[Heathkit] HW-7 MOSFET question

[Dennis Berry]

I've a question on replacing the 40673 in an HW-7.  I've gone through my old
handbooks and looked on the web, but there is little to no information about
how to properly bias a dual gate mosfet.  Just sample circuits.  So, I'm
trying to figure out how to take whatever dual gate I find and use in the
circuit.  Should be able to be done, yes?

Now I know there are 40673's still to be had.  But heck, most of the fun is
trying to understand and learn, right?

I figure some of the newer dual gate mosfets in SMT packges should work
also.  It looks like their max voltage is lower than the old RCA part.  The
design process is applicable for any type device really.

It looks to me like the drain has about 10 volts on it after the 100 and
1500 ohm resistor.  And it looks like the gate 2 has about a 4.6 volt dc
offset with the LO injected on top of it.  On my HW7, that looks like almost
4 volts of LO signal riding on G2.  First question is does this mean  I need
to see >4V of signal on G1 to overcome the bias on G2?

Do these voltages sound right for others that have played with the HW-7?
The HW-7 schematic shows 3.3 on G2 and 12 on the D circuit, with a DC input
of 13 volts.  So I guess my readings at 13.6 are within reason.

Perhaps someone can help explain the low level signal on G1 (basically right
from the antenna peaked with a tuned circuit to G1) and how that
'mixes'/'amplifies' with 3.3 volts on the G2?  I haven't found an
explanation for that part of the circuit yet.

It would seem that to replace the 40673 with a newer BF988 should be easily
do-able, just once I understand the biasing.

And, I bet there are many that would like to understand this, so if someone
would be so kind as to share some wisdom on this topic, a lot of us would be
greatful, I'm sure.

Thanks to all in advance,

Dennis