The bias voltage is measured from drain to ground. So you measure between, from the trimpot lug going to the fet, and ground. Hope this helps.
Mind; I build the public domain version; I don't know how the commercial is build. Maybe a different approach to both FET.
How do I measure the bias voltage? All I know is that the FET has 3 pins and a flat on the body. Do I measure pin to ground or pin to pin? I know sod all about the electronics of these things I'm afraid so I'd be very grateful if you could be very specific.
Thanks
Hywel Harris