Topband: Using a 40673 in a KD9SV 160m pre-amp?

[Steve Ireland]

G'day,

A friend of mine Phil VK6APH has recently built the famous KD9SV variable
gain 160m preamp, as published in Ham Radio October 1989.

As it was difficult to obtain an NEC 3SK88 that Gary used in his original,
he substituted a RCA 40673.  Although this is a very similar dual gate FET
to the 3SK88, it has internal diode protection.

Phil was very active on 144MHz back in the 1970s - the heyday of the dual
gate FET - and recalls the diode protection used on some dual gate FETs
meant that they could be prone to intermodulation/cross modulation problems.

Is it worth Phil trying to get hold of a 3SK88 to use instead of the
diode-protected 40673, or are the cross modulation/intermodulation problems
associated with the latter likely to be minimal?

The reason for using the KD9SV premap with its tuned input and output is
because other 1.8MHz preamps tried have been affected by the presence of
strong local medium wave signals, causing cross modulation/intermodulation
to occur.

Vy 73,

Steve, VK6VZ