twin BJT guitar amp stage

[Steben]

Another take on solid state guitar circuitry.
As many might know transistors are in essence much less linear amplifiers (exponential response) than tubes, especially triodes (+/- square law).
A way to reduce non-linearity is using feedback on high gain structures. But the best guitar tones are not linear and triodes usually are the holy grail.
Based on some input recently I started thinking of the following circuit in which BJT's are fully used in their advantages. It is basically an emitter follower (buffer) and a feedback common emitter not much different than the clean channel Peavey darlington stage. But both biased with a lower voltage, putting the working point in a less linear region. It forces the circuit into distortion yet the feedback reduces it.
This stage gives high input impedance without fets but uses the slightly reduced non-linearity of BJT's to mimic the square law response.
It uses BJT in a way less sensitive to the specific current gain given the emitter follower and feedback and is completely free of finicky characteristics like jFETs have (Vgs(off), current, ...)

The Fourier analysis on the right shows the calculated harmonic components at different biases with 0.05V signal. These values are not stellar as in clipped waveforms. They are there to compare to tube performance and to search for a profile similar to what is called a pleasant "clean" sound.

[antonis]

It should be interesting to have Fourier analysis for x10 and/or X20 signal voltage, also..

[Steben]

It should be interesting to have Fourier analysis for x10 and/or X20 signal voltage, also..

50mV input, feedback resistor from 50k to 200k:


feedback resistor 100k, input from 50mV to 350mV:

[Kipper4]

Does t sound good?

[Steben]

Does t sound good?

still tweakin'. But the key sound is the Peavey Bandit clean channel. Transtube is a much underrated design... Now I am pushing the bandit with a soul food on low drive and clean channel volume on 10. It sounds great, lush clean to full creamy drive compression with guitar volume. All the clipping comes from the power amp emulation, the preamp does not clip.

[Kipper4]

And if you play it through a solid state clean amp does it still have that tube feel ?

[Steben]

And if you play it through a solid state clean amp does it still have that tube feel ?

"Tube feel" does not come from preamp circuits.  At least not IMHO. They can add harmonic profile. Just as preamp tubes do. I never thought of tube feel with preamp tubes.
Compression comes from the power amp or power amp emulation.
In other words: this stage is all about basic tube (clean) harmonics, not tube feel. If that makes any sense at all. 

I remember seeing many guitarist complain about the distortion channels on many SS amps, while calling the clean channel ok. That clean channel actually has as good as no harmonic content as such. Those amps are completely other babies than emulating circuits.
It goes along with my love for transtube circuitry and the question I always have in my mind: did you ever play a bandit on 10?

[Kipper4]

No such luck, oddly enough never played a Peavey.

[Steben]

No such luck, oddly enough never played a Peavey.

Bandits are rather cheap. Still think many don't appreciate the things because they need to have volume high as a tube amp, which many don't.
I get the designers in going all the way, but I don't get the lack of using the good stuff at low volume which is what good tube amps can't.
I added a master volume control after the emulation and that problem is solved now at room level.