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Tone control squeal and motorboating

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  • #1

    Tone control squeal and motorboating

    Just finished a 5E5 project. If you google the Fender 5E5 schematic and layout it's exactly what I built.
    2 things going on which I thought I'd try to get input on.
    First off, with nothing plugged into the amp, I'm unable to turn the "instrument volume" control to zero without getting an extremely loud noise something akin to a 2 stroke engine revving. LOL, seriously, it's nuts. Guess it's commonly called motorboating.
    Secondly, with a guitar plugged into the "instrument" channel, the tone control can't be turned past 5 without encountering the wickedest squeal I've ever heard. If the guitar's plugged into the "microphone" channel, the tone control is totally fine.

    I was thinking bad tone cap and possible ground problems, but after an afternoon of changing stuff around, no impovements. Thought I'd just throw it out there. Thanks for any help.
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  • #2
    Sort out the motorboating first - it is usually something not right in the power supply because of inadequately unfiltered or improperly rectified B+, so the rectfier and/or the filter caps.

    Since it is a new build, go back and triple-check that you have everything wired in correctly;

    - especially the PT and rectifier (HT goes to pins 4 and 6, the 5V heater goes to pins 2 and 8, with the B+ coming off pin 8 and going the junction of the first filter cap(s) and the OT CT and the first dropping resistor)
    - and the filter caps (make sure they are oriented in the correct polarity (-ve end to ground, +ve end to the B+)). Are the filter caps the right voltage rating? (you want 450V minimum). Those first two filter caps are in parallel (you could go with 2 x 20uF in there).

    Have you got about the right amount of DC voltage on the B+? It should be steady.

    Be careful and remember to safely discharge those filter caps before going back into the amp.
    Building a better world (one tube amp at a time)

    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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    • #3
      I didn't look it up, but if there is a NFB on that amp, reverse the OT primary leads
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        Can you post a photo of the vol/tone pot wiring (the whole circuit would be good)?

        Record the dc voltages at power & preamp tube pins (plates, screens on the power tubes & cathodes) & post them.

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        • #5
          Thanks for the replys gents. Really appreciate it. OK, first off, can't post pictures. Dropped my camera while taking pics of the build in progress and haven't had time to get a new one. As far as cap values etc, I used 450 volt/ 16uf Spragues and 2 watt resistors. Preamp are orange drops and Solen.

          OK, PT and RectTube are wired correctly. I've quadruple checked them. Checking all voltages against the Fender layout, all of them are higher. They are all consistently higher by the same percentage. Even the pre-amp voltages. I'll post actual values soon. I'm tied up with work for a few days here.

          I tried the OT lead switch before I posted, no difference. I had read that it was more pertaining to volume control squealing at max volume, but I thought I'dtry it anyways.

          I might have given the impression in my first post that the amp doesn't work. I must clarify, that it works. Actually sounds beautiful! It's a louder Tweed Deluxe! It's just when you turn down that instrument volume control, then the motorboating starts. Plus if I turn up the tone too much it squeals.

          As for the input jacks, this is exactly how I wired mine.
          http://bp3.blogger.com/_4u9l5q2yFBA/...hing+jacks.jpg

          Thanks again guys.

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          • #6
            If you're sure that the vol & tone pots are wired correctly, I'd start looking at layout...routing of grid wires (preamp & output), keep them away from plate & heater wires, keep the grid wires as short as practical, sheild wherever possible. The amp's susceptability to high frequency problems suggests layout issues.

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            • #7
              Thanks MWJB. Very astute suggestion. I must admit, it might not have crossed my mind to look for layout problems.

              Thanks again!

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              • #8
                Well, as you suggested, definitely some pre-amp layout issues. I got the amp to the point of just beginning to "squeal" or feed-back, and did a little probing with a chop stick. Located 2 wires which were obviously not routed correctly. Thanks again for that tip.

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