Solidstate vs Tube Gain Staging

[Littlewyan]

I've started going down the rabbit hole of developing overdrive circuits for pedals, in particular pedals that emulate valve amps. Something I've noticed looking at other designs is they don't always match the same number of distorting stages as the amp but still do a good job emulating them. Why is this? Using logic you'd think you'd require the same number of stages. One example is the SLO Overdrive Pedal, this only has two distorting stages when the amp has 3 (4 if you split the last gain stage and cathode follower), but still sounds close.

Interested in others thoughts on this.

[Stevem]

Have you looked at how much gain / signal level gets dropped when passing thru a passive tone stack when even just adjusted to normal settings?

[LOUDthud]

Can you provide a schematic or link to the SLO pedal ?

[Littlewyan]

Have you looked at how much gain / signal level gets dropped when passing thru a passive tone stack when even just adjusted to normal settings?

Yes but not really sure how this links in with how many stages are distorting. In an SLO the tonestack is at the end of the preamp, once all the distortion has been done. Unless you crank the master of course but it's not really an amp for that. And there's a big difference in how many stages distort a signal versus how much you distort a particular stage. The dynamics are very different.

https://www.pedalpcb.com/product/pcb596/

[Stevem]

A lot depends on what type of distortion sounds good to your ears.

As a tube clips harder and harder it produces odd and even order harmonics and each in vearing degrees .

These two types of harmonics can aid or detract in what you’re looking for sound wise.

To avoid types of distortion that sounds bad to most folks and how also certain types of distortion can reduce sustain ( the main reason we like distortion) you need to use only a certain level of distortion/ overdrive from each gain stage that you may need to get the type of harmonic overdrive you might like.

[R.G.]

It does make a kind of sense that similar sounds would need similar numbers of stages. The flaw in that line of thinking is that all stages are not equal, and even more so when the stages being compared are thermionic versus solid state.

Tube stages, especially triode stages as used in guitar amps, are very similar. When you pick the operating mode - common cathode, common plate, or common grid - you largely pick the range of gains that are possible, given that the 12AX7 is by far the most common tube used for signal stages. There is a tiny range of transconductance ("Mu") available, and all the tube stage gains involve mu as the major parameter. Mu is the ratio of plate current change divided by the change in grid voltage. Pretty much everything else falls out as a result of using resistors around the tube to use that amps/volt transconductance. The typical mu of a 12AX7 is in the range of 1ma per volt to 1.5ma per volt.

This is most certainly not the case for semiconductors. Semiconductors have much wider variation in their effective transconductance, and generally a much higher transconductance in general. To pick a gee-whiz number, a big MOSFET may have mu in the region of one amp per volt, a thousand times higher. Then there's that fact that a huge range of semiconductors are available - bipolars, JFET and MOSFETs are just the main classes, and there are hundreds or thousands of types inside these groupings.

In my mind, this makes the question of how many stages moot. All stages are dramatically not the same from tube to solid stage.

I believe a better question might be that of the output voltage size and waveform.

[Helmholtz]

. There is a tiny range of transconductance ("Mu") available, and all the tube stage gains involve mu as the major parameter. Mu is the ratio of plate current change divided by the change in grid voltage. Pretty much everything else falls out as a result of using resistors around the tube to use that amps/volt transconductance. The typical mu of a 12AX7 is in the range of 1ma per volt to 1.5ma per volt.

You mean the transconductance gm. With tubes it's defined as gm = d(Ip)/dV(gk). Unit is A/V or Siemens (S).

(Mu or µ denotes the voltage amplification factor, defined as µ = d(Vp/d(Vgk) = rp * gm. Unit is V/V.
It corresponds to the max. possible voltage gain. Typical µ of a 12AX7 is 100.)

[R.G.]

yeah what you said...

[LOUDthud]

Looking at the link the OP posted in reply #3, I see only opamps and diodes. Not a single transistor or FET. Is there no schematic available ?

Edit: AH, there is a schematic: https://docs.pedalpcb.com/project/LeadS ... dalPCB.pdf

[R.G.]

Ah.Thanks L.T.!

@OP: An opamp is typically three stages internally, a differential amplifier, a voltage gain stage and a high(er) current buffer output stage. This is all obscured by feedback into a composite stage until such a frequency that the internal gain reduces and lets the internal stages show through.

Feedback composite stages are another reason that all stages are not equal.