swapping transistors

[Al Heeley]

When deciding to swap out or replace transistors, what are the characteristics that you look for on the data sheet to make the decision?
If your circuit calls for a J201 and all you have is a BS170 or a 2n3904 or a 5088, for example, can anyone help me understand how to tell which one would be the best alternative? I'm just thinking about the mainstream op-amp types that virtually every circuit calls for.
Not too good yet at ploughing thru all the specs and graphs over 7 pages of a fairchild datasheet.
What are the key features to be aware of?

[kupervaser]

j201 is a jfet for starters, you can't put a 2n3904 npn transistor there. So first you check if it is a jfet or a transistor or something else. If it is a transistor you need to kno whether it is npn or pnp. Most transistors are substitutable. For example if you have a 2n390n transistor you need to substitute, you can take any small signal npn transistor and it will work.

[Al Heeley]

So is there a list somewhere which advises interchangeability between like types?
It comes back to my original question - are there figures on the data sheets I should be looking at to compare and ascertain suitability?

[trjones1]

I guess the question is what you're trying to accomplish by switching transistors.  Do you want higher gain, lower noise, different sound?  Generally any bipolar transistor will be interchangeable with another bipolar, jfet with jfet, mosfet with mosfet, etc., they'll all just give you slight variations in tone.   The things to watch out for are polarity (NPN v. PNP for bipolar, N channel v. P channel for fets) and pinouts.  As an amateur, pinouts are probably the most important info you get from a data sheet. 

Beyond that, gain is really one of the only bits of technical data on the datasheet that most stompboxers are going to care about.  We're not putting transistors through too much strain, so their characteristics at the edges of operation are not too important. 

Since most datasheets won't come out and tell you whether a transistor is bipolar, jfet or mosfet, there are a few tricks you can use to figure it out.  If the datasheet has the circuit symbol, just look to see what kind of transistor it represents (you can find this info on google).  Otherwise you can look at terminology in the datasheet.  If a transistor is identified as NPN or PNP, that means it is bipolar.  N-Channel and P-Channel means FET.  Of course if you work with this for just a little while you'll begin to memorize the different common types and what they're good for.

Just off the top of my head, the transistors in each of these categories are freely interchangeable with each other, as long as you have the right pinout:

NPN bipolar transistors
2N5088
2N5089
2N3904
2N2222
2N5013
BC108/109
PN2369A
MPSA18
MPSA13 (this is a darlington - 2 transistors strapped together - but it is interchangeable with the other transistors if you want much higher gain)

PNP bipolar tranistors
2N3906
2N5087

N-Channel Jfets
J201
2N5457
2N5458
MPF102

N-Channel Mosfets
2N7000
BS170

There are obviously bunches of other devices in each category, but these have been more than enough to keep me busy for years.  I think that you start reaching diminishing returns when you're swapping out different transistors trying to find the perfect one.  Yeah, you could spend 4 more hours going through every transistor Mouser stocks to get a tiny edge in tone, but in real life music playing situations that little difference is pretty meaningless.

[petemoore]

  For 'common' bipolars and opamps the header of the data sheet usually tells enough to tell if it's what you need.
  In the case of Jfets it's another matter, better off reading about them at GEO and doing some preliminary testing for Vp and Idss [runoff groove has a 2 resistor and a switch circuit for this].
  Same thing except different for Ge's, they might not like to be biased, they drift, they leak a lot [GEO leakage tester...hardly worth the trouble unless you have a nice set of them.
  All the other type components that 'get you through' are much more consistant in performance etc.

[Al Heeley]

Thanks for the info, the main thrust of the question was availability issues with some of the parts in the UK, you know when the circuit calls for a couple of trannies, you don't have the exact ones and you don't want to place an order for $20 of other stuff as you're only short of 2 transistors but you have 6 or 7 other types in your box. We get really stung opn postage for small lots import stuff. I hate having to pay £5 postage for a component worth $1.

[kupervaser]

This is why we are telling you that almost all small signal transistors are substitutable.
You just sloughter some old radio and get out whatever is in there, check if that is npn or pnp, check the the pins position and just plug it in there and see if it works.
That;s about it.
Well as some people said above, just make sure you substitute jfet with a jfet, mosfet with a mosfet, etc