Output impedance in mixed-mode feedback production guitar amps: kinda high?

[Kaz Kylheku]

I've been looking at various schematics of production amps that use voltage + current feedback: Marshall Valvestates, Fender Deluxe 85, Trace Elliott SuperTramp, 1979 Roland JCM-120 and others.

I build a simplifed facsimile of their feedback loop, with some details omitted like Zobel network and caps, and go through a procedure of finding a load resistor value which cuts the open load voltage in half.   That V/2 resistor value serves me as a kind of rule of thumb nominal output impedance.

I'm seeing pretty high values for this, like 24 or more in some cases; way in excess of nominal speaker impedances.

It seems like designers of some guitar amps have gone to town with "goosing" the current feedback. Definitely not keeping the Zout below the nominal speaker impedance.

Is that the way to do it?

[Loudthud]

My method for finding output impedance is basically to setup the amp with an 8 Ohm resistive load at about 1 Watt. Measure the output Voltage, then connect an additional 8 Ohm resistive load without changing the generator or any controls and measure output Voltage again. Use Ohm's law to calculate output current from Voltage and Resistance. The output impedance is <Change in Voltage> divided by <Change in current>.or:
(Voltage@8-Voltage@4)/(Current@4-Current@8)

Expect to get different readings at different frequencies with Presence and Resonance controls in the feedback network. Yes, guitar amps have high output impedances, try measuring an amp with NO feedback !

[Kaz Kylheku]

Use Ohm's law to calculate output current from Voltage and Resistance. The output impedance is <Change in Voltage> divided by <Change in current>

So what Zout do you shoot for?

[g1]

As far as intent goes, don't forget this is pretty much the same thing as damping factor.
Tube amps are relatively loose in this regard.  Some of those amps are trying to sound like tube amps, the Valvestate in particular.

[Kaz Kylheku]

As far as intent goes, don't forget this is pretty much the same thing as damping factor.
Tube amps are relatively loose in this regard.  Some of those amps are trying to sound like tube amps, the Valvestate in particular.

I've run some simulations of somewhat simplified Valvestate circuits. Output impedances are as high as 25Ω. (In terms of that being the purely resistive load that cuts voltage in half relative to open.)

I'm experimenting with a (simlarly derived) Zout of around 18 Ω. I like some aspects of it, but the mid scoop is so wide that (my hearing tells me) it's cutting out upper mids.

[Loudthud]

So what Zout do you shoot for?

My latest solid state power amp measured 60 Ohms. Sounds very tube like