Dual Gate N−Channel MOSFET TO72 Type Package Absolute Maximum Ratings: Drain−Source Voltage, ... Gate 2 Leakage Current IG2SS VG2S = 5V, VG1S = VDS = 0 − − 10 nA In figure 4 below I have depicted the schematics of the two most popular types. A J-FET and a dual gate mosfet. Typical types might be MPF-102 for a J-FET and the old RCA 40673 for the dual gate. Figure 4. - schematic of J-FET transistor and dual gate mosfet transistor. The FET of course is characterised by its extremely high input impedance.
Those RCA 40673 Dual gate MOSFET's For a dead receiver pre amp card the Dual gate MOSFET burned out. (shorted?) The FET has 10vdc gate protection , internal diode clamps. Almost like TVS's function, but can be blown up. (TX'ing in to it would smoke em', as would lightening hits)