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gibson vintage amp help

grssr

New member
Joined
Jan 18, 2006
Messages
104
bigjim, what sound will the ga 8t give you. do these break up early?
also is your's a discoverer?
 

RAB

Active member
Joined
Mar 17, 2005
Messages
2,122
I've owned several tweed GA-20T Rangers, GA-18T Explorers, GA-19RVT Falcons and my current GA-6 Lancer...they all sounded great...the GA-6 Lancer has, as Big Jim notes, plenty of gain and is also the loudest of the aforementioned amps...
 

FiveG

New member
Joined
May 17, 2004
Messages
282
What's the gain like on the GA-5T? And any suggestions on boosting it?
 

blewsbreaker

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Joined
Dec 29, 2003
Messages
1,113
Well said! IMHO both of these amps are great values under a grand if in original shape. A lot of those $500 amps, have replaced speakers, transformers, and most of the board replaced. What really are you buying at that point? Speaker selection is huge in these amps. Be sure to get the correct speaker and some good NOS preamp tubes!

Some eye candy anyone? A few from my stash.:2zone Sorry guys, I love these Gibby amps and can't help myself. My wife thinks I have a compulsive disorder, think?:hmm
A '59 GA-20T
60GA-20Front.jpg


A '58 GA-20T, I traded this amp for a brown Princeton about a month ago to a good friend! I miss it already. He has a great collection and has wanted it for a few years. Oh well.
GA-20Front.jpg


A '56 GA-20T, check that square plastic tag logo!!!! Original!
IMG_0880-1.jpg


A '60 GA-6
60GibsonGA-6.jpg


A '55 GA-6
55GibsonGA6Front.jpg

Those are awesome Akstrat... Which one is yer fav?
I had a mint 58'Ga-20T just like yours with original cover that I regret selling...I kept my 61' GA-20T which is a mutherfucker. Have you tried popping in a low-wattage Celestion in place of the Jensen's? I think they work great in the GA-20T's. I also like dropping in a GZ-34 in place of the 5y3 to get a bit more output.:dude:
 

akstrat61

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Joined
Feb 16, 2004
Messages
1,845
blewsbreaker Those are awesome Akstrat... Which one is yer fav? QUOTE said:
They all have their unique tone. A sleeper that isn't mentioned much is the 1x10 GA18T. I have Celestion in that one and it is really a Deluxe killer. From what I have read, this circuit is much closer to the tweed Deluxe, other than it hosts a 10 inch! The tweed GA18's are really worth the price of admission.:wail

I have to learn to shut up!:lol Sorry guys!
 

Boytbpc

New member
Joined
Mar 17, 2017
Messages
2
Necrobump! Hi, I just joined. I've lurked for a while, since getting my Les Paul, but now that I've acquired a Gibson GA-6 Lancer, I figured it was time to join and post. For the guys that own one, can anyone explain to me how the Phase Inverter works in one of these? I'm confused looking at the schematic.
 

B Ingram

Member
Joined
Jan 3, 2016
Messages
730
... I've acquired a Gibson GA-6 Lancer ... can anyone explain to me how the Phase Inverter works in one of these? ...

Gibson changed specs of a given amp model frequently, and didn't always publish a schematic for every variation. That said, I found two different GA-6 Lancer schematics and linked below (distinguished by the choice of preamp tubes):

GA-6 Lancer, 12AY7 6SL7

GA-6 Lancer, 12AX7

While the details differ, fortunately the basic phase inverter type is the same between these schematics. Bottom line, a phase inverter seeks to create equal-and-opposite output signals to drive the output tube grids.

The two GA-6 schematics show a paraphase inverter. In this inverter, the preamp signal is applied to a gain stage (in this case, one side of a dual-triode) and is amplified. This takes care of generating one-half of the needed "equal-and-opposite" push-pull drive signals for the output tubes.

The paraphase inverter makes use of the fact the plate output of a gain stage is opposite-polarity of the grid input signal. So to get the opposing signal for push-pull drive, another gain stage is used get get an output signal opposite the first signal. You can see the plate output of the upper-half of the 6SL7 passing through a 0.05µF coupling cap and passing on to the upper 6V6 grid. But it also turns down through the resistors and is applied to the lower half of the 6SL7. The plate output of the lower-half will be opposite polarity of the upper-half's output, providing the needed opposite drive signals to the 6V6's.

"Wait a minute... Doesn't the lower-half of the 6SL7 amplify the applied signal? Wouldn't that make this second output bigger than the first output??"

Sure it does. So before applying the signal to the 2nd half of the paraphase inverter, the signal is reduced by a voltage divider. Here it is made of 220kΩ and 6.8kΩ resistors, which reduces the signal to 6.8kΩ/(220kΩ+6.8kΩ) = ~0.03 of the output of the upper half. 1/0.03 = 33.3 and so the design of the inverter assumes gain of the lower half will be ~33 to get the "equal" of "equal-and-opposite" output signals.

As a final note, the two halves of the paraphase inverter share a cathode resistor. If it were a much larger value (or replaced by a constant-current source) it could help enforce equal outputs by forcing the opposing currents of each side of the paraphase to be the same. Except we don't have that here. Instead, what it does is save the cost of one resistor, and tends to eliminate the need for a cathode bypass cap. There might be a trace of local negative feedback for very unequal output signals, and a little gain reduction. However, this is mostly a production expedient that lowers cost.

This paraphase will actually do a somewhat poor job of achieving "equal-and-opposite" output signals, because it is not a self-balancing or self-correcting form of a paraphase. An assumption is made that the signal reduction of the 220kΩ/6.8kΩ voltage divider exactly matches the gain of the triode in the lower-half of the paraphase. That's a poor assumption, as all tubes vary around average characteristics, and some have higher or lower gain than other samples. And a switch was made to a 12AX7 (which has an in-circuit gain closer to 60 than 35 for the 6SL7), but the voltage divider values stayed unchanged.

BUT!!! Many people like that about the paraphase and feel they get a more interesting sound, even if the amp has a little more distortion than it would if the phase inverter outputs were perfectly matched.

I hope that helps you see how this circuit works!
 
Last edited:

Big Al

Well-known member
Joined
Apr 24, 2002
Messages
14,537
Vintage 50's Gibson amps have gotten pricey. For the dough I think the 63ish Gibson Crestline amps are hard to beat. My two picks for value, utility and function for your needs I'd suggest the Falcon RVT and Discoverer T. The Falcon is my favorite.

For a little more dough nothing beats a late 50's GA8 Gibsonette, the single knob one.
 

F-Hole

Well-known member
Joined
Sep 2, 2015
Messages
2,171
GA 2-RT, 16 watts, reverb and trem....and it breaks up like a Deluxe/18 watt hybrid.
 

Boytbpc

New member
Joined
Mar 17, 2017
Messages
2
Thanks for the info. That was really helpful.

Gibson changed specs of a given amp model frequently, and didn't always publish a schematic for every variation. That said, I found two different GA-6 Lancer schematics and linked below (distinguished by the choice of preamp tubes):

GA-6 Lancer, 12AY7 6SL7

GA-6 Lancer, 12AX7

While the details differ, fortunately the basic phase inverter type is the same between these schematics. Bottom line, a phase inverter seeks to create equal-and-opposite output signals to drive the output tube grids.

The two GA-6 schematics show a paraphase inverter. In this inverter, the preamp signal is applied to a gain stage (in this case, one side of a dual-triode) and is amplified. This takes care of generating one-half of the needed "equal-and-opposite" push-pull drive signals for the output tubes.

The paraphase inverter makes use of the fact the plate output of a gain stage is opposite-polarity of the grid input signal. So to get the opposing signal for push-pull drive, another gain stage is used get get an output signal opposite the first signal. You can see the plate output of the upper-half of the 6SL7 passing through a 0.05µF coupling cap and passing on to the upper 6V6 grid. But it also turns down through the resistors and is applied to the lower half of the 6SL7. The plate output of the lower-half will be opposite polarity of the upper-half's output, providing the needed opposite drive signals to the 6V6's.

"Wait a minute... Doesn't the lower-half of the 6SL7 amplify the applied signal? Wouldn't that make this second output bigger than the first output??"

Sure it does. So before applying the signal to the 2nd half of the paraphase inverter, the signal is reduced by a voltage divider. Here it is made of 220kΩ and 6.8kΩ resistors, which reduces the signal to 6.8kΩ/(220kΩ+6.8kΩ) = ~0.03 of the output of the upper half. 1/0.03 = 33.3 and so the design of the inverter assumes gain of the lower half will be ~33 to get the "equal" of "equal-and-opposite" output signals.

As a final note, the two halves of the paraphase inverter share a cathode resistor. If it were a much larger value (or replaced by a constant-current source) it could help enforce equal outputs by forcing the opposing currents of each side of the paraphase to be the same. Except we don't have that here. Instead, what it does is save the cost of one resistor, and tends to eliminate the need for a cathode bypass cap. There might be a trace of local negative feedback for very unequal output signals, and a little gain reduction. However, this is mostly a production expedient that lowers cost.

This paraphase will actually do a somewhat poor job of achieving "equal-and-opposite" output signals, because it is not a self-balancing or self-correcting form of a paraphase. An assumption is made that the signal reduction of the 220kΩ/6.8kΩ voltage divider exactly matches the gain of the triode in the lower-half of the paraphase. That's a poor assumption, as all tubes vary around average characteristics, and some have higher or lower gain than other samples. And a switch was made to a 12AX7 (which has an in-circuit gain closer to 60 than 35 for the 6SL7), but the voltage divider values stayed unchanged.

BUT!!! Many people like that about the paraphase and feel they get a more interesting sound, even if the amp has a little more distortion than it would if the phase inverter outputs were perfectly matched.

I hope that helps you see how this circuit works!
 

crashbelt

Active member
Joined
Apr 10, 2016
Messages
103
Thanks for the info. That was really helpful.
I have 4 Gibson tweed era amps all classic tones.

GA5 LPJ Junior 4w 1955 - practice amp for home oval Jensen - careful not to blow it up!
GA8 Gibsonette 8w 1959 - dual single ended sweet tone for band rehearsal and very small gigs
GA18T Discoverer 14w 1960 - main gigging amp for pub/club gigs awesome tremolo
GA77 Explorer 25w 1960 - larger gigs 1 x 15" speaker very hard to find these

It took some time to get this line up here in the UK - but I wouldn't use anything else!!
I gig a 63 345 59 LPSpecial and 66 Custom Tele - very different guitars which all work
great with all the amps and no pedals!!! Guess I'm a bit old school.
 
Joined
Mar 31, 2020
Messages
33
I have 4 Gibson tweed era amps all classic tones.

GA5 LPJ Junior 4w 1955 - practice amp for home oval Jensen - careful not to blow it up!
GA8 Gibsonette 8w 1959 - dual single ended sweet tone for band rehearsal and very small gigs
GA18T Discoverer 14w 1960 - main gigging amp for pub/club gigs awesome tremolo
GA77 Explorer 25w 1960 - larger gigs 1 x 15" speaker very hard to find these

It took some time to get this line up here in the UK - but I wouldn't use anything else!!
I gig a 63 345 59 LPSpecial and 66 Custom Tele - very different guitars which all work
great with all the amps and no pedals!!! Guess I'm a bit old school.

any pictures of the GA-18T explorer?
 

agogetr

Active member
Joined
Jan 22, 2019
Messages
451
the early fifties brown one with the g in the middle. was it the br9? great early breakup like your lookin for. dont rule out the old eh150 they crank for little guys
 
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