Its definitely a depletion mode MOSFET, thats what the symbol means. Enhancement mode devices have a broken line for the channel, depletion have a solid line.
There seems that there is a bias problem with the connection in series of all the diodes of the Thermaltrak transistors. This was an error in the first amplifier schematics published using these transistors.
True and neccessary for the design involved.
I've made a very rough and haphazard scetch to understand the (nonamplifying) endstage with the twisted and curled temp-diodes.
All boiles down to the very current source.
Indra is right, but serves better targetting.
What's available to Unimog considering low Id dep mosfet's to tackle the 'mystery 1041' ?
Attachments
Other then all those pot-based bias configurations, I like this high power intinsic correcting biasing without hassle. Smart design. Dubble control. Hats of gentlemen!
The amplifier now appears to be running well with Q7 temporarily replaced with a potentiometer. So the depletion mode mosfet seems to have been the problem. Now I am concerned about what else was changed before the amp made it to me, so I am looking through the operation of the rest of the circuit. It can be fully powered up now, which has improved test conditions a lot. I appreciate the recommendations for an appropriate fet and will set up a little test circuit for it. If I run into trouble with that circuit I will post again. Thank you all for your assistance!