volt filament heater (at the bottom of the drawing), and the secondary AC in the middle with its center tap shown grounded.

In the third switch position - 'On', the signal is no longer grounded out (the green wire effectively stops at a dead end), the amp is on, and you hear nothing because the black (-) isn't connected to the speaker, it's connected to the chassis effectively grounding the signal out again. The fourth switch position is the other polarity position; if humming in position three, the idea was to bump the switch over to position four to kill the hum .... without having to turn the amp off, pull the plug, turn it over, plug it back in, and turn the amp back on.

The 'left'/magenta tap is connected to the middle/blue/(+) via the nfb resistor. So long as the black wire is grounded to the chassis, there won't be any sound in either switch postions 3 or 4. The left terminal is where the yellow/white wire should be connected and it's other end on the tone expander switch.

But but but but but ... before you go any further with this amp, you have to get a 3-wire cord on it and that .0? cap, shown on the left side of the dotted box as connected to ground, has to go. You also have to get a new power switch; you don't need all the contact points and they get old and ratty. 'Standby' allowed the player to unplug without nasty noise; after all, the signal path in Standby is grounded so whatever junk is happening at the input jacks isn't amplified. With a new Off/On switch, if you want to unplug, just turn the amp down. If you want to test your amp using the middle (+) and black (-) terminals, that's one thing; if you're going to do rehab work on it and want to use it, the next step is the switch and cord ... nothing else.

12AX7: if that tube has a diamond and the name Telefunken etched on it and it is in good condition, it might be worth $40-$50 or more. No point in letting it stay in the amp at some degree of risk when you can stick a sacrificial Sovtek there; we're testing, rehabbing, not starring at Lincoln Center .... yet .... and no, can't upload file of my GA30's tone ... am not yet enabled and, for the level of effort to get there .... I will promise you though that if you get it running including cord, switch, and power supply caps, you won't be disappointed. Besides, it won't sound like mine; it's full of 6EU7s and, not to embarass, the quality of my amp's output and rectifier tubes can't be exceeded. :wink: CJ
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
Bump: the black/(-) side of the OT is already grounded by the little wire that's soldered to the top of the transformer frame.
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
All done with the electronics ... switched out the 25uf electrolytic cap for an audio-grade 4uf/250v non-polarized cap and pretty much got what I was after; the 8" Jensen no longer breaks up early and, mated with the P12N, they are the equivalent of a 20" speaker spraying out 15 watts of lush Gibson tone. The cap was sized to block frequencies below 4khz so the 8" speaker only sees mid-range and up and doesn't bog down with bass:

4ufcap.jpg



The amp presented with unbalanced channel-to-channel volume. I thought it might have been a problem in the technical sense but now I think that there is a limit on signal strength at the input of the reverb can; too hot and mush comes out the other side. As with JP's Mesa, the tube that transmits the 'verbed signal in this amp also runs at noticeably low voltage suggesting that, once 'verbed, you have the same problem; push it too hard and it muds up.

In the pic below, the dotted green line represents Channel 2, the reverb channel. It shares V1 (magenta circle/arrow) with Channel 1 and by looking at the voltage chart, both halves have the same plate voltage, no difference there. The red circles identify V2 and the first voltage imbalance is reflected in the voltage chart; one half of V2 is running at 75% of the other. Channel 2's signal goes to V4A - green circle and arrow - where the plate voltage has been run up to 250v so that it has enough energy to drive the reverb transformer and induce a stepped-down voltage on its secondary winding. If the V2 voltage were much higher, then the signal at the input of the reverb tank would be correspondingly higher and that looks like something they were trying to avoid.

volume.jpg



The blue circles represent V3 and I think that the drafting boo-boo that I was crabbing about above, while annoying, doesn't really matter that much; that is, the plate voltages are also reversed. V3B, which is recovering and sending the 'verbed signal, is operating at its design 120-odd volts. The voltages for V2B and V3B are set by the 470K resistors ID'd with the yellow arrow. So .... wassit all about Alfie? I think some operational limitations of reverb mean that even with the reverb switched off, the electronics that produce the reverb effect also limit total gain in Channel 2.

When the amp showed up, I also thought the amp's output was constipated ... veiled, as if it had a blanket thrown over its grillcloth. Part of the problem was identifying the drifted resistor that allowed Channel 1's V1A voltage to be way too high; plenty of apparent 'gain' but at the expense of tone. The amp now has completely new power supply filter, cathode bypass, and inter-stage coupling capacitors - no more chocolate drops with their murky indiferent tone - and a magnificent, period-perfect, industrial-strength 250 ohm cathode resistor courtesy of PhillySteve's Amps and Day Care Center.

resistor.jpg




The amp is currently running on a pair of black-plate smoked-glass RCA Radiotron 6V6s laid on me by Coastie the Extraordinary Mostie; they are warm, dark relative to current JJs, and very pleasing to the ear:

rca01.jpg



This tweed GA30RV has been re-re-listed again ... w/ its limit-pushing BIN of $1,100 ... based on the prior year's tweed Gibson eBay transactions, he's about a year late. The auction amp is in slightly better cosmetic condition than mine but the electrolytic cap he's using to block bass to the amp's 8" speaker is the wrong type and wrong value. Its filter caps have been updated but not recently and no effort to get rid of the chocolate drops in the audio chain.

Click here for a little GA30RV time-travelling

I've tested denatured alcohol on my tolex and some of the tone cheese is coming off. Left to do also is the footswitch which somebody's dog worked over a little bit but the amp is a big, strong piece of vintage Gibson electronics; very sensitive to pick attack, warm, crunchy breakup, and for a 6V6 amp, a reasonable amount of headroom thanks in part to the use of a 12AU7 for the driver with its lower-than-a-12AX7 gain characteristics. The auction amp would need the nearly-unavoidable $200 in bench work but, if bought and spent, the owner would have a mighty fine amp. CJ
 

Walter Broes

Enlightened Member
Joined
Jun 13, 2005
Messages
5,892
Reaction score
1,957
Location
Antwerp, Belgium
capnjuan said:
:twisted: I hate you :twisted: :evil:

Kidding of course, great work on the amp, and a great read as usual, wish I lived a little closer so I could sample your work.
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
Walter Broes said:
wish I lived a little closer so I could sample your work.
Hi W: thank you, and so do I. All of us can argue all day long about what good tone is but the baseline test ought to be whether an amp causes listening pleasure when played through; this one and the other Gibsons do ... more than a Fender? Less than an Ampeg? ... who cares. Best. J
 

BluesDan

Senior Member
Joined
Jul 3, 2007
Messages
1,407
Reaction score
0
Location
New York
capnjuan said:
Walter Broes said:
wish I lived a little closer so I could sample your work.
Hi W: thank you, and so do I. All of us can argue all day long about what good tone is but the baseline test ought to be whether an amp causes listening pleasure when played through; this one and the other Gibsons do ... more than a Fender? Less than an Ampeg? ... who cares. Best. J

Well Walter.....I was thinking the same thing, but decided I just HAD to sample one of the Cap's fine amps, so when one became available to me, I jumped, and damn glad I did. This amp is now giving me mucho listenin' pleasure, it has THAT sweet tubey goodness for sure. My "new to me" Gibson GA18, refurbed & restored like only "JuanzAmpz" can:

P1010026.jpg


I'll be starting a thread with a review......this amp rocks!
 

capnjuan

Gone But Not Forgotten
Gone But Not Forgotten
Joined
Nov 29, 2006
Messages
12,952
Reaction score
4
Location
FL
8) 8) 8) 8) 8) 8)

Glad you like it Captain Dan :D CJ
 
Top