Transistors on a Rotary Switch

[lucidgenius]

Hey All,

I'm a long time lurker, but new member to the site. I have a quick question regarding a npn fuzz face build.

I've wired up a switch between transistors for the Q1 section of the circuit. I wired all the collectors and emitters together and have a rotary controlling just the input to the bases.

The thing farts out and takes a heck of a lot of pounding to get any input signal.

Any ideas?


I appreciate any help.

[eatyourguitar]

your just stacking leakage by doing it that way. in theory it should not be a problem since silicon has virtually no leakage but it sounds like you are having problems with it and thats what matters. I guess you only have one pole on your rotary switch? most people switch transistors by disconnecting any two pins. you only disconnected the base. you need a 2P6T to switch 6 transistors. you should also consider that you have almost no way to bias things correctly if you just keep swapping transistors. did you even bias your Q2 collector to be half your supply? maybe you should try building a correctly biased fuzz face before you shoot for the moon?

[lucidgenius]

...

[eatyourguitar]

ok good to know. did you bias the Q2 collector to be at half the supply voltage? every resistor and transistor in a fuzz face affects the base bias of Q1. Q1 collector voltage will bias Q2 base. Q2 base will bias Q1 base. it is a loop. as long as your Q2 resistor value is tweaked in a regular fuzz face to bias Q2 collector at half the supply voltage, you should be able to swap Q1 with just the base being lifted like you say.

1) Bias Q2 collector
2) connect another Q1 CE in parallel to the first Q1 CE but leave the new base disconnected.
3) check if Q2 collector bias has moved or is it still the same?
4) if it moves, you have a leaky Q1. check them both for leakage.

[lucidgenius]

Thanks for all your help thus far, but I must be missing something...

1.) None of the Q1 transistors show leakage.
2.) Resistor Values: R1 = 33k / R2 = 10K / R3 = 100K
3.) I'm measuring around 8.8 - 9.0v on all the Q1 & Q2 collectors.

[eatyourguitar]

I see your problem right there. you probably have the same 9v at that cap C7 also. the cap blocks DC. thats what caps do. imagine what would happen to a transistor if you connected the collector AND emitter +9v with no other connections. what would happen? answer = nothing. you have no dc ground for fuzz face Q2 (Q8 in your schematic). try a 470R or 1k or 5k from Q8 emitter to ground. next select the resistor for R2 that puts your Q8 collector at half the supply. 10k is a starting point. if your your Q8 collector is 6v, try a larger resister to get it back down to 4.5v... if your Q8 collector is 3v, try a smaller R2 till you get 4.5v. when you do this you will have a fuzz pedal and your mostly good to go. any values you change after that require you to bias the Q8 collector all over again by swapping R2. let me know if that worked.

[lucidgenius]

I forgot to rename those transistors. Just ignore my typo, It's Q1 & Q2 like you mentioned.

1.) I have .5 volts on the C7 cap input.
2.) I added a 1k from the Q2 emitter to ground and it only dropped the collector voltage from 9.0v to 8.68v

Something is obviously wrong.

[Dr Satan]

The C7 cap "+" goes to the wiper (2) of the Fuzz pot. Pins 1 and 2 are not tied together. The "-" side of C7 goes to pin 3 of the Fuzz pot and to ground. I don't know what "Tune" is supposed to accomplish, but maybe you want it in series with the Fuzz pot instead of parallel? I would take it out until I got things working how they are supposed to then try adding it between the emitter and the Fuzz control or between the Fuzz control and ground. Or maybe even off of the wiper of the Fuzz control to the "+" side of C7.

[lucidgenius]

I appreciate your help.

[Dr Satan]

Ah, I see. You've got the "Tune" pot wired wrong in the diagram. Pin 3 of the "Tune" trimmer should go to the "-" of the cap, not the "+"

[eatyourguitar]

you could try and work backwords from Q2 starting in a saturated state.

R1 = 10k
R2 = 10k
R3 = disconnected
add a 1k resistor from the fuzz face Q2 (Q8) to ground. remove the gain pot and c7 completely for now.

this will get you down very low maybe 0.9v at the collector of Q2 (Q8). this is a good thing. it means we turned the transistor ON and it is in saturation mode. now that we have had it off and on, getting it to fuzz is all about finding the resistor values that puts it somewhere in the middle.

1) post all your voltages for Q1 E, B, C + Q2 E, B, C
2) add R3 back in. 100k to start
2) post all your voltages for Q1 E, B, C + Q2 E, B, C

[lucidgenius]

Thanks for your help guys, I'm sure learning a lot. I really appreciate your time!

1.) Schematic has been bread-boarded with the "Tune" trimmer removed and I have the appropriate measurements. (duh)
2.) I added the tune pot back in with pin 3 now going to ground (instead of to the "+") - measurements still good.

The schematic I posted above shows the "Tune" - Leg 3 going to the "+" side of C7 cap... is this blocking the current from going to ground? Would this cause 9v at the collector?

[eatyourguitar]

The schematic I posted above shows the "Tune" - Leg 3 going to the "+" side of C7 cap... is this blocking the current from going to ground? Would this cause 9v at the collector?

yes this is what I was trying to say earlier. the cap blocks DC so it acts like a 1000Meg resistor. this almost like an open circuit. that is why the voltage drop across the cap is almost equal to the voltage drop across your power supply. kinda like a battery not connected to anything, you can measure the voltage across the battery terminals and it will still be 9v with infinite resistance (air) between the battery terminals. in reality the meter is another 100meg resistor but the idea is similar.

[lucidgenius]

Yeah, I tried adding a resistor to ground off that emitter leg like you said but it didn't change the voltage.

I'm gonna pull leg 3 from the "Tune" pot on my pcb board and wire it straight to ground. I'm not sure what the heck is going on.

[McSpunckle]

Did you try it with the switch removed and just one transistor wired up? Transistors have some capacitance and such between the emitter and collector. I wouldn't think it'd cause much of a problem, but... you know. Things are weird sometimes.

And what are the transistors? The Mutli-face has some MOSFETs and JFETs thrown in, which would cause issues if you only disconnect one end.