Transistors and Transformers

[lowell]

I've long been curious how one goes about interfacing a transistor and a transformer. Specifically designing a solid state pa to work with an OT. With tubes it's easy because of the ra in the datasheet. With transistors I don't see such a spec. I DO however think that a bjt transistor's gain largely depends on Rc. Is this correct? Possibly there is a "range" of values for Rc that can be used and it depends on the tone/gain that one is after? Therefore then the OT Z ratio is largely open to taste?

Maybe someone can point me to some popular know amps that utilized this arrangement?

[Steve Conner]

You might as well ask how you go about interfacing a transistor to a speaker. A speaker still looks like a speaker when seen through a transformer, it is just 1500 or 5000 or whatever ohms instead of 8.

So all of the math for regular solid-state amp design applies. The transistor has a current limit and a safe operating area, both specified in the datasheet. You plot these out on a graph, then choose a loadline that won't infringe them. I like to only use half of the available dissipation, to allow for imperfect heatsinking and overloads.

Finally you hook up a protection circuit that will stop them being infringed even if some musician decides to hook up a completely wrong load. Tubes can normally survive this without protection, the exception being the open circuit, but transistors can't.

I personally believe, if you want transformer mojo in a SS amp, the place for it is an interstage transformer, ideally powered by a tube.