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Volume pot help (Rat content)

Discussion in 'Burnt Fingers DIY Effects' started by tacomamc, Feb 13, 2018.

  1. tacomamc

    tacomamc Tele-Afflicted

    Jul 2, 2009
    Northeast Pa
    I finished up two builds today... except for painting the enclosures. The first is a Devi Ever Soda Meiser build on vero board. The other is a Rat built on a PCB I got off eBay. Both sound great, and I’m
    Quite pleased with them.

    However, the Rat volume pot is a 250K (log) while the other pots in it are both 100K (log). The volume is quiet until the last bit of the turn where it kinda jumps. Overall volume is ok. I can order another pot, and probably will....

    Conversely, the volume on the Soda Meiser jumps to very loud early on in its rotation and isn’t gradual at all. This pot is a 100K (log) also. What should I put in this to make the volume climb more gradually?
     

  2. luckett

    luckett Friend of Leo's

    Jun 14, 2011
    .
    Maybe you got some bad or mislabeled pots. Log pots for output level should all pretty much work the same way. I'd remove the pots and measure them to see what's going on.
     
    GuitarsFromMars likes this.

  3. tacomamc

    tacomamc Tele-Afflicted

    Jul 2, 2009
    Northeast Pa
    Ok, thanks. I’ll do that
     

  4. matmosphere

    matmosphere Tele-Meister

    351
    May 27, 2015
    District of Columbia
    Generally the volume pot on a Rat is 100k not a 250k. I’d just swap it and see if that takes care of the issue.


    Increasing the value of the volume pot on the soda mieser might help. If I remember correctly it’s a loud pedal, I’m pretty sure my build acted about like that.
     

  5. matmosphere

    matmosphere Tele-Meister

    351
    May 27, 2015
    District of Columbia
    Generally the volume pot on a Rat is 100k not a 250k. I’d just swap it and see if that takes care of the issue.
     

  6. luckett

    luckett Friend of Leo's

    Jun 14, 2011
    .
    A different value pot isn't going to change the output level. It's just acting as a voltage divider, regardless of it's value.
     

  7. matmosphere

    matmosphere Tele-Meister

    351
    May 27, 2015
    District of Columbia
    It should make a difference with the rat. I’m pretty sure there is a mistake on the bom or something because every Rat schematic I’ve seen has a 100K volume pot.
     

  8. tacomamc

    tacomamc Tele-Afflicted

    Jul 2, 2009
    Northeast Pa
    Yes, the pcb I have called for a 100K audio pot, but I used it in the Soda Meiser build that I did earlier in the day. I will say tho, that this is the third Rat I’ve had or tried, and I love it. Previous amps I tried it with were a Peavey Envoy and a VHT Special 6. I really didn’t find the rat very useful at all with either amp setup. However, with my Night Train G2, it’s very useful. The only “mod” I’ve done with the circuit is to use a 100R in place of the 47R. I read somewhere here on the TDPRI that it helps tame some harshness, which is one complaint I had with previous Rats. I think that’s the only other deviation from stock that I made. So it could be that mod or my amp setup, but either way, this pedal sounds great.
     

  9. luckett

    luckett Friend of Leo's

    Jun 14, 2011
    .
    Switching to a 100k log pot from a 250k log pot won't change the output or taper at all (if the manuf. made them with the same taper). The pct. of total resistance will be the same at all positions of knob rotation resulting in a voltage divider that functions identically for a 250k and a 100k. The voltage output will be same at every knob position whether it's a 100k or a 250k. At max level, it's essentially a short from the coupling cap to the output jack. They will both allow the full voltage to pass to the output so there is not a difference in the max output level between a 100k and a 250k.
     

  10. matmosphere

    matmosphere Tele-Meister

    351
    May 27, 2015
    District of Columbia
    Yes I get that but, if I’m not mistaken, in a circuit like this the total resistance does matter. There is only so much current in the circuit and if you have too much resistance at any point in the circuit then the current can’t effectively pass through that part of the circuit.
     

  11. matmosphere

    matmosphere Tele-Meister

    351
    May 27, 2015
    District of Columbia
    I’m just a hobbyist so I’m not sure that’s the problem in this case but without going to the trouble of looking at the schematic and working out the numbers it seems to make sense that this could be causing the issue.
     

  12. tubejockey

    tubejockey Tele-Meister

    284
    Nov 25, 2015
    the bozone
    Try a little squirt of DeOxit first.
     

  13. tacomamc

    tacomamc Tele-Afflicted

    Jul 2, 2009
    Northeast Pa
    I’ll try the deoxit.... thank you.

    Tonight I was messing around with it a bit and it seems almost like it has a cocked wah pedal sound to it.... is this because I changed the 47R to a 100R? I made no other changes to the circuit. What do you recommend?
     

  14. matmosphere

    matmosphere Tele-Meister

    351
    May 27, 2015
    District of Columbia
    Once again, not claiming to be an engineer here but...

    I just glanced at the schematic. There is two 47r resistors, one in the power section and one in the gain section. Which one did you replace?

    I don’t have my Rat here but if I remember correctly you can turn the distortion all the way down and still use it as a boost. If the 47r is the one in the gain/clipping stage then I’m pretty sure turning the distortion knob down completely removes that resistor from the circuit. If it still has that wah sound with that knob down then you need to look elsewhere.
     

  15. luckett

    luckett Friend of Leo's

    Jun 14, 2011
    .
    A cocked wah has a very narrow and tall resonant frequency. Changing the rat to a 100R will make the mid hump wider and flatter vs a 47R, which would be less like a cocked wah. The rat has a prominent high mid peak, but it's not nearly as extreme as a cocked wah.
     

  16. tacomamc

    tacomamc Tele-Afflicted

    Jul 2, 2009
    Northeast Pa
    Ok, thanks guys, I appreciate your help. I replaced the 250k pot with a 100 k audio and it’s now working properly... ie, the volume climb is gradual. I also replaced took out the 100r and put the stock value 47r in there. The cocked wah sound has gone, and the pedal definitely sounds better. However.... it seems like there isn’t as much volume on tap As I think there ought to be. I’ll play more with it tomorrow at a higher amp volume and see if it still seems that way. Thanks again!
     
    matmosphere likes this.

  17. tacomamc

    tacomamc Tele-Afflicted

    Jul 2, 2009
    Northeast Pa
    Couldn’t wait, went ahead and cranked it up a bit. The rat is just slightly below unity gain when the pedal volume is maxed out.... definitely not boosting the amp, at all. Where should I look to address this? I’ll post pics of the pcb shortly.
     
    Last edited: Feb 16, 2018

  18. luckett

    luckett Friend of Leo's

    Jun 14, 2011
    .
    The output is limited by the clipping threshold of the diodes. You need to raise the clipping threshold for more volume. Try 2 diodes in series or red LEDs like the turbo rat.
     
    matmosphere likes this.

  19. tacomamc

    tacomamc Tele-Afflicted

    Jul 2, 2009
    Northeast Pa
    So, put two diodes in series in place for each of the two that are there now (4 total vs the current 2)?
     

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