I also included trimmers for Q1 to Q4 to allow you to bias each individually to around 4.5-5V. I just selected a trimmer that easily covered the drain resistor shown and so allow you to tweak significantly above and below the suggested resistor value. And again I included an axial cap because there was room for one that I have in stock, but a radial will do the job perfectly well if you don't have the axial.
I couldn't find any videos on this but a demo can be heard here.
And a layout based on the original scheme by GT Labs which John K thinks has a preferable drive control, and also including the original passive values with just a couple of tweaks:
and for those who would also prefer fixed resistors rather than trimmers this version uses the original drain resistor values, but I would suggest socketing as shown so you can fine tune the bias.