Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on March 09, 2018, 06:34:45 pm
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A true closet find Gibson GA 20. Don't see a schematic for this one. The preamp uses 12AY7-12AY7-12AX7. Those are the preamp tubes that are in it. Though i'm not sure what this amp calls for. Ain't it a beauty?
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More:
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Preamp:
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I take it's not the trem version? the Gibson site is pretty good for dating and other info. I have a newer G20T schematic, but without field coil speaker. I've fixed a few and ALWAYS find something that doesn't match, Gibson is well known for their un-documented features. Take some time, make your own.
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Yes it's the non trem model. OT is mounted on the speaker, but is wired same as any push pull OT. Strange seeing the B+ (CT) running all the way from the speaker mounted OT to the 5Y3 and then the plates to the power tubes. Not a field coil speaker. I thought that as soon as i saw it.
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Awesome!!!
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As mentioned above- Your model may differ.
silverfox
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nice score!
use NOS parts except for electrolytic caps.
--pete
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I had a non-trem one like that, apart from it having chrome front panel, being very worn and having various ad hock repairs, eg replacement OT and speaker.
Mine had those lovely bumble bee oil filled caps with the bump at the end; unfortunately most all of them passed dc on to the next stage :sad:
So I'd be very wary of them / NOS / ANOS replacements.
Each input was arranged like the top 2, with the 3 'instrument' input stages all sharing the same plate resistor, and the 'mic' input stage having its own plate resistor, then V/T controls arranged as per the trem schematic.
Mine came with 2 12AX7s and 1 12AY7, the latter being used for 2 of the instrument inputs; I've no idea whether that was correct or if all 4 input stages should have been 12AY7.
Beware of that version of the paraphase phase splitter; the shared, unbypassed cathode is fine for regular audio use, but a type 1 or 2 master vol reveals that it overdrives horribly, putting out blips of astable oscillation that make for an unpleasant 'almost digital' overdriven tone, even when masked as normal by lashings of power tube grid clipping.
The simple fix is to bypass the cathode, or ideally split and bypass both cathodes.
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Check your amp against this schematic. I don't think it's the same as you have.
A few years ago member "Bruno" restored an amp like you have. He drew a schematic and posted it here, but unfortunately, it's been purged. I would send him a pm. He was last active March 08, 2016.
http://el34world.com/Forum/index.php?topic=11837.0
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Also with mine, the heater winding CT was referenced to the power tubes' cathodes.
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Thanks for all the info guys! I have not had a chance to get in there today but on a quick glance i see (1) 1M input grid r and (3) 470K input grid r's.
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Blues harp players love those old small amps. They put lower gain preamp tubes in to help stop their Hi-Z bullet mics from feeding back.
Maybe a harp player owned that amp at 1 point?
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Hi
One more GA20 schematic, that I have found on my computer.
It seems like Gibson had different GA20s
/Mats
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Hi
One more GA20 schematic, that I have found on my computer.
It seems like Gibson had different GA20s
/Mats
I cant get the file of your schematic to open in windows photo viewer or paint. It says the file format is not valid even though it is a jpeg in properties.
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Hi
Sorry I use MAC here
I will get back
/Mats
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Hi
One more try
GA20-version
Hope its Ok this time !
/Mats
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Hi
One more try
GA20-version
Hope its Ok this time !
/Mats
WOW! You nailed it! Spot on schematic! Incredible! You should add this to the hoffman schematics library. Thank You!
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Hi
Glad that it helped you.
Hope that the GA20, this version, will bring sweet tones after restoration.
/Mats
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Mats, where did you find that schematic?
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I found this info on the Gibson Garage site. Called the Tweed 2 Tone!
This amp was introduced in 1950. This amp is almost identical to the Epiphone EA-25 Century amp. (www.drtube.com (http://www.drtube.com), March 28, 2003)
1956 [GA-20] (see pics). All tube with 2x6V6 power tubes, 12AX7 preamp tubes, tube rectifier. Original Jensen 12" Alnico 5 speaker [...]. 2 channels (microphone and instrument) with seperate volume controls. Single tone control. 4 input jacks. [...] This is a killer sounding 50's amp for blues or early rock tone. It sounds very Fenderish with the 12AX7s and 6V6 power tubes. Great Elmore James slide tone. (ebay, June 21, 2003)
You made my year! Just finding old schematics and confirmation is what it's all about for me.
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Yes, well done, that’s the schematic for the one I had too :thumbsup:
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Hi
I dont know where I found that GA20 version-schematic. I must have been on some forum/discussion.
I saved it, I think, because of the 12AY7 preamp tubes and the grid leak bias. Here its 470k and one 1Meg. Usually you would see 5Meg or 10Meg resistors
(like that 1956 GA20 schematic here).
I wonder which version that was produced first ?
/Mats
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Of the original 'dated' parts that were remaining in my amp, the pot codes were
134724 1M lin
134724 1M lin
134713 250k ~10% audio
Astron Minimite 20uF 25 WVdc ecap
517
So a late 1957 / early 1958 manufacture seems likely?
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Hi
I dont know where I found that GA20 version-schematic. I must have been on some forum/discussion.
I saved it, I think, because of the 12AY7 preamp tubes and the grid leak bias. Here its 470k and one 1Meg. Usually you would see 5Meg or 10Meg resistors
(like that 1956 GA20 schematic here).
I wonder which version that was produced first ?
/Mats
pic of this amps speaker dates it to 1956, 2nd week.
--pete
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Good to know DummyLoad.
The pots are dated 1954 but that does not set anything in stone. The speaker is original unless it was changed early in it's life. By the way, if you ever go looking for the second 10K load resistor your going to have a lot of frustrating fun. It is buried back behind the lower .05 grid coupling cap. I mean barely a trace of being able to see it even with a mirror and light.
I recapped the amp and it has a huge hum. On the search now for bad coupling caps in the preamp and throughout.
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I take it's not the trem version? the Gibson site is pretty good for dating and other info. I have a newer G20T schematic, but without field coil speaker. I've fixed a few and ALWAYS find something that doesn't match, Gibson is well known for their un-documented features. Take some time, make your own.
Are you sure it´s a field coil speaker?
A lot of Gibson amps had the OT on the speaker in the 50´s.