


Since your MOSFET is a 1972 vintage, it's probably not a very good one by 
today's standards.  If you have an idea about its voltage and current 
ratings, e.g. 60VDC @ 6A, you can probably get away with replacing it with 
anything with better specs.  Early MOSFETS had a gate-source voltage rating 
of approximately +/- 20 VDCmax, and they would usually turn completely "ON" 
at +10VDC.  Otherwise, MOSFETS are not really mysterious -- they're more or 
less voltage controlled current sources.  If the MOSFET in your circuit is 
used as an open-loop, voltage controlled current source, you may have to 
experiment with various gain-altering techniques.