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Gibson G30 SS amp

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  • Gibson G30 SS amp

    I just finished some electronic restoration of a ~1972 Gibson G30 Solid State guitar amp made by CMI for Gibson.

    I will admit to the usual prejudices concerning SS guitar amps, but, IMO, you can dial in some pretty musically useful sounds on this one. It's not hard or edgy like some and compresses nicely when you play it loud. Tremolo and spring reverb work well. It certainly sounds better in stock form than some of Gibson's mid-60s tube amps with all the T-filters.

    FWIW, I clocked this one at 18 Watts before clipping while setting the bias for symmetrical clipping, as per instructions on the schematic. As is often the case, people read the power consumption figure of "38 Watts" on the sticker and think it's a 38W amp. I could tell just by looking that that wasn't the case here.

    It uses complementary output transistors, but does something I haven't seen before; one of them is NPN silicon, 2N3055, while the other is PNP germanium, 2N2147. Was a mix of Si and Ge output transistors ever common?

    Like other early SS amps, it suffers from a bit more hiss than one might like. I quieted this one down significantly by shotgunning some critical resistors with low-noise metal films. It could potentially be quieted further with lower-noise transistors, but I'm not sure I want to start searching for equivalents.

    One tip: The schematic commonly circulating for the G10/20/30 amps shows no component values, which is frustrating, but the schematic with full component values should be found glued to the inside of the back panel.

    In one position, a 390k resistor on the schematic was a 1M resistor on the PCB, definitely original. Since this is on the voltage divider for output stage biasing, I suspect that they may have tailored that value based on the transistors.

  • #2
    Yes, not all solid state amps suck.

    Back when this amp was designed, NPN power transistors were easier to manufacture with Silicon, while PNP power transistors were easier to manufacture with Germanium. And by easier, I mean cheaper. While not really common, I've seen a few examples of this design. The Heathkit combo amp used this and there was at least one Carvin bass amp that I remember. It was probably more common on home stereo amps.

    A hundred years ago I had one of the Carvin amps come in for repair after another shop had installed a Silicon PNP transistor, without adjusting the bias resistors. It had a little fizzy crossover distortion that sounded like a bad speaker. It took a few minutes, but I remembered the circuit and was able to rebias it to work correctly.

    A lot of that era Gibson solid state schematics will have a resistor or two that are marked as factory selected, probably to adjust for different transistors.

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    • #3
      Originally posted by Rhodesplyr View Post
      One tip: The schematic commonly circulating for the G10/20/30 amps shows no component values, which is frustrating, but the schematic with full component values should be found glued to the inside of the back panel.
      Glad you repaired it

      Can you please take a readable picture of said schematic and post it here?
      Thanks.
      Juan Manuel Fahey

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      • #4
        The thing that makes this schematic hard to read is when you don't have the parts list. Here's the best I ever found.

        I had a G10 and it was unfortunately one of a few failed repiars I've had over the years. It hissed really loud and I replaced Q5 MPF161. Sounded SO good and the noise floor was really low. Unfortunately the hiss came back two weeks later and I was never able to banish it again. Spent MANY hours trying to sort this one. I offered a refund for my original repair and my customer was so kind he refused it. If only I knew that RG BFC to transistor base trick back then I probably would have had a better shot.

        Anyway.....
        Gibson G10,G20,G30.pdf
        Click image for larger version

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        • #5
          Originally posted by J M Fahey View Post
          Can you please take a readable picture of said schematic and post it here?
          I will scan and post :-)

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          • #6
            Not trying to steal your thunder but I posted the schematic in the previous post. Is it different or am I mistaken??

            If you have a parts list available that isn't so hard to read .........???

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            • #7
              On this one, first, the output capacitor had a broken lead, so that was the first step. Then there was a lot of hiss with some "shuffling" sounds mixed in. That's the best description I can come up with. The volume control had no effect on it, but the Treble control did, so that helped me to localize it. The problem with SS circuits is that the input/output isolation isn't as good as in tube circuits, so you have to think carefully about whether noise is coming from the output of one stage of the input of the next.

              I targeted all the current-carrying resistors in the vicinity of the noise, and that strategy succeeded. There is still some transistor hiss, but it's acceptable. I think it's from the Reverb Isolation transistor, a TIS98.

              Just my experience, but CC resistor noise often seems to be a much bigger problem in SS circuits than in tube.

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              • #8
                Originally posted by nsubulysses View Post
                Not trying to steal your thunder but I posted the schematic in the previous post. Is it different or am I mistaken??
                The one in question is different in that it has the component values right on it.
                "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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                • #9
                  Originally posted by Rhodesplyr View Post
                  I will scan and post :-)
                  As promised, scanned and image-corrected for legibility: Gibson G10/G20/G30 Schematic and Parts List from a 1972 G30.

                  Click image for larger version

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                  • #10
                    Originally posted by nsubulysses View Post
                    The thing that makes this schematic hard to read is when you don't have the parts list. Here's the best I ever found.

                    I had a G10 and it was unfortunately one of a few failed repiars I've had over the years. It hissed really loud and I replaced Q5 MPF161. Sounded SO good and the noise floor was really low. Unfortunately the hiss came back two weeks later and I was never able to banish it again. Spent MANY hours trying to sort this one. I offered a refund for my original repair and my customer was so kind he refused it. If only I knew that RG BFC to transistor base trick back then I probably would have had a better shot.

                    Anyway.....
                    [ATTACH]36099[/ATTACH]
                    [ATTACH=CONFIG]36100[/ATTACH]

                    What is the "RG BFC to transistor base trick" you speak of??

                    I'm repairing one of these currently.
                    Got it up and running well, repaired power supply and reverb.
                    I'm going to isolate the mains AC off the pc board, wiring it directly to the switch with an in-line fuse as mentioned elsewhere here.

                    This amp also has a significant amount of HISSSSSS, worse with the treble turned up so I was going to replace the current carrying carbon resistors also as you mentioned.
                    If I put a scope probe on the base of Q7 the hiss goes away but it messes with the bias of that stage, causing the amp to sound distorted with reduced output.

                    Is this "BFC" mentioned a Big Frickin' Cap connected somewhere?

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                    • #11
                      Well the AC section turned out nice, getting the mains off the pc board and away from the first stage knocked down any induced hum.

                      Tried replacing both Q1's and their voltage feed resistors but it had no effect on the hiss. I could always bump up C5, the 330pf treble cap.
                      I'm sure the amp is bright enough.

                      Ah, "Big Frickin' Cap" it is, to find the noisy stage- after doing some searching-
                      Last edited by drewl; 08-28-2018, 08:21 PM.

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                      • #12
                        Hmm, it seems the hiss is actually a buzz from the AC from the volume pot/power switch.

                        Experimenting with a strip of cardboard with copper tape inside can knock it way down.

                        Need to get the mains farther away from the preamp/input Jack's somehow.

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                        • #13
                          Well to wrap this story up for future readers-
                          With shielding and replacing a noisy resistor on the base of the reverb isolation stage also marked Q1 got the hum and his way down to where it should be.

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