I built the one from Elektors book once upon a time. 303 or 305 or 309 circuits or whatever it was called 
I remember that when biased to 400mA the distortion was very low and... well I was not an audiophile back then, but to me it sounded ok.
Bias compensation was done with an NTC (connected to some pins on the LM391) attached to the heatsink.
Without the NTC on the heatsink thermal runaway occurred.
I recently bought an amp that was horribly built but had 4 of these in sockets so I took them and will maybe use them some day, unless they go to some repair project
If someone is really interested I will try to find the book and post the schematic here.
I remember that when biased to 400mA the distortion was very low and... well I was not an audiophile back then, but to me it sounded ok.
Bias compensation was done with an NTC (connected to some pins on the LM391) attached to the heatsink.
Without the NTC on the heatsink thermal runaway occurred.
I recently bought an amp that was horribly built but had 4 of these in sockets so I took them and will maybe use them some day, unless they go to some repair project
If someone is really interested I will try to find the book and post the schematic here.
Will take a few days for me to find it.
Meanwhile...I am 99% sure it is this one:
https://www.electroschematics.com/guitar-amp-lm391/
Meanwhile...I am 99% sure it is this one:
https://www.electroschematics.com/guitar-amp-lm391/
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