Footswitchable master volume cut mod

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by krueger, Jan 18, 2015.

  1. krueger

    krueger Member

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    Hi!

    If you got an amp without an fx-loop, with master volume, and run the pre-amp dirty, it can be difficult to bump the volume up or down and keep the same level of dirt. A feature I would like would be a footswitchable cut to the master volume. I'm saying cut because in my simplified understanding you somehow could switch in an added resistor to basically simulate the volume being turned down a bit. You would then use the cut engaged on rythm parts and then press the footswitch for a perceived boost in volume for solos.

    Would this be a feasible mod to non fx-loop master volume amps?
     
  2. walterw

    walterw Gold Supporting Member

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    right there with you!

    i did something like this to my marshall 2203, and it made all the difference in real-world gig use.

    mine is a switchable volume boost, which i use for solos and (with my guitar turned down) for good strong clean sounds.

    this thread even ends with a nice little schematic that looks like what i did, a relay to switch in resistance between the master volume's ground lug and ground, making the master less effective and the amp louder.
     
  3. walterw

    walterw Gold Supporting Member

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    the other way, maybe more useful overall, is to just get a nice effects loop installed, like the metro amps loop for marshall-style amps.

    that gets you access to the signal after the preamp distortion, so you can not only easily use any ol' clean boost pedal for solo jumps, but you can put delay and reverb pedals in there too, which sounds way better than delay or reverb in front of the distortion.
     
  4. krueger

    krueger Member

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    Yes, something like this is what I'm after. I guess you somehow need to involve relays to avoid running the signal or ground out to a footswitch and back?
     
  5. pdf64

    pdf64 Member

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  6. rumbletone

    rumbletone Silver Supporting Member

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    Why not just add a bypass to the relevant resistor that is controlled by a vactrol - you could even add a pot to control the amount of boost (i.e., by attenuating the amount that is allowed to bypass). All that runs to the footswitch is the circuit that lights the led in the vactrol (no audio), and it's generally easier/simpler to implement than a relay.
     
  7. krueger

    krueger Member

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    As an electronics newbie I only vaguely understand what you all suggest, except that it's doable in one form or another, level of sophistication and solution differing.

    With a normally closed 1/4" mono telejack in series with the master volume pot, couldn't you implement this in a very basic form directly in a foot switch? First position of foot switch just closes the circuit, other position wires in a resistor or potentiometer for the desired cut. What would some drawbacks be, apart from the obvious of sending out the signal out to the foot switch and back?
     
  8. pdf64

    pdf64 Member

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    Not sure whether it will be obvious to you that there will likely be significant treble loss from sending a high impedance signal down 20' of cable?
    A low pass filter is formed, with turnover frequency maybe as low as 1kHz.
     
  9. krueger

    krueger Member

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    Ah, ok. So to do something like my suggestion you would need to have something more or less like a proper fx loop?
     
  10. pdf64

    pdf64 Member

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    Yes, a (proper!) fx loop will have a low impedance 'send' to minimise such treble loss.
     
  11. tresspassor

    tresspassor Member

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    This is done as a solo boost on some amps. You just need insert a resistor or even an addition potentiometer for user control from the master volume pot's leg to ground, and use a relay to bypass it. Assuming that the master volume is pre-PI type. Yes, you have to do this with a relay (mechanical or solid-state). There's no way that you will have success running the signal through a 40 feet of cable to your footswitch.

    Weber and Skipzcircuits sale footswitch relay kits that could be used for this purpose.
     
  12. schmidlin

    schmidlin Member

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    What tresspassor said. Bear in mind you will need to source a low-volt DC power supply to run your relay or optoisolator (both are dirt cheap if you buy them individually.)
     
    Last edited: Jan 19, 2015
  13. krueger

    krueger Member

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    Thanks for the advice!
     

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