"homemade" darlingtons

[psychotree]

hey so i was wondering if anyone has tried this, or could give me some advice.... I needed a MPSA13 (darlington pair) for a clone of the tea philter that i'm building, and i couldnt find one, so i read somewhere, but couldnt find much info on it, that I could "make" a darlington pair by just using 2 NPN's and soldering the emitter of the first to the base of the second, and then us it as "1" tranny, base, from the first, emitter from the second and the collector from both also soldered together..... so i used 2 MPSA18's that i had right there at hand..... i havent put it into any circuit yet, as i am still fabricating the enclosure, but i was just wondering if anyone has tried this, and if you had any tips or advice, maybe a different tranny i should be using for this or something, i dont know, i guess just want to see what someone else thought, cause i couldnt find much about it online.

Thanks guys and gals,

--trav

[smallbearelec]

I needed a MPSA13 (darlington pair) for a clone of the tea philter that i'm building, and i couldnt find one, so i read somewhere...that I could "make" a darlington pair by just using 2 NPN's, soldering the emitter of the first to the base of the second, and then us it as "1" tranny, base, from the first, emitter from the second and the collector from both also soldered together....

That is how to make a darlington pair, yes. But I'm not sure how stable the composite would be in this case, because you are starting with two, pretty high-gain, devices. If I create a Darlington, it is usually to turn a pair of low-gain parts into something that would be difficult or expensive to buy:

https://www.smallbearelec.com/HowTos/BreadboardGeDarlingtonRMs/BreadboardGeDarlingtonRMs.htm

The MPSA13 is so inexpensive that I would rather spend a little money for it than buy.

[PRR]

It's fine and is widely used where they have a big bin of single devices and don't want to manage another bin for Darlingtons. They cut the PCB for two devices and stuff it.

In DIY I'd DIY it once, and add a bag of MPSA13 to my next order-list. Hog-tying a pair into a Darlington is not that hard, but IMHO really no fun, and easy to get wrong. Also confuses the next guy to work in there (which could be me, a decade later, when I forgot what I did).

[psychotree]

cool, hey thanks for the response guys, i appreciate the input!

[bool]

You make a discrete (ok, "homemade") darlington pair only when you know exactly why you did it (which happens often, but ... ). For a guitar effect, MPSA13 is your friend ... or a BC517.

[teemuk]

...I could "make" a darlington pair by just using 2 NPN's and soldering the emitter of the first to the base of the second, and then us it as "1" tranny, base, from the first, emitter from the second and the collector from both also soldered together...

Yes, that is a Darlington. It performs somewhat better if you also add a "pull down" resistor from emitter of the driver transistor to the emitter of the output transistor. Many integrated Darlington's have that one integrated to the chip as well.

[psychotree]

oh right on, thanks bool, what would you say a ballpark would be on the value of the pull down resistor?

[bool]

Extremelly top-secret ballpark darlington pulldown resistor values:

10k-100k for guitar circuits.

100R-1k for small poweramps

10R-100R for bigger poweramps

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1Meg for maximum fat mojo due to skewed base charge at turn-off