Having trouble adding a starve pot to pnp fuzz face build. Best way to fix it?

[Bandwagonesque]

Hi everyone. I've been running into an issue in getting a voltage starve pot to work with a pnp fuzz build and was hoping someone could shed some light on what im doing wrong which i realize may be quite obvious. I managed to get this to work just fine on a recent npn fuzz face build where I cut the positive power wire coming from the power jack and sending that to the 3rd lug of a b10K pot (in my instance, tried 5 but 10 worked great!), then taking the 2nd lug and sending it to where the 9V input is on the circuit, where the power jack I just cut and send to lug 3 would've gone.

This worked fine for my npn build but when i tried it with my pnp build i ran into an issue of the pot working only for a moment. what happens is ill turn the pot down and it does something, then ill turn it back up but then it wont work again. its really weird, and in that instant i have it turned down i didnt have enough time to see if it actually worked or not. i realized i may need to flip my wires around with grounds being flipped. this didn't seem to work either in fact it did the exact same thing as before. so im at a loss and wondering if i need to try again.
theres nothing weird other than a in4001 diode for polarity protection on my pnp build.

can anyone tell me the essential way of adding a voltage starve pot to a pnp fuzz, or the right and best way to do it in general? im going for that starved fuzz factory video game sound if that helps. thanks everyone and hope youre all safe and healthy.

[r080]

This thread has some ideas:

https://www.diystompboxes.com/smfforum/index.php?topic=76150.0

I don't know if it is intended to be functionally similar to what you are doing, but it appears the Fulltone '69 only has a resistor on Q2.

http://generalguitargadgets.com/pdf/ggg_ff5_sc_b69.pdf

Searching for the term "sag" might come up with some ideas as well.

When you say it doesn't work, do you mean the pot affects the sound how you are expecting, then it goes back to normal sound? Is the pot making the whole effect stop passing sound?

[antonis]

theres nothing weird other than a in4001 diode for polarity protection on my pnp build.

A shematic of particular pot wiring should be helpfull..

[Bandwagonesque]

This thread has some ideas:

https://www.diystompboxes.com/smfforum/index.php?topic=76150.0

I don't know if it is intended to be functionally similar to what you are doing, but it appears the Fulltone '69 only has a resistor on Q2.

http://generalguitargadgets.com/pdf/ggg_ff5_sc_b69.pdf

Searching for the term "sag" might come up with some ideas as well.

When you say it doesn't work, do you mean the pot affects the sound how you are expecting, then it goes back to normal sound? Is the pot making the whole effect stop passing sound?

thanks a bunch for the help and suggestions, turns out I wasn't even doing it right on my npn build so i kind of revereted back to that fuzz now but i wouldnt have found that out had i not read that article you suggested. till i get this right on this build im gonna set aside the PNP. but now im running into a small but obvious sonic issue.

I wired up my starve pot per this instructional. https://moodysounds.com/wp-content/uploads/2014/10/dyingbatterysimulatorpot.pdf

sticking a 3.9K resistor to lug 1 of a B10K pot, then lug 2 of that pot to the 9V on the circuit and then lug 3 to the positive 9v jack terminal. it works great and gets raspy and spitty but now I seem to top off when the pot hits 3-4 o'clock, then the rest of the throw lowers in volume just a bit. the instructional suggested a b25K pot but didn't have one of those. used a b20K and wasn't happy, tried b5K but settled on b10K as having the best throw but all of them suffered from this problem of it hitting '10' around 3 or 4 o'clock and then going back down to '7' for the remainder of the throw till the end around 5 oclock.

can anyone tell me how I can get over this small hump? I feel i'm reallllll close to putting a lid on this npn build?

[antonis]

sticking a 3.9K resistor to lug 1 of a B10K pot, then lug 2 of that pot to the 9V on the circuit and then lug 3 to the positive 9v jack terminal.

Your circuit has positive or negative ground..??

[Bandwagonesque]

hello, this one im working on is negative ground standard NPN. forgot to mention that my build is pretty much a normal NPN layout with high gain bc108's.

only mods i made were changing the 470ohm resistor with a 1.2k resistor, and putting trims in place of the 33K and 8.2K. biasing q1 at 1.2v (can't get any lower around .5-.7v) and q2 at 4.5.

im going to assume that 1.2 resistor may be what is causing my issue now that im speaking/typing it out?

[antonis]

OΚ then.. Pot wiring is correct..
(in the mean of +9V when fully CW..)

Supposing you want to simulate a dying battery effect, voltage shoud lie between 9 and 6 Volts (or so..)

You can work on voltage divider: V_lug2 = 9 X (R_lug2-lug1 + 3k9) / (R_Pot + 3k9)
(neglecting current drawn by circuit..)

P.S.
3k9 resistor is there just to prevent Power supply shorting to ground when Pot set fully CCW..

[Bandwagonesque]

thanks alot. i tried it out again today but measured voltage and i'm getting 9.5 cw and tapers off normally all the way down to zero. theres no noticable hump on the meter when I hit the sonic hump at around 5 o'clock. I was expecting the voltage to peak at around 9.5 at around 3 oclock like im hearing and then taper down a bit as the knob stopped at around 5 where im hearing the dip as well. but on the meter it goes smoothly, while im hearing otherwise.

So you're saying I need to implement a voltage divider circuit to get it to die at 6v and not Zero? it was only today that I learned that a 9v craps out at around 6V so this makes total sense now. That same person told me that most everyone who adds a voltage sag to a fuzz doesn't do it correctly to get it to dip to 6V like it should. I was going off of the Fuzz factory approach and was told 'that's not the optimal way to do it'.

if thats the case and im understanding you right ill get back on that. Truth be told im going for that fuzz factory-esque spit, and im awfully close so don't mind negating the use of a voltage divider circuit to do it the right way, i'd be happy to just be rid of that dip at the top of the pots throw. But if this'll solve that its worth the shot

[antonis]

So you're saying I need to implement a voltage divider circuit to get it to die at 6v and not Zero?

6V is an arbitrary minimum value..
Pot + 3k9 resistor DO form a voltage divider..
9V enter into lug 3 and a lower value voltage comes out of lug 2..
(without 3k9 resistor and Pot set at 12 o'clock, circuit voltage should be IDEALLY 4.5V..)
If you want circuit voltage to be as close as possible to the one calculated from voltage divider formula, you have to make divider resistance 10 times lower than circuit's one..
(or current through divider 10 times bigger than circuit's one..)