1952 Gibson GA20 users

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1952 Gibson GA20 users

Postby Alain Landry on Wed Jan 20, 2016 4:58 pm

Just bought a 1952 Gibson GA20 amp. It sounds amazing, but almost no clean headroom. It is 12 wattish and I believe that its lacks of clean sounds at moderate volumes will be a no no for live use, unless I a/b it for dirt with a cleaner amp. But break-up and distortion sounds are to die for... Anyone else using those old GA Gibsons???
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Re: 1952 Gibson GA20 users

Postby stratele52 on Fri Jan 22, 2016 9:25 pm

Is the speaker is good ?

Check tubes and output tubes bias, filter caps, B+ voltage . This amp may need to see a good tech.

Schematic here , I see a error in wiring from 6V6's cathods. The 680 ohms must go to ground, no cathodes.
http://el34world.com/charts/Schematics/ ... _GA-20.pdf
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Re: 1952 Gibson GA20 users

Postby pdf64 on Sun Jan 24, 2016 6:28 pm

I don't see an error, but then I don't see a 680 ohm resistor either!
I think you may mean the 220 ohm power tube shared cathode resistor?
If so, the connection of the heater winding CT to the power tube cathode is a method of reducing hum by dc elevation of the heater circuit.
See http://www.aikenamps.com/index.php/technical-q-a

Q: Why do some amps connect the center tap of the filament winding, or the junction of the two resistors off the filament string, to the cathodes of the output tubes?
A: In a cathode-biased amplifier, the cathode is at a positive voltage, somewhere around 10-40V with respect to ground. If you elevate the filament "reference" above the potential of the cathode by connecting the center tap to this point, you can effectively reduce the amount of hum coupled into the tube. This is because the filament is now positive with respect to the cathode, so the cathode doesn't attract electrons (i.e. hum) from the filament. This is a very inexpensive and easy method of reducing the hum in an amplifier without having to go to a DC filament supply.

Regarding the old GA20, some of its parts are likely to no longer be serviceable.
Carbon comp resistors may drift up in value, electrolytic caps develop high ESR and may as well not be in circuit, or leak dc and overheat, and the bumble bee coupling caps can leak dc and screw up the operating conditions of the next stage.
They would be unlikely to put out 12 watts though, even when new.
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Re: 1952 Gibson GA20 users

Postby stratele52 on Sun Jan 24, 2016 6:40 pm

Sorry , yes it is a 220 ohms resistor.
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Re: 1952 Gibson GA20 users

Postby Alain Landry on Mon Jan 25, 2016 6:24 pm

You are getting to technical for me...À technicien went through the amp. Old Gibson catalogs were advertising the BR6 amp for 10 to 12 watts and the GA20 from 12 to 14 watts. But you are right, it definetly sounds like a weak 12 watts.
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Re: 1952 Gibson GA20 users

Postby pdf64 on Mon Jan 25, 2016 7:55 pm

I think it would be doing well to get 10 watts.
They tend to put out the same power overdriven as clean, due to all the screen grid sag, so are very touch sensitive, similar to eg a Trainwreck.
If it's still got the original speaker in there, then a high efficiency speaker would allow it to put out a fair bit more SPL.
To do that a Celestion Blue may be great match, or perhaps one of the G12Hxx range.
Or a 4x12 cab!
Or just mic it up.
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