Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on February 13, 2016, 02:33:34 pm
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I have a tremelo problem on a GA20 T.
I have replaced all the cracked and split open bad capacitors under the board. They were all seriously (SHOT).
There is no oscillation or voltage pulsing in the trem circuit. I have tested the 6SQ7 tube but am uncertain even after looking at the tube data which pin or pins are the plate voltage pins. This tube has 2 diodes in it as well. I am not knowledgeable about this tube.
Otherwise the amp sounds fantastic.
If i ground the 47K trem resistor i assume that it turns on the trem. But has no effect at this time. I don't have another 6SQ7 to throw in it at the moment.
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Unfortunately, the tube is the biggest suspect. That trem circuit needs a strong tube.
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I have some of those metal can tubes here but no 6SQ7. I'll look again/
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Agree w/Sluckey. Phase shift oscillators (characterized by those three caps in series between plate and grid) do not start reliably, they are very dependent upon the gain of the tube. Even under ideal conditions (new tube, new caps) they need a few cycles to get going. You can prove this to yourself in a Fender by swapping in a 12AT7 and then a 12AU7 for the normal 12AX7 trem osc tube. The 12AT7 *might* work, the 12AU7 probably won't, even if its brand new. If you have a weak tube the trem osc may never start. I wouldn't go farther changing parts until the tube, the most likely culprit, has been OK'ed.
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I found an NOS 6SQ7 online for $2.00. Nice and inexpensive. :worthy1:
Will get back with the results next week sometime. Thanks all!
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New 6SQ7. No trem. :cussing:
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Found an error on one of the .047 trem cap lugs i soldered to. My bad. Working fine and strong. This amp sounds killer!
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What's the tube line up and the speaker, Jensen, alnico, 12"?
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What's the tube line up and the speaker, Jensen, alnico, 12"?
The tube lineup is:
12AY7 - 5879 - 6SQ7 - 12AX7 - 6V6 - 6V6 - 5Y3 / A GZ34 is in it now raising the plate voltage to 332 VDC.
The speaker in this one is a 1963 Jensen C12P. It sounds really good. The Alnico has vanished.
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Have you been able to find a schemo for this amp?
Or is it 1 of Gibson's famous missing links?
If it's a missing link and sounds great, then maybe, PLEASE, draw a schematic of it while you have it?
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Found the schematic posted in hoffman schematic section/ :worthy1:
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Nice! :icon_biggrin:
Soooo, how does she sound?
Hows the trem?
Hows the non-trem channel?
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Nice! :icon_biggrin:
Soooo, how does she sound?
Hows the trem?
Hows the non-trem channel?
Both channels sound loud and have great tight yet full bottom end. I am amazed at how good it sounds. I like it better than the real 1959 Tweed 5E3 i had. You have to hear one of these amps to believe how good they do sound. Tremelo is beautiful and not obnoxious. I had been hearing how good the early Gibson GA20 amps sounded compared to the later 1961 model year ga20. I believe this one is 1958.
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There were a number of ~20w 2x6V6 amps Gibson made that they lost the schematics for. Some never had changes made to the schematics that Gibson shipped.
Story goes that while Clapton played through a tweed Champ and BF Princeton Reverb for the Layla album, Duane played through an unknown '50's 2 tone Gibson. :dontknow:
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Didn't Ry Cooder sing the praises of the GA-20? I have a vague recollection (old Guitar Player interview?) of him thinking this was the amp Elmore James used to get what Cooder called James's "beautiful sound". Probably not the T version and not sure how much difference there would be or the different eras of GA-20s, not to mention all the undocumented Gibson changes.
Anyone recall Cooder/GA-20?
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I'm Head scratchin, if the 6SQ7 is the trem osc, what *role* does the *RC stack* on the 5879 tube play?
thanks
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It's a hi-pass filter to get rid of any unwanted low frequencies from the LFO.
It lets the guitar ac signal go through but not the LFO's signal itself.
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yes, high pass with a steep slope. bode plots at midpoint and 4th.
--pete
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What is "Bode Plotter"?
A hasty search didn't find an application which looks like that one.
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What is "Bode Plotter"?
A hasty search didn't find an application which looks like that one.
NI Multisim ver.14
an example using MATLAB.
! No longer available (http://www.youtube.com/watch?v=-HMhKVZ0EtQ#)
--pete
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Thanks guys, was guessing as much, but the 'ol brain kept jumping back to a 2nd LFO
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My brother has a GA20T that used to be my uncle's just like that amp in your picture. I overhauled it for my uncle and all caps in the amp were bad also. This one has the original Jensen P12R in it and the speaker kicks ass...very nice sounding. The amp is similar to a 5E3 but the trebles are a bit more harsh and the breakup isn't quite as sweet. I have a real 1956 5E3 to compare it to. The GA20T is probably a little lower power too as the output transformer is slightly smaller than the 5E3. Its a nice amp though.
Greg
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This one came in with no power or output transformer. Pretty scavenged. This amp now has a MM power transformer and a 5E3 output. I'm surprised at this wonderful old school tone. I wish they still made the 60% taper pots so i could build more accurate clones.
There's something special about the old potentiometers. Know anyone that has them?
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Thanks, Pete.
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Thanks, Pete.
you're welcome, sir.
--pete