Transistor and Tube Biasing

[EricKnabe]

So here's the question. If you take a standard tube common cathode stage, and then, changing nothing else but the B+ supply, place a transistor in the tubes place, and then you set up the base bias resistors so that the operating point is in the same place on the load line, will the operation characteristics be similar? According to an online circuit modeling software the answer appears to be yes. A hot biased stage with significant gain clips symmetrically and by changing the emitter(cathode) resistor to a cold clipper, the operating point shifts and clipping becomes asymmetrical. But will that hold up outside the realm of an online spice type simulation?

[antonis]

FET or BJT replacement..??

For FET, you should delete grid stopper resistor and undersize a lot plate (anode) resistor..
(depending on new B+ value..)

For BJT, you should also alter bias configuration..
(maybe keep grid stopper resistor & place a feedback resistor from Collector to Base, deleting grid pull-down resistor..)

But, simulators are far away from real world so breadbording is slippery slop resort..

[Joe]

I've found my simulator to be accurate enough, but it depends on the circuit. Unless we're talking about something like an op-amp (or chip power-amp), either trim-pots are required, or testing voltages on a breadboard for the exact transistors and resistors in question. The specs can vary enough to throw things off.

There are many ways to bias transistors. Years ago I had the idea of using a diode to keep the base current to a minimum, in tandem with a high-gain, lower-noise transistor (2N5089). This was the result.

I don't remember all of the details, but another idea worth looking into is the original Peavey "Trans-Tube" transistor stage, which used a diode to ground instead of in series. I've bread-boarded that before and it sounds pretty good.

[swamphorn]

It looks to me like you're using the reverse saturation current of a diode? I like that idea! The only thing that would worry me is variance in the reverse saturation current from diode to diode. I also wonder if this idea would work well with lower-gain transistors using leakier diodes?

[PRR]

> will the operation characteristics be similar?

Yeah, kinda, in a general way.

WHY are you asking the idiot simulator? Why are you asking a forum? Ask the transistor!! Spend a buck, set it up, listen.