Yes, you are right.it looks like a mosfet is being used with an op-amp in a "voltage stabilizer" type circuit.
They are enhancement types.Looking at the diagram, the discrete semiconductors seem to be drawn as depletion mode MOSFETS. The problem though, is that when I've been looking up the parts numbers, the only parts I've found with suffixes that match these items are not depletion mode mosfets. In the case of the 2N6761 the Fairchild spec sheets list it as an enhancement mode device.
The symbol drawn indicates so by not showing an internal diode getting into (or out of) the gate symbol (because there's none there); the parallel gate line showing it's insulated from the channel (by a thin glass layer) and the channel drawn with an interrupted line, meaning the "natural" state is open.
The schematic is quite dated/venerable/obsolete/vintage, take your pick, even in the way it's drawn, maybe that confuses you.
Probably any good op Amp (think TL071) will work there.the proper supplier/part number for the semiconductors listed in the voltage stabilizer circuits shown below? So far I've determined that the OP176 is an obsolete product by Analog Devices. Its been replaced by the OP184. I've had no luck ID'ing Q8 (6761) or Q9 (8401).
I've found the 2N6761 to be a 4,5A 450 or 500V device, quite obsolete by the way.
2N6761 Datasheet pdf - N-Channel Power MOSFETs/ 4.5A/ 450V/500V - Fairchild Semiconductor
I'm sure an IRFP450 will replace it nicely.
Too sleepy (it's 04:50 here now) to follow 2N8401, although i'm sure I'll find it in my ooooollllddd D.A.T.A. handbook rather than on the net. (yep, it's *that* old).
If you found its datasheet, post it here.
These supplies or regulators are very old, needlessly complicated, and they look like the stuff used by the NASA or Vietnam era planes, probably worth U$24.000 each , meaning "taxpayer dollars" and "1968 dollars", not joking about that.
What do you need to be done exactly? (As in Volts in->Volts out @ so many milliAmperes)
It can be done today with *very* simple circuits, although they'll have an error of, say, 500mV , for less than 10 bucks each.