Vox Pathfinder 10 modded into faux power engine for low volumes

Discussion in 'Amp Central Station' started by fozhebert, Oct 28, 2018.

  1. fozhebert

    fozhebert TDPRI Member

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    Surprised by the outcome of this experiment.

    I always found that my Pathfinder 10 never took gain very well. So I tried quite a few things. Now here is what I've come up with:
    1. replaced the stock speaker with a 8" from a dead G-Dec jr (more full range and flat)
    2. shorted out C6 for more low end
    3. Jumpered from the junction of R9/R10 to the junction of R13/14 (bypassing the EQ)
    The result is a fairly flat low wattage amp that takes my Zoom G1on very well. My Zoom G3xn sounds not as good through it (amp sims anyways). So the G3xn will be off-loaded.

    Now I have a simple rig. I like the limitations imposed by the G1on in terms of only having 5 slots, enough to try stuff with, but not too many to go overboard. I also like the Interface of the unit as well, simple and straightforward, every amp has gain/bass/mid/treble/presence and cabinet. The kicker I found out is, for my setup, to use the cabinets but keep the output type to combo amp.

    Anyone else Frankenstein their small amps into something completely different using the original circuitry?
     
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  2. GTG_Gopher

    GTG_Gopher Tele-Meister

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    How did you fit the 8” speaker in there? Do you have pictures?
     
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  3. fozhebert

    fozhebert TDPRI Member

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    Sorry, I have no picks, but all you need to do is chisel out a bit of the top support bracing (it's mdf) for the top part of the 8 inch speaker. I used a wood chisel. Took 5 minutes.
     
  4. J-Flanders

    J-Flanders Tele-Meister

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    About bypassing the EQ:
    I've done this before, thinking the same, and intuitively it also seems you do, and you might still like the outcome but jumpering does not bypass the tonestack/EQ.
    The output of U1B still sees the load of pretty much every part of the tone stack.
    I erased the treble pot as I don't know if that is actually bypassed by jumpering or if it's still (partially) part of the load.
    [​IMG]
     
  5. fozhebert

    fozhebert TDPRI Member

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    Interesting point.

    What I noticed was a large increase in gain and no dip in the mids (like when the eq is used). The eq controls also have no effect.

    What would you recommend to bypass all eq then? I don't fully all the inner workings of amp design.
     
  6. J-Flanders

    J-Flanders Tele-Meister

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    Edit: after checking in the sim it does seem that jumpering actually does make the eq ineffective. (I've had other circuits where it did not)

    Making a cut before R10 and after R13 and then using a jumper or a DPDT toggle switch would really bypass it. But if you like the way it sounds now, there's no need.

    I always sim these things in an online simulator (you need the java plugin for it)
    The working circuit in Falstad (online java applet) :
    https://tinyurl.com/ycpofotq

    I tried lots of mods to my PF10. The most useful one as far as distortion goes is adding diodes (with low forward voltage: 1N34 or 1N60(p)) to the feedback loop of the first opamp.
    It greatly removes the crackly fizz on the OD channel when going for edge of breakup tones.
    It gives a nicer and warmer crunch on the clean channel when turning up the gain. Best to install on a switch, otherwise the clean channel will always distort a little.

    [​IMG]
     
    Last edited: Oct 29, 2018
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  7. J-Flanders

    J-Flanders Tele-Meister

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    Checking the eq-bypass in the sim it looks that you can get a slightly flatter frequency response when not bypassed and turning treble to 0 and bass to around 1.
    It's only 1db difference in mid scoop though, but a huge 16db difference in output level:

    TS bypassed:
    10kHz - 1kHz => 27,5db - 21,7db => 5,8 db mid scoop
    [​IMG]

    TS treble at 0, bass around 1:
    10kHz - 1kHz => 9,9db - 5,1db => 4,8 db mid scoop
    [​IMG]

    The more you turn up the gain, the flatter it gets by the way. Pretty flat with gain maxed out but at that point you'll clip the first opamp.

    EQ bypassed and gain close to 0:
    [​IMG]


    EQ bypassed and gain at 10:
    [​IMG]
     
    Last edited: Oct 29, 2018
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