Adapting this Magnavibe schematic to use a lower gain transistor.

[kforte318]

I'm trying to use the VCA of this Magnavibe schematic, but I'd like to adapt it to use a 2n5088/9 instead of the MPSA18. Is it possible? The schematic uses some pretty uncommon resistor values, so I assume messing with those might help.

[Kevin Mitchell]

The MPSA18 are darlington I believe.

This link is a schematic of a darlington transistor. It'll give you a good idea on what to do.
LINK HERE

[kforte318]

Oh! That's insanely helpful, thanks! Let me give that a shot and see how it goes.

[armdnrdy]

I posted this in another thread a few years ago:

Darlington pairs are available as integrated packages or can be made from two discrete transistors; Q1 (the left-hand transistor in the diagram) can be a low power type, but normally Q2 (on the right) will need to be high power. The maximum collector current IC(max) of the pair is that of Q2.

[Kevin Mitchell]

So are you suggesting that kforte318 (poster) use something like 2n3904 for the lower transistor(Q2) in the substitution?

I've worked on quite a few vibes. Never worked one up with darlingtons though. I've used 2n5088 paired and 2n3904 paired with no issues.

But then again, this isn't your typical vibe (realistically not one at all). I believe it's been mentioned that the original Univibe used darlingtons. -random fact

[armdnrdy]

http://www.cc.gatech.edu/classes/AY2013/cs3651_summer/docs/Darlington_Pair.pdf

[Kevin Mitchell]

 

My bad! Didn't get that quite right. I suggest putting something like a 4k7 resistor from C of the lower transistor to +9v.

[armdnrdy]

Hopefully R.G. will pop in to clarify.

I believe that he went over this in a thread a few years ago.

[nocentelli]

MPSA13 are darlington, MPSA18 are just a standard npn bjt with a gain similar to that of the 5088. In fact, I've seen a magnavibe circuit with the MPSA18 just swapped for a 5088 and the emitter and collector resistors swapped for 10k instead of the 7k5.

[Kipper4]

There are some 2n5088 phase shift oscillator circuits on tinternet

Ergo

http://www.learnabout-electronics.org/Oscillators/osc31.php

[armdnrdy]

The MPSA18 are darlington I believe.

This link is a schematic of a darlington transistor. It'll give you a good idea on what to do.
LINK HERE

Kevin,,,,you're grounded! 

[Kevin Mitchell]

Kevin,,,,you're grounded! 

Is that an electrical joke or a line from home alone?

My apologies! Simple mistake I should have researched the parts before suggesting anything. I was wrong to assume it was darlington. MPSA13 and MPSA18... they're one of the same if you really squint your eyes....