A simple circuit with an optocoupler creates a “tube” sound

[Steben]

One thing I found puzzling was that it had voltage gain, despite being set up common collector. You can see this in LTspice sims too, so it must be a known property of the optocoupler. Any idea what's going on there?

common collector (or emitter) implies common to two circuits ... here, the circuits are decoupled

so, voltage in >> LED current >> photons/lumens >> base current >> hFE times base current times emitter resistor >> emitter voltage

potential for overall voltage gain production is there provided product of all transfer ratios amounts to something greater than unity


the bipolar transistor is working as a current amplifier here, and not a voltage follower since there is no voltage source at the base

Thats quite non linear.
2nd harmonics, here we come

[Eb7+9]

Thats quite non linear.
2nd harmonics, here we come

and not just any kind of non-linearity ... it's the soft clipping that matters, the expo-like part near the origin

exactly the same "outcome" as my jFET-based idea: http://www.lynx.net/~jc/transferCurvature-TubeSimulation.html

amazing what a search on tube or triode emulation yields these days ...

[Steben]

Thats quite non linear.
2nd harmonics, here we come

and not just any kind of non-linearity ... it's the soft clipping that matters, the expo-like part near the origin

exactly the same "outcome" as my jFET-based idea: http://www.lynx.net/~jc/transferCurvature-TubeSimulation.html

amazing what a search on tube or triode emulation yields these days ...

Although linearisation by means of a source resistor is exactly what brings a jfet curve towards a triode curve 
Do we really want all those 2nd harmonics each stage?
Not just clean but even in overdrive mode. High distortion level combined with a lot of 2nd harmonics is fuzz land.