[HBR] Need help with noise

[Walt Hutchens]

Ian said:

> With the RF amp replaced by a 5pF capacitor (turning the two LCs
> into a sort of double tuned preselector), the receiver behaves way
> better, with what I regard as normal S/N for a not-very-narrow BW
> system (the IF filter is 2.5kHz wide).

EXCELLENT!

The blind alleys I've slept in myself are easier to recognize than the
others ...

> My intention for the RF amplifier was to provide a small amount of
> gain (for poor antennas) with a manual RF gain control to act as an
> attenuator for strong signals. I expect the 6EJ7 to have good intermod
> characteristics, if I can get it tamed.

Why not a 6EH7 there? That eliminates any question of matching AGC
characteristics (with the IF stage tubes) and the 6EH7 is a
superlative large signal performer, especially at higher bias (AGC)
voltages. The gain is less than the 6EJ7 but that too is in the right
direction.

If the RF stage is fixed tuned (and must thus be broad) then much of
the gain reduction can be done by loading the tuned circuits as in the
HR-10. (Maybe a combination of loading and stagger-tuning to give a
sharper drop-off outside the band you want to cover.) If it's tunable,
then you want as high a Q as you can get (since it can't be high
enough to make tuning awkward) and gain reduction can be accomplished
with cathode bias on the tube and mismatching, perhaps by link
coupling to the two coils. Of course that complicates the
bandswitching for an all-band receiver.

Another variable is the 1st mixer: Since you have excess gain to play
with you could think about a lower gain mixer; maybe a 12AT7, maybe
even a 12AU7.

> Will get back to this receiver in a week or two. I think it will work a lot
> better if I can, indeed, rethink the front end.

It sounds like that's the only major issue.

Walt Hutchens