where does distortion come from??

[txstrat]

The biggest difference is the circuit around the first gain stage. 220k plate resistor and (shared for two triodes) a 1.5 k cathode resiistor (double it's value for only one triode) - plus the tone stack, that makes a big difference too.
Then you can expect a nice overdrive sound when you turn up the voulme.

[pdf64]

Regards negative feedback, the 820ohm resistor is only half the picture, the resistor to ground is 47ohms on the ab763 deluxe, whereas its 100ohms on most other ab763 schematics (probably to compensate for the 8ohm ouput of the deluxe compared to the 4ohm output of most other amps). The 100ohms will give less gain than the 47ohms.
I much prefer to get rid of that negative feedback loop altogether. Peter.

[txstrat]

The 100ohms will give less gain than the 47ohms.
I much prefer to get rid of that negative feedback loop altogether. Peter.

That's right.
No nfb will give you an earlier breakup of tone in this case.

[loudthud]

One thing that rarely gets checked is the heater voltage. It's taken for granted on most amps, but with so many companies making transformers these days, you never know unless you actually measure it at each socket. It can make matched tubes unmatched and preamp tubes lifeless. Bad solder joints, wire that's too small, weak tube sockets and distorted waveform coming from the PT can reduce the voltage getting to the tubes. It may go down when the standby is put in play mode (especially with solid state rectification).