RF interference in Marshall 18 watt

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Jan 28, 2018.

  1. itsGiusto

    itsGiusto Tele-Meister

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    I've had a Marshall 1974x for about 4 years. As long as I've had it, I could hear radio on the tremolo channel whenever the footswitch is plugged in. The RF vanishes if I unplugged the footswitch. This would only happen when I play the amp in certain locations, including my house.

    For a few years, I never really bothered using the normal channel, but I've just started to recently, and I really like it! The only problem is that whenever I plug a cable into the normal channel input jack, I get RF on that channel too.

    See here for schematic of this amp: http://www.thetubestore.com/lib/the...Marshall/Marshall-18-Watt-Schem-Schematic.pdf

    I'd like to eliminate this whole RF problem. The conventional wisdom seems to be one of two things:

    1) Solder a 50pf cap from the tip to the sleeve/ground of the input jack. This essentially creates a low-pass filter to get rid of the RF
    OR
    2) Disconnect the wire from each input to the first tube grid it encounters. Solder a 68k resistor from that wire to the grid, in series.

    Option 1 sounds like less work, but which option has less impact on the tone of the amp? Would option 1 make the amp sound dark? Would option 2 lower the gain of the normal channel? Or are they both negligible impacts on tone?
    Also, would either option 1 or 2 also work for if I tried it out on the the input jack for the tremolo footswitch?
     
  2. corliss1

    corliss1 Friend of Leo's

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    Does the amp have a shield on the chassis? As in, the side that exposes the electronics, there is some type of metal material there? Everything you can think of has been used before - a sheet of tin, foil tape, chicken wire, but it's important that there is *something* there to shield the electronics.

    I'd start with that before trying other things.
     
  3. itsGiusto

    itsGiusto Tele-Meister

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    Thanks! There's a big piece of aluminum going across the chassis back, so that's not the issue. I didn't think it was that, anyway, since the radio interference only happens when a cable is plugged into either the normal input or the tremolo plug input - this indicates, at least I think, that the cables themselves are picking up RF outside the amp, and injecting it in.
     
  4. corliss1

    corliss1 Friend of Leo's

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    Good to confirm that was in place.

    So most every amp has the "grid stopper" resistor in place. Pretty much any Fender schematic has the 68K resistors at the inputs, which is there mostly to cut out any non-guitar frequencies from getting in. I would try adding those instead of a cap, although you may need to try one (or both) plans to get it quieted down.
     
  5. tubejockey

    tubejockey Tele-Meister

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    Yes to both. However, a 5pF cap on the input would be more than sufficient to ground out the RF without impacting tone. 50pF is really too big. Grid stopper resistors are not a bad idea, most manufacturers used them for a reason, but yes it will lower your gain slightly.
    The footswitch in that schematic is tied across the audio line, so you will need to use a shielded cable to the switch, and make sure the metal switch box is also grounded. A better option if you're handy is to use a relay or FET to do the switching inside the amp and let the footswitch be just the controller. That way the audio never leaves the safety and comfort of the chassis. A FET, with the proper timing, would have the added benefit of no "pops" while switching.

    **EDIT** On closer inspection, I spoke too soon about the footswitch being across the audio line. It only grounds the grid of the trem oscillator. But the above comments still apply, as it could still be a path for RF to get into your circuit.
     
    Last edited: Jan 28, 2018
  6. itsGiusto

    itsGiusto Tele-Meister

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    Thanks! Is there any reason why a 5 pf cap from tremolo input to ground wouldn't work?
     
  7. itsGiusto

    itsGiusto Tele-Meister

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    Can anyone think of a good fix I can try for the tremolo channel footswitch input that doesn't involve me using a shielded cable or major rewiring? I'd like to be able to still use the footswitch that came with the amp.
     
    Last edited: Jan 31, 2018
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