50kra or 50ka for intensity pot w/ switch???????

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Jeff Phebus, May 18, 2011.

  1. Jeff Phebus

    Jeff Phebus Member

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    So I want to mod the trem circuit in my Vibrolux to a switchable pot. my schem says the normal pot is 50kra but Hoffman sent me a 50ka spst. Is this going to mean the knob will be backwards or do I wire it differently?
     
  2. SatelliteAmps

    SatelliteAmps Member

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    You wire it the same. Not a a whole lot of switchable ra pots.
     
  3. pdf64

    pdf64 Member

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    I advocate that a 100k linear pot is used for the trem intensity control; it results in all of the tonal benefits, and most of the gain increase, of switching the 50k pot out of circuit, whilst retaining the trem function via the footswitch.
    When I had the trem control on a switched 50k pot, there was a big disincentive in switching the trem back in, because the amp seemed to become limp and crippled when I did so; the 100k linear pot (same as reverb intensity) gets rid of that. Pete.
     
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  4. Jeff Phebus

    Jeff Phebus Member

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  5. pdf64

    pdf64 Member

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    Sorry, to answer your original question, if it's a regular audio taper then it will be difficult to get good resolution / control of the trem intensity. You could wire it backwards to get the resolution, but that may be too confusing in use.
    As you've got it you may as well try it, both ways, before giving up on it; but it can be a drag to keep pulling your amp to bits to experiment.
    Weber sell a switched 50k RA pot but the switch quality was poor - mine failed hence I looked for another solution.
    Another option would be to build a suitable switched pot from 2 donors, eg buy a CTS pull pot 1M and CTS 50kRA, then swap the tracks over. It's not super tricky - if you can solder ok then you should have sufficient fine control. Need some good, fine-nosed pliers and side cutters. Pete.
     
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  6. phsyconoodler

    phsyconoodler Member

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    Quote:"You could wire it backwards to get the resolution, but that may be too confusing in use."

    Reversing the wiring simply makes the pot work backwards,it won't change the taper.
    You could simply take the pot apart and scrape off a small portion of the carbon trace at the zero setting and it would effectively remove the resistance of the pot from the signal path.Some guitar players do this for the tone pots.
     
  7. pdf64

    pdf64 Member

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    Give it a try. By wiring it backwards, the user approaches the gentle end of the slope where it needs to be.

    Great idea - I never thought of that! The intensity control doesn't need to function as a pot, so a variable resistor would be ok. Pete.
     
  8. walterw

    walterw Gold Supporting Member

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    i was just looking at this for my newly revitalized '70s pro! (my '66 SR has the switched trem pot, and it definitely livens up things when it's disengaged.)

    so we're just using two lugs of the pot (like a tone pot), and disconnecting it entirely increases the amp gain?
     
  9. pdf64

    pdf64 Member

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    As I see it, just disconnecting the ground to the trem intensity control should work fine, no need to scrape the track like a no-load tone pot.
    When wired as a pot, the 50k load to the 12AX7 stage that feeds it is significant; it drags down the gain and output voltage swing.
    I can't think of any other commercial 12AX7 common cathode stage that drives a 50k load, in the main signal path? Pete.
     
  10. Jeff Phebus

    Jeff Phebus Member

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    I'm confuzed, Doug Hoffman said this is the pot everyone uses (50ka spst) for this mod. does it just work in reverse of the original 50kra if wired up the same?
     
  11. walterw

    walterw Gold Supporting Member

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    in general, a "reverse audio taper" pot works the same way as a regular pot when wired the same way.

    it just has a very different sweep, jumping in effect where the regular pot was gradual and vice-versa.
     

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