But the transistor circuit designers have gone out of their way to put in "tube like" behaviors which result in extra circuits and components that don't appear in the tube amp, and which would not appear in a "high fidelity" transistor amplifier.
One very common trick that has appeared in transistor guitar amps from the 1970's and onward is current feedback. The return terminal of the speaker doesn't go straight to ground, but feeds a tiny current-sensing resistor, like 0.1 ohms. Signal from the current-sensing resistor is combined into the negative feedback. This gives the amp a nonzero output impedance against the speaker, which changes the frequency response: more power is driven at the frequencies where the speaker impedes more, like its resonant frequency (about 70 to 100 Hz for a 12" guitar speaker) and high frequencies (due to voice coil inductance: rises from about 2 kHz up or something like that).
This doesn't reproduce everything that happens with tubes, but it goes a long way.
I built a circuit like that into an off-the-shelf amplifier, with a switch. I tell you, whenever I switch that off, it's not long before it goes back on again. Without the current feedback, it's sounds blatty/tubby and lifeless. It's not just the frequency response, because even if we dial in a similar EQ curve before the amp to eliminate the difference, it's not "it".
The TubeWorks people have an interesting design in the MosValve 500 amp. Rather than using current feedback, that amp places the MOSFET output stage outside of the negative feedback loop. Negative feedback is drawn from the VAS (voltage amplification stage) before the output stage. That means that the speakers will see the impedance of the MOSFETs. Plus the supposedly "tube like" overdrive characteristics of the power devices will come into play when that thing is cranked. Because they are outside of the NFB loop, it will just be soft onset clipping. Here is the important thing: unusually, the VAS and the output stage are on separate power rails, and those of the VAS are a significantly higher voltage (+/- 93V versus +/- 71V). So it is hard to make the VAS clip; it requires a much higher signal than what it takes to make the output stage go into progressive clipping. When an output stage is included the feedback loop, like in almost every amplifier out there, the amp is perfectly linear up to the limit, and then clips really hard.
One very common trick that has appeared in transistor guitar amps from the 1970's and onward is current feedback. The return terminal of the speaker doesn't go straight to ground, but feeds a tiny current-sensing resistor, like 0.1 ohms. Signal from the current-sensing resistor is combined into the negative feedback. This gives the amp a nonzero output impedance against the speaker, which changes the frequency response: more power is driven at the frequencies where the speaker impedes more, like its resonant frequency (about 70 to 100 Hz for a 12" guitar speaker) and high frequencies (due to voice coil inductance: rises from about 2 kHz up or something like that).
This doesn't reproduce everything that happens with tubes, but it goes a long way.
I built a circuit like that into an off-the-shelf amplifier, with a switch. I tell you, whenever I switch that off, it's not long before it goes back on again. Without the current feedback, it's sounds blatty/tubby and lifeless. It's not just the frequency response, because even if we dial in a similar EQ curve before the amp to eliminate the difference, it's not "it".
The TubeWorks people have an interesting design in the MosValve 500 amp. Rather than using current feedback, that amp places the MOSFET output stage outside of the negative feedback loop. Negative feedback is drawn from the VAS (voltage amplification stage) before the output stage. That means that the speakers will see the impedance of the MOSFETs. Plus the supposedly "tube like" overdrive characteristics of the power devices will come into play when that thing is cranked. Because they are outside of the NFB loop, it will just be soft onset clipping. Here is the important thing: unusually, the VAS and the output stage are on separate power rails, and those of the VAS are a significantly higher voltage (+/- 93V versus +/- 71V). So it is hard to make the VAS clip; it requires a much higher signal than what it takes to make the output stage go into progressive clipping. When an output stage is included the feedback loop, like in almost every amplifier out there, the amp is perfectly linear up to the limit, and then clips really hard.
The schematic: https://img.photobucket.com/albums/v202/xr3drumx13/MV500Sche...
Tube amplifiers "do their thing" without extra gimmicks like this, though.