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Repairs, hum, and the noise floor on a Kalamazoo Bass 30

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  • #16
    I just want to note here that Justin "discovered" a technique that was actually used by a major amp manufacturer. Mesa! Mesa had an amp model (I can't remember which though) that had an unterminated lead off the filament supply to one of the preamp tubes. Just a three to four inch lead with the end clipped flush and (I assume) moved around to "buck" hum (as Justin accurately put it). A brilliant idea IMHO . There are a S ton of aspects to a design and idealizing all of them is damned hard even for an experienced manufacturer like Mesa. Best practice SCHMEST practice. Justin used a solution that was employed by (perhaps) the greatest patent holder in amp manufacturing. That's not cheating, that's just clever.

    EDIT: In fact I tried to add to Justin's "reputation" points based on his post, but I was thwarted by a notification stating I had to spread them around more before giving them to him again. I guess I just want Justin to know this.

    Last edited by Chuck H; 11-19-2019, 04:54 AM.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A


    • #17
      A short piece of wire is called a gimmick capacitor.

      I remember a Mesa that had a short piece of bare wire soldered to a socket pin. I mean like one inch long. Just bare wire sticking straight up. And indeed, it was there for hum abatement. I can imagine a young tech, spots that and feels superior, and snips it off. Wondering why the guys made the amp sloppily. And then have no idea where the hum came from.

      Noise reduction by cancellation is certainly a valid solution to hum, and darn clever.
      Education is what you're left with after you have forgotten what you have learned.


      • #18
        I've seen the skinny wire wrapped tightly around thhe signal wires in later SF Fenders before. Never seen a "gimmick cap" though. I ended up using about a foot of wire instead of the usual "shortest route possible." Had to reposition a ground on a Bassman 50 once & it killed a thuddy attack, which I guess was an oscillation

        And, thanks Churck, I got your message!

        There are a ton of tricks out there; a lot are listed in those Golden-Age tube books. All of which say keep your input tube as far as possible from trannies & don't run long signal leads next to power supplies... And a Gibson says D'oh!

        "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
        "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
        "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -


        • #19
          Some of those 'gimmick' caps are etched right into the circuit board. Usually a kind of squared spiral looking pad that goes nowhere.
          "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


          • #20
            One of my favorites of those is the Peavey Classic 30. And you can see it on the board layout drawing. Over by V1, the input tube, you can see in the traces a bunch of parallel lines in the traces. Connects to pins 1 and 2. Definitely done on purpose, but not on the schematic.

            You also can see traces used as shields in some circuits. A circuit might result in two long traces carrying signal and running parallel along the board. There will be a third traces between them. COnnected to ground somewhere, that third trace really goes nowhere, but it acts as a shield between the two signal traces. I cannot recall a good example at the moment.
            Education is what you're left with after you have forgotten what you have learned.


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