Transistor substitution help needed

[Dr.Pain]

Hey guys, I'm sonsidering a build right now, but my local place does not stock on eof the transistors so I was wondering if you could maybe help me out with a substitution. The scematic calls for an MPSA18 which I believe is an NPN.
Another question - the same schematic calls for a 2N2222A, but my store lists nothing under that, but has an MPSA2222A/2N2222. Are these the same?
Thanks in advance.

[Ripthorn]

Just thinking off the top of my head, but I think the MPSA prefix denotes a Darlington transistor, which is totally different from your typical 2n2222 type NPN.  I could be wrong, but I know the MPSA13 is a Darlington.  If that is the case, just ask them for an NPN Darlington to see what they have.

[Dr.Pain]

So these darlingtons can't be replaced with something more common then?

[.Mike]

Hi there,

I think most people here would recommend that for a beginner build, you should use sockets for the transistors. Not only are you less likely to fry a transistor while soldering it into place, but you can try several different transistors in your circuit. Just a tip.

For the MPSA18, try a 2N5089. A lot of circuits I've seen call for using either of those.

I don't think MPSA18 is a Darlington. According to the datasheet, the HFE for the MPSA18 is around 500-1500. The MPSA13 datasheet actually states that it is a Darlington with an HFE of 5,000+.

For the 2N2222 vs 2N2222A, I believe there are minor differences that may or may not make a difference in your circuit. Transistors are cheap. Pick one up and try it, and if it doesn't sound right, look for something different.

I'm sure someone will correct me if I'm wrong.



Mike

[R.G.]

Mark's response is correct.
MPSA... is not necessarily a darlington. The MPSA13 and MPSA14 are, but the MPSA18 is a single transistor designed specifically for audio, with high gain and low noise.

PN2222, MPS2222, etc. should be fine for a 2N2222. The 2N part is metal cased, the others are plastic cased.

[Dr.Pain]

Whoa, thanks a bunch, Mike and others! I just checked and my store does not caryy 2N5089s, but I will just try out the 2N5088 instead (I looked up the datasheet and it just has a lower hfe). I guess I will then also try the 2N2222/MPSA2222A as well, can't hurt.
New question now - what kind of switch could I use to change between 3 different resistor values in a circuit? I was thinking a 2P3T, but that can't be right. Thanks again, everybody.

[Ripthorn]

You could do a SP3T or DP3T, either will work (the DP3T will just have an unused pole).  Looks like I was wrong about the MPSA prefix, but then again, I don't have experience with too many different transistors.   Good thing there are smarter people than I here.

[Dr.Pain]

Yeah, I thought about a SP3T, but wasn't sure. My store's catalog lists a 3-Pin ON-OFF-ON switch which I am assuming is a SP3T. Thanks a bunch everybody. 

[Ripthorn]

On-Off-On is typically an spdt toggle switch with a center detent where the center lugs make no contact.  With that you could have a set resistor when set to off and then the two on positions could be used for switching different resistors in parallel with the set one to produce lower effective resistors.  So it can be done, just takes a slightly different approach than using a rotary.