Spec'ing transistors

[JonFrum]

I've been building tube amps, and I'm just starting trying my hand at pedals. I was able to learn online how to read tube data sheets and plate curves to choose a tube for an amp circuit. Is there similar info online for choosing transistors? To make it easy, what about a how-to for replacing a transistor in a know circuit. I don't mean just a substitution list - I want to know WHY another transistor will work in a given circuit.

Any suggestions? A book would be good too.

[R.G.]

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

The secret to replacing transistors is largely a secret of knowing how much is enough. To replace most transistors for audio, all you need to know is:

1. What is the maximum voltage which the transistor will have to withstand? The Vceo needs to be significantly larger than that.
2. What is the maximum current the transistor will have to pass in the circuit? The Icmax needs to be significantly larger than that.
3. What is the maximum power that the product of voltage and current that the circuit will force the transistor to dissipate; Pcmax must be bigger than that, and you may need a heatsink to get to that Pdmax.
4. What is the minimum gain that the transistor needs to have?

Unlike tube circuits, transistor circuits are largely dependent not on some special specification of the transistor, but on the transistor being able to withstand the power/voltage/current, and having enough gain. If it has vastly more gain, that's usually all right, because the vast majority of transistor circuits have local feedback to make them come out about right if you just have enough gain.

This is NOT true for radio frequencies in general, but to most transistors, the highest audio is indistinguishable from DC.

[JonFrum]

Great answer - thanks.