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Gibson GA-75 question on schematic

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  • Gibson GA-75 question on schematic

    I have an early GA-75 which resembles the schematic I'm attaching. I don't understand the section of the PT feeding the 6L6's among other things but I'd like to start with that. I want to test the PT with a variac first and just don't understand what I'm looking for from that part of the PT.

    Any help greatly appreciated!

    Attached Files

  • #2
    There is normalcy where the plate voltages are fed through the output transformer with one stage of filtering, 20uf capacitor. There is a choke and another 20uf filter that feeds the screens voltages which is not unusual either. Then there is the center tap of the heater circuit which ties directly to the cathodes, which is maybe where your having confusion? You'll see a 250 ohm cathode resistor with a 20uf cathode bypass cap which goes to ground, which is normal. The only semi-different thing there is the center tap of the heaters connected directly to the cathodes. Which I believe just elevates the heater circuit to a DC voltage rather than ground or zero volts. This helps reduce hum or buzz related to the heater circuit.
    When the going gets weird... The weird turn pro!


    • #3
      It might also help if we knew what the problem is with the amp or what we're trying to fix. Or, are you posting to simply understand the circuit?
      "Yeah, well, you know, that's just, like, your opinion, man."


      • #4
        Thanks for the replies. Yes, it was the cathodes tied to the center tap of the heaters. I want to get this amp playing but it came to me as an unknown and there is not much info out there about this amp that I could find. I don't have a problem as of ..yet, but I'm not very experienced, and wanted to move slowly so as not to burn anything up. I've built an AX84 HO amp and experimented on it and my Super Reverb (main amp).

        I've been repairing my own equipment and that of friends, this forum has been invaluable to me. I try not to waste your time or ask the same stupid question twice!

        I've been trying to physically trace all the point to point components and see how they match up to the schematic. Also tested the PT with low volts and it appears to work. Tested at 30Vac and got 1.41v for rectifier heater, 1.76v for tube heaters, and 178v across the high voltage. Tested without any tubes.

        I have all tubes per schematic but haven't fired up with tubes in yet. I do not have the choke shown in the schematic and the OT appears to have been mounted on the chassis instead of the speaker as shown in the pics I've seen on the web. Looks like almost all original electrolytics except the can cap which has early sixties date.

        Sorry so long....I don't have a problem at the moment other than the lack of details I'm used to in the modern schematics. Any observations or thoughts on details maybe lacking the schematic would be very helpful.

        Assuming it is wired correctly, will this thing make sound with these old caps? Is noise the main worry or am I risking frying something without proactively changing some of the electrolytics?

        Thanks, I'll post pics tomorrow.


        • #5
          Pictures for anyone interested...
          Attached Files


          • #6
            Power cord

            Originally posted by clinkous View Post
            Pictures for anyone interested...
            I realize the thread is a bit old so I apologize 1) if I happened to miss anything and 2) for jumping in late. I just acquired one of these and stumbled on the thread while looking for comparisons of upgrade parts.

            But I noticed in this one that the 3-prong power cord is not wired safely:

            1. The hot (black) wire is always wired to the fuse first; THEN the switch.

            2. The neutral (white) wire is always wired directly to the power transformer. A tie point can be used but creates an unnecessary non-insulated exposed point, so soldering the transformer and neutral wires together directly, covered with heat shrink is safer and "cleaner".

            3. The "death cap" is useless. as is the ground switch, unless parts are used as a tie point. Clip off the cap and toss it after the thing is wired safely.

            4. I can't see how the ground wire is attached, but it seems headed to a power transformer attachment bolt (hopefully a tie point with a star washer and chassis paint scraped away). That's typical and fine. Directly soldered to the chassis is best but takes one big mutha iron!

            Thew main point, thought, it that basic electrical safety rules (and UL) require the hot side to be first fused, then switched - but never in "continuous connection". The way this one is wired the power transformer is always "hot" when the amp is plugged in - even if the fuse blows!

            Please keep yourself safe and change the wiring - and just bypass that ground switch altogether to keep things neat.


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