[R-390] 6DC6 Replacement

[Bob Camp]

Hi,

More or less in the order asked:

> I am looking for:
> equal or better sensitivity.

Certainly a reasonable thing to want.

> equal or better gain.

Probably not a good idea to go for more gain. You will degrade the radio's
performance.

> lower device noise.

Good idea, but it's the same as sensitivity.

>

It would probably be a good idea to add similar or better AGC action and
higher intercept point(s) to the list.

The R-390A is already chalanged in the intercept point race, and several
radios of the same era (R-390 not an A) beat it in RF selectivity. If you
degrade the AGC things only get worse in this department.

So how to come up with a solution:

1) A reasonable RF FET these days has at least 4X the gain of a 6DC6. Noise
figure is not to hard to beat with a FET.

2) There isn't much that will run at high voltage anymore. The days have
long past when you could get RF small signal parts that ran on 120 volts or
so. The large signal stuff has enormous input and output capacitances.

3) The input and ouput capacitances of a single FET are a bit higher than a
6DC6 so if it a plug in mod you will have to get creative.

I suspect that leads you pretty quick to a cascode JFET arangement. That
would get the output and feedback capacitances up into a reasonable range.
Ouput admitance might be an issue but I doubt it.

That gets us to AGC. Probably time for some kind of op-amp and PIN diode
arangement in the middle of the cascode. Sounds kind of messy.

In order to get all that running and not mess up the second and third order
intercepts you would need to fiddle around quite a bit.

    Good Luck!

        Bob Camp