Transistor Substitution ??

[griff10672]

How do you guys know what transistors you can sub in and out ??  Is it as simple as matching the same specs as the stock one ??

I would like to take a stab at some old circuit designs and obviously aren't going to find original parts ....

[Plexi]

Hi there.
First, I see if it is NPN or PNP.
Second, check the hfe, where and what are they doing in the circuit.
Then, google it (transistor, substitutes and circuit).

[griff10672]

Thanks Plexi !

I ended up finding a few decent articles/ blogs on the subject ... and was just learning about hfe ....

[R.G.]

Remember Keen's Second Law: When in doubt, whip in a 2N5088.

Well, OK, it's more complex than that.  But not a whole lot. As noted earlier, first figure out if it's an NPN or a PNP, then the voltage it operates from, and then the power level. For (1) small signal (2) low-ish voltage  (3) low power (4) audio/low frequency circuits, almost any bipolar transistor of the right polarity, with enough current gain (beta, hfe, etc) will work somewhat, if not perfectly.

This is because there is inherently a lot of variation in the gain of bipolar transistors, so most WELL designed circuits are designed so that a wide range of gains will work. There are exceptions, but not a huge number. If you're after the absolute, cork-sniffing best performance, all bets are off and even the nominally correct part number may not work correctly. It may have to be hand selected.

You can in fact build the vast majority of pedal circuits using only the 2N5088 for NPN and the PNP 2N5087.

This is NOT true of JFETs or MOSFETs. That's a whole different kettle of fixh.

[Plexi]

I thought Keen's Second Law was with 2N3904 [emoji16]

[R.G.]

No, It's the 2N5088.

The 2N3904 is an acceptable substitute if you can't find a 2N3904. 

[Plexi]

So...may I say BC550?
[emoji4]

[R.G.]

Only south of the equator.

[mac]

Only south of the equator.

BC and japanese down here, mostly.

mac

[duck_arse]

only southern cork sniffers use/have/reach for BC550. grunts down under opt for BC548B.