"Pete KE9OA" wrote in message
...
The RF preamp is a 3SK195 and the 1st mixer uses a pair of these
balanced for RF and driven in parallel by the LO. Is there any point in
paralleling a second 3SK195 (piggyback style) on the preamp? on the
mixer? Or changing out the transistor type? How would one determine/set
the correct operating point?

The radio doesn't seem bad in this respect. I haven't had one for
several years, so I wouldn't be able to give an intelligent answer
without the schematic. You could look at the Icom R75 schematic and see
how they implement their RF amp. I believe that they do use the
configuration that you suggest.

I really meant piggyback! No additional components but probably would
have to change out the resistors to set the operating point correctly.
Wondered if it was a crazy idea or had merit.

I am not sure about that one.

Pete, I looked up the Icom R75 schematic as you suggested and I see that
they parallel 2 2SN2171 FET's, pin-to-pin, in the 1st RF stage. However,
it's a grounded-gate JFET configuration while the DX-394's 3SK195 is a Dual
insulated Gate MOSFET common source (unless I have the source and drain
mixed up).

Do you think there would be any reason that it is not feasible to parallel
them? There is a 100ohm resistor in series with the primary winding of the
output transformer on the drain, and a 100 ohm resistor and transistor for
AGC in series with the source. If I cut the 2 100ohm resistors down to 50
ohms, I should get about the same operating point as with one transistor,
right?

BTW, the 3SK195 is described as being for TV Tuner, VHF RF Amplifier
Applications, FM Tuner Applications and exhibits Superior cross modulation
performance, Low reverse transfer capacitance: Crss = 0.015 pF (typ.), and
Low noise figu NF = 1.1dB (typ.)

The R75 uses MA77 silicon epitaxial planar band switching diodes on the
outputs of the preselector. These are similar to the DX-394's HSU277's.
However, on the input side, it uses 1SV265 silicon epitaxial PIN diodes -
from the spec sheet, 2.5ohms @10mA, 0.26pF @13V. Now that's interesting -
using the PIN diode on one side only.

73, Tom