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  • #16
    Chassis is steel; guessing anty sheild I install should also be steel & not aluminum? Or at least magnetic.
    As I suspect capacitive/electric and not magnetic coupling, any conductive material should provide adequate shielding.
    Last edited by Helmholtz; 01-15-2020, 07:14 PM.
    - Own Opinions Only -

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    • #17
      A few random observations...

      Setting it up to squeal but then setting the guitar across the room (maybe 10' away) & no squeal. I feel like it's ground-related, though, as I can get it to screech by grabbing the tube & then if I grab the strings/controls/bridge it stops. I'm picking up a loud buzz too, mainly through the bridge pickup, that also ceases when I grab the strings. So I still feel like there's something there... And why the pickup transmits more noise when I get my hands near it or grab the bobbin is kinda annoying.

      I'm gonna use shielded cable for the Cut control runs; as for the rest of the amp, it's tight & there aren't really any long signal runs due to the tube layout. There's not really enough room to get any shielded wire in there.

      Thanks for all the ideas, & some of these I'll have to file for "future use" on build #5. Otherwise, it's clean up my mod-mess & finish the cosmetics for this guy. And explore my guitar wiring & probe my grounds.

      Justin
      "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 -

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      • #18
        I feel like it's ground-related, though, as I can get it to screech by grabbing the tube & then if I grab the strings/controls/bridge it stops. I'm picking up a loud buzz too, mainly through the bridge pickup, that also ceases when I grab the strings.
        Grabbing the tube increases coupling capacitance between the plate and the PU (the human body is a conductor!) while grabbing the strings grounds your body and thus reduces coupling.
        BTW, grabbing the other power tube should not produce squeal as its plate voltage is out of phase.
        Last edited by Helmholtz; 01-15-2020, 09:21 PM.
        - Own Opinions Only -

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        • #19
          Ah, that makes sense.

          I stuck a bit of insulation in the input jack to defeat the shorting feature, that enabled noise. I also both used shielded wire for my Cut runs, then used regular with a shield of copper tape around it. No success. Put a grounded steel shield between pickup & power tubes, no luck.

          I may pull out my Bassman & compare some things, using same speaker. Kinda wish I'd bought a scope way back when I hd money instead of buying more amps...

          Justin.
          "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 -

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          • #20
            I don't know if I understood correctly but I would check the path and physical orientation of the OT input wires. Even disconnecting the feedback lifting a pin on the input resistor to the circuit. Sometimes rare things happen in that area.

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            • #21
              Update.

              So I dug my Jet with the crappy Celestion out of the closet & rigged it up with a series of jumper wires & such. Holy $#!+ this amp was brite! After looking at about 4 different tone stacks, and 18 working hours later, I finally got pissed off enough to just make the damn thing a straight-up Fender 50W Normal channel w. a Mid control & permanently set Cut across the PI.

              I'll just say I have a useful tone stack with lots of spread & this little tird gets loud AF. Tweaked a few caps but otherwise, yeah. That was easy... Three years later. Oh well, at least I'll get a decent amount of money on this one. If I average it out over the 3 years, I think I'm up to maybe $1.50/hour.

              But the damn thing WORKS.

              Justin
              "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 -

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              • #22
                Originally posted by Justin Thomas View Post
                ...I'll get a decent amount of money on this one. If I average it out over the 3 years, I think I'm up to maybe $1.50/hour.
                I think the most I ever made was about $5/hour. The least I ever made was probably an actual negative figure. It probably averages out to about $1.50/hour. Welcome to the industry! You've made it
                "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

                "If you're not interested in opinions and the experience of others, why even start a thread?
                You can't just expect consent." Helmholtz

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                • #23
                  Originally posted by Mick Bailey View Post
                  I've had a few amps for repair where when the shielded back panel is removed they exhibit guitar-specific electrical feedback. One Fender in particular played like a theremin with hand proximity to the output tubes, but only with the volume turned down. Full up or at zero, no noise. With a different guitar, no noise at all.

                  I also has a standalone tube reverb that did this and I kept coming back to that for ages before finding it was the recovery tube coupling cap at fault. Why that only fed back with certain guitars is a mystery.

                  Do you have foil screening in the enclosure or back panel?
                  I like that, and amp that plays like a Thermin ! I could use that at one of my gigs when we are playing the crazy part of Led Zeppelin's "Whole Lotta Love" !

                  Usually I just get some feedback and harmonics,and run up the repeats and level on my digital delay, and use my whammy bar to warp the sound further.

                  This would be a step up, no Joke !
                  " Things change, not always for the better. " - Leo_Gnardo

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                  • #24
                    Originally posted by Justin Thomas View Post
                    Update.

                    So I dug my Jet with the crappy Celestion out of the closet & rigged it up with a series of jumper wires & such. Holy $#!+ this amp was brite! After looking at about 4 different tone stacks, and 18 working hours later, I finally got pissed off enough to just make the damn thing a straight-up Fender 50W Normal channel w. a Mid control & permanently set Cut across the PI.

                    I'll just say I have a useful tone stack with lots of spread & this little tird gets loud AF. Tweaked a few caps but otherwise, yeah. That was easy... Three years later. Oh well, at least I'll get a decent amount of money on this one. If I average it out over the 3 years, I think I'm up to maybe $1.50/hour.

                    But the damn thing WORKS.

                    Justin
                    $1.50 and hour for doing what you love. Sounds like a fair exchange in my book !

                    I've run into the same amp tone tuning problems as you before, and I've only just kicked the habit of tuning my project amps with a single Jensen test speaker. I always have to go back and change something later on when I match the amp with a much bigger "real" cabinet that has darker speakers. The single 12" Jensen C12Q on my bench is convenient as it's in a small cabinet with a small footprint, and not too loud for testing, but it also is a bit too bright for a rock amp. So I stopped being a bit lazy, and now hookup the project amp to a Well worn Twin Celestion cabinet, before I consider the tone changes done.

                    Good Luck with that new Beast Justin !
                    " Things change, not always for the better. " - Leo_Gnardo

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