Love Pedal Purplel Plexi 800

[vick]

I am playing with the Plexi 800 circuit and a had a couple questions.

I read the post about putting another resistor in series with Gain 3 to stop the squeal (150r or lessish)

1.  The 470r resistor seems to be setting the minimum gain. couldn't you just increase the value of that a bit instead of using two in series?


edited scratch part 2 I was backwards on the restiznce at lug 3
2.  Also this seems opposite of what I would expect.  Pot turned all the way down less gain setting, the only resistance coming from the 470r.  Pot all the way up most gain 1k pot + 470r resistor in series (1.47K).  It seems to me you would need to lower 470r to eliminate the squeal.

[vick]

okay scratch the second part

 lug 3 with the pot dimed out is 0 resistance so upping resistor should stop the squeal.

[induction]

The resistor has to be in series with gain 3, because at minimum gain the negative leg of the 47u cap is shorted to ground and the 470R does nothing. (Parallel resistors: Rpar = R1*R2/(R1+R2). Plug in R1=470 and R2=0, and see what you get.)

You can, however, simply increase the 10R between pin 8 and the 47u cap. When I breadboarded this, I bumped it up to 100R to eliminate the squeal. This will also change the frequency response, though (you'll bleed less treble to ground compared to bass, which results in less bass in your signal. If you like the original frequency response, replace the 47u with 22u and you'll get pretty close.

Obviously you can tailor this cap to your liking: larger cap = less bass, smaller cap = more bass. (This is the opposite of how coupling caps work, because you're bleeding these frequencies to ground instead of passing them through the signal chain.)

[vick]

thank you very much I will give it a try