NPN vs PNPs

[archer]

Hello everyone !

It's my first post here, so I hope I'm posting in the right section and i'll try to be as clear and concise as possible.

I'm building a tubescreamer clone and I'm getting no sound out of the effect. The reasons may vary but in the proccess of debugging i found that instead of NPN transistors I've used PNP ( with the same voltage characteristics etc.).
Do I need to alter the placement of the transistor? for example do I need to place the PNP's collector where originally the schematic places the NPN's emmiter?
And if the  NPN is connected to an electrolytic capacitor, do I need to reverse the polarity of the cap ?

Thanks in advance! 

Archer

[kaycee]

None of the above will help you in this instance. You need NPN transistors for this application. The good news is that just about any Silicon NPN you can lay your hands on will do the job though.

[archer]

ok.. well that's some good news for debugging!! thank you very much !! I'll be interested (for learning and curiosity purposes) as to why NPN s are needed exclusively though !

[kaycee]

That's above my pay grade really

But it's basically a convention so that the vast majority of circuits operate with a negative ground and NPN transistors work in that instance. For PNP transistors you need a positive ground circuit (generally, there are exceptions and other uses etc).

This is greatly simplified, there are circuits that run bipolar power supplies, and have both positive and negative grounds., now if you really want a headache, ask what ground is

[R.G.]

ok.. well that's some good news for debugging!! thank you very much !! I'll be interested (for learning and curiosity purposes) as to why NPN s are needed exclusively though !

The short answer is that the circuit is designed that way.

It is possible to redesign the tube screamer to use PNPs, even serveral different ways that I can think of, but it is not possible to get it to work by simply substituting in PNPs.

The detailed answers require a lot of background that is not a simple thing to get in a few posts in a forum. It starts with PNPs and NPNs needing a different and opposite power supply voltage, and 'inverting' the resistors that bias it and set it up for gain. It goes on from there down several different rabbit trails.

[PRR]

If you have a polarity-sensitive device, and need the "other" polarity:

* if the device has two terminals, you just turn it around

* if the device has _three_ terminals, there is no general way to reverse the polarity. (In special cases there may be a way, but not for PNP/NPN transistors in linear {audio} circuits).

As said above, instead of getting the other polarity of transistor, you can "turn around" the battery (and all other polarized parts). With pure battery operation this is practical. However everybody likes power supplies. And these are (in pedal world) universally negative-ground and can not be "turned around" if other pedals share the same power supply.