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5E5-A Clone Build, Oscillation and Other Questions

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  • 5E5-A Clone Build, Oscillation and Other Questions

    Hi, first-time poster.

    I've built a couple amps before, but I've never owned or even played a tweed. Built a Weber 5E5-A Tweed Pro clone with Heyboer transformers. It mostly works but there are a few problems. If this should be in another section let me know/move it.

    Fender Schem:
    http://music-electronics-forum.com/a...e5-a_schem.gif

    Checked the voltages compared to the Fender schematic. The only one that was a concern was the cathode of the cathode-follower driving the tone stack, which was at 160V instead of 130V

    Here are the problems/questions

    1) Distortion starts at 3 on the volume knob with a P90-equipped guitar. Maybe this is normal? The distortion seems to come from the phase inverter. The PI grid is not very distorted, but it is quite distorted on the power tube grids. By 12 the distortion is crazy. I do have a 12AX7 in V2, as I only own one 12AY7 (which is in V1; the kit comes with all 12AX7s)

    2) When the Bass control is turned all the way up, the volume cuts out almost entirely. Acts normal otherwise. Is this normal on a bass/treble knob tweed?

    3) Oscillation when presence and tone controls are turned DOWN.
    There is a serious ~16 KHz oscillation when the presence and tone controls are both turned down, when they are both around 2-3. The oscillation kicks in suddenly when turning either knob down, and then backs off gradually as the knobs are turned back up. They interact with each other; if one of them is higher, you have to turn the other lower to start the oscillation. It only happens when a speaker is connected, not with a resistive dummy load.

    I've tried changing tubes, pushing the wires around, tried moving the OT leads around (did not swap them), tried running the treble-pot-wiper-to-grid wire away from the board and everything, but it always acts the same. Made sure the pots were the right value, checked board components /wiring a lot, can't figure it out. The treble pot and presence pot capacitors are grounded at the same joint on the brass plate, could this be the problem?

    It goes away when the NFB wire is removed from the speaker jack.

    Any thoughts/comments appreciated

  • #2
    Originally posted by RC-Cola View Post
    It goes away when the NFB wire is removed from the speaker jack.

    Any thoughts/comments appreciated
    That strongly suggests that the feedback phase needs to be reversed. You can swap wires at the output tube plates, the output tube grids or the connections at the speaker jack unless you have multiple taps. Swap them at only one place.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

    Comment


    • #3
      Originally posted by loudthud View Post
      That strongly suggests that the feedback phase needs to be reversed.
      You're the man! Thank you for the quick reply, that was it. I hadn't tried it cause I thought it would just immediately go into wild oscillation if it were backwards. Guess the Heyboer and Weber OT wire colors are different.

      Anyone with a 5E5-A (or 5E4 or 5F7) have an opinion on the bass control / distortion?

      Comment


      • #4
        I built a bandmaster (not a weber kit but still similar, and with HB trannys) which is the same circuit except for the local feedback over the cathode follower, and the NFB resistor. Mine had oscillations, and I switched the inputs to the power tubes and that solved it. It sounds great. As for the distortion, put a 12ay7 in the preamp, also I don't think It calls for a 12ay7 in the second stage, just the first one. My BM doesnt. But only 12ay7 in the first section. THe bass control does cut a lot, shouldn't be a problem, but check the cap and resistor values around it just in case, and make sure its wired right, otherwise if it works dont worry about it.

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        • #5
          Originally posted by RC-Cola View Post
          Anyone with a 5E5-A (or 5E4 or 5F7) have an opinion on the bass control / distortion?
          There is a miswire with the bass control circuit. Check and recheck. Then, if you haven't found it, check again. Repeat as needed.

          The high cathode follower cathode voltage and higher than average gain are likely due to the 12ax7 in V2. If your able to check for clipping at individual stages, does that mean you have a scope?
          "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

          Comment


          • #6
            Originally posted by Chuck H View Post
            There is a miswire with the bass control circuit. Check and recheck. Then, if you haven't found it, check again. Repeat as needed.
            Will do.

            Originally posted by Chuck H View Post
            If your able to check for clipping at individual stages, does that mean you have a scope?
            Scope, function generator, and "stethoscope" amp a la Doug Hoffman. And something of an electronics background.

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            • #7
              Are your power tubes clipping before the PI starts to clip?
              "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

              Comment

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