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Gibson GA-20 1st version

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  • Gibson GA-20 1st version

    I found a Gibson GA-20 and am considering the preamp. It uses one 6SJ7 for a microphone channel and another 6SJ7 for 3 instrument inputs. However, both tubes are grid-leak biased and that is my area of concern. I read positive opinions about using the mic channel for a boost to guitar signals (early distortion) but I am considering a cathode bias of the instrument channel to provide a contrast to the mic channel. It seems redundant to bias both channels identically with 4 inputs...sort of a waste of potential. I will post a cropped image of the preamp tubes and want to hear if you have opinions/suggestions for amending these inputs to provide options. Thanks!Click image for larger version

Name:	GA-20 6SJ7 cropped.gif
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ID:	871220

  • #2
    Yeah i would cathode bias at least one tube, maybe both. See the Gibson schematic just posted at the end of this thread.
    http://music-electronics-forum.com/t8060/

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    • #3
      Yep! That is one of the options I am looking at. I think I can find several Gibson amps with different setups for a 6SJ7 preamp. I don't expect that cathode bias will sound a whole lot different from the Mic channel, but from what I hear, it should be a bit more flexible with pedals. What do you think about using different input resistors with each of the three inputs? If I chose, say a 47k...68k...and 100k resistor for each one, would they yield a variety of responses? I have a few Supro amps and was not a big fan of grid-leak pushed into overdrive. It sounded harsh and splatty to my ear. I converted them to cathode bias and chose to replicate the Supro 1616T whose tone was exceptional. After the conversion, I really loved the smaller Supros too!

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      • #4
        I am in the process of rebuilding this amp. I did choose to retain the grid-leak bias of the microphone channel but I opted to wire the instrument channel as cathode bias. I am using a 1K cathode resistor bypassed with a 25/50 volt sprague atom. The screen resistor is still a 1meg and the screen cap a .05uf. Both channels have 220k plate resistors which is another change from the original schematic. This was a suggestion from other GA-20 owners. When the amp was being disassembled, I had removed and discarded the Grey Tiger capacitors. It was my assumption that they were original to the amp (early 50's) and they were probably leaky (or would leak when I relied on them). I went a fished them out of the garbage can when I read a few posts and realized some folk pay big bucks for these things. When I retrieved them from my garbage can, I found that they were not the correct schematic value anyway. I had 2 ea. .05uf caps and 2 ea. .1uf caps where I should have had 4 ea. .05uf caps. I'm gonna test them and if they are good (not leaky) I may use the .05's and sell the .1's. I can't imagine how the mismatch affected the performance of the amp. They were either used as coupling caps or maybe input or screen caps. Hmmmm.

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        • #5
          Gibson schematics changed each time the wind blew. Chances are they used whatever they had available at the time. The grey tigers are wax capacitors, even if they test good do not use them. If people are buying them, then sell them or put them back in the garbage can.

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          • #6
            Got the amp all wired up and it sounds great! I bought a Weber 12A125 and rethought the layout before I started. The amp used a 14 position terminal strip to place all the cathode, plate and rail resistors. Some of them would have been darned near impossible to access. I wired them from the tube pins to the terminal strip and located the power rail next to each tube socket (for plate resistors) while the coupling caps and volume pot lines run in between them. The amp is pretty quiet and the voltages seem reasonable. The microphone channel is thick and chewy (grid-leak bias) while the instrument channel is more articulate. I used Orange drops (715) for the 1st coupling caps but chose 6PS orange drops for the PI caps. I am happy with the result and look forward to more time to play. This amp was a real fixer...I found it in a music store...it was not for sale until I asked....It was actually two amps in one. The previous owner had removed the 12 inch speaker and built a shelf upon which he installed a Maestro GA-1RT Reverb-Echo amp. It appears as though his intent was to add reverb to this amp and use an external speaker. When I saw this two-headed monster, I was stunned. The owner was asking a reasonable amount of money for the amp(s) and I could not resist...I figured the GA-1RT could be fixed and sold to cover the purchase price...Sometimes a bargain finds you....

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            • #7
              I had a PA that was grid leak biased that I was converting. It had issues. I don't recall exactly what. That was a while ago, but cathode bias was a big improvement.

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              • #8
                I have played the amp for awhile now and it is sounding better and better as he speaker breaks in. It did have a strange issue, though. When I first turned it on, the volume was very strong but it had a high ringing feedback sound not associated with the open strings of the guitar (even when the strings were muffled). If I played the strings hard, the amp would crackle and the volume would drop about halfway. I suspected a cold solder joint and found that wiggling the 6SL7 caused the volume to jump. Upon closer inspection, I found the shared cathode ground wire was wrapped around the terminal but not soldered. I fixed my mistake and the amp has lost the crackle/volume drop. I docked my own pay and placed a disciplinary warning in my employee file. The 2 volume pots are still a bit squirelly...I replaced one because the old one had ceased to function. I am beginning to suspect the mic channel pot is a problem spot.

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                • #9
                  Originally posted by dkevin View Post
                  I have played the amp for awhile now and it is sounding better and better as he speaker breaks in. It did have a strange issue, though. When I first turned it on, the volume was very strong but it had a high ringing feedback sound not associated with the open strings of the guitar (even when the strings were muffled). If I played the strings hard, the amp would crackle and the volume would drop about halfway. I suspected a cold solder joint and found that wiggling the 6SL7 caused the volume to jump. Upon closer inspection, I found the shared cathode ground wire was wrapped around the terminal but not soldered. I fixed my mistake and the amp has lost the crackle/volume drop. I docked my own pay and placed a disciplinary warning in my employee file. The 2 volume pots are still a bit squirelly...I replaced one because the old one had ceased to function. I am beginning to suspect the mic channel pot is a problem spot.
                  Get yourself some Deoxit and spray it on the pots.

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                  • #10
                    I sprayed the pots as part of the teardown/cleaning process. The replaced pot was completely non-functional. The remaining pot has some dead spots and behaves oddly. I have read some things about two-volume pot amps and the interaction between the pots. From all the pictures I can find, this amp seems to be wired the same as the others.

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                    • #11
                      As a follow-up, I replaced the mic volume pot and all the gremlins left with the old pot. The pots are much less interactive now. The amp still sounds good but I am wondering about the decided lack of headroom in the amp. The tubes are strong and the components are good but the amp struggles to make undistorted sound. I checked the voltages and found my plates for the 6SJ7's are only running at 49vdc with the screens at 15vdc or so. I wonder if the preamp is "Browning-out". I did alter the plate resistors to 220K (from 470k) and made the instrument channel cathode biased (instead of grid-leak biased). The distortion is most obvious on the mic channel. I am gonna take a closer look at my preamp rail and figure out why these two tubes are showing such low plate voltages.

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                      • #12
                        Originally posted by dkevin View Post
                        I am gonna take a closer look at my preamp rail and figure out why these two tubes are showing such low plate voltages.
                        I'm not super-familiar with this particular amp, but it's not that unusual to find lower plate/screen voltage in pentodes. I have worked on a 1953 GA30, and it used cathode-biased 6SJ7s. It is hard to find 6SJ7s that work in these amps without being obnoxiously microphonic. I had to select from a batch and use silicone damper rings on the shells.

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                        • #13
                          For comparison purposes, the Gibson Les Paul Jr amp has a grid-leak biased 6SJ7 in the preamp and the plate voltage is 165vdc; the screen voltage is 20vdc and the cathode is grounded (0vdc). The supply voltage for these preamp tubes is similar.

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