What the heck is the transistor pairing in a percolator?

[midwayfair]

I thought this was a type of Darlington, but going back to the schematic, it looks like maybe it isn't. What is this pairing called, and where can I learn how it does its thing? I'm partly curious because it offers a way to use PNP germaniums in a negative ground circuit. Thanks!

[electricstorm]

I would have to see the schematic you are looking at, but I think one is set up as a buffer with gain and the other as a Colpitts VCO. Several VCO circuits are configured like this. The configuration is a method of reducing current consumption by having two transistors arranged in a way so that they share the same biasing current. This allows the two trannies to use the same amount of current as one device would use. One tranny is an input emitter follower and the other is an output buffer with some gain.

I could be wrong though, as I may be looking at a different schematic than you are. Maybe someone else will chime in.

Jim

[oldhousescott]

Yeah, it is a clever little circuit. Like Jim said, Q1 and Q2 share the biasing, from R4 through Q2 through Q1 through R2 to ground. In AC terms, C3 looks like a short so you just have two common emitter gain stages (with no emitter resistor) cascaded into one another: Q1 -> Q2 through C4.

[midwayfair]

hey, Jim, it's Bean's: http://www.madbeanpedals.com/projects/PepperSpray/docs/PepperSpray.pdf

And yeah, the NPN transistor in the Percollator does look a lot like a Colpits oscillator based on some radio schematics I found on Google just now, with the guitar pickups standing in for the inductors they all have.

Thanks for the answers, guys. These mixed polarity circuits are still mysterious to me. Might be something I can do with this info. Hmm.

[Bret608]

I'll be excited to see what you do with the information! I remember the thing with the Barge Concepts attempt at a Percolator clone where they went with two NPN transistors because they convinced themselves that an NPN-PNP mix was impossible. Apparently there was an earlier version that actually did have two NPN silicons, but Interfax later abandoned it because the sound wasn't as good.

Not to do a total thread hijack here, but I'd be curious to hear the story of how someone laid hands on the Albini specs. Did he really let someone take apart his personal Percolator?

[Effectsiation]

Not to do a total thread hijack here, but I'd be curious to hear the story of how someone laid hands on the Albini specs. Did he really let someone take apart his personal Percolator?

I'd read the whole thread over here:
http://www.freestompboxes.org/viewtopic.php?f=19&t=6675&hilit=harmonic+percolator

[Bret608]

Hey thanks, I had been too lazy to read through the whole thing before! That's awesome that Mr. Bill ("from Guitar Works in Evanston") who Albini mentions in the gearwire video is the one who chimed in with the part values. I'm having a great time rediscovering my Pepper Spray now that I have the transistors oriented correctly (after nearly a year of not knowing that they weren't!).

[midwayfair]

Mark Hammer renewed the interest in this thing over on DIYSB, too. I still love this box; I think one of my next breadboard obsession (after the thing I'm currently working on is done) will be a low-medium gain overdrive based on it, basically a Skreddy-ization of the circuit. It has some great OD tones hiding in it, but it can go one step further and maybe find ways around the noise issues without destroying the high-end content.

[Bret608]

That sounds really cool, Jon! If you were to successfully Skreddy-ize it and make a vero layout, that would be absolutely epic. I agree that this circuit has major overdrive potential.

[midwayfair]

That sounds really cool, Jon! If you were to successfully Skreddy-ize it and make a vero layout, that would be absolutely epic. I agree that this circuit has major overdrive potential.

I don't do vero, but I'll certainly post a perfboard layout when it's done.