[HBR] Need help with noise

[Walt Hutchens]

Ian said:

> The two 6EH7s in the IF strip are AGC controlled.

> The first mixer uses a 6DJ8 with the second triode as cathode follower
> (a Pullen mixer, with the different gms of the two stages arranged via
> different plate voltages on the two triodes).

This is likely the problem.

I once lived in an urban area where my full wave horizontal loop for
80M would deliver half a volt RMS at 75 ohms on an open circuit. Even
in rural places with less flat-out garbage in the ether there's too
much power coming from a resonant antenna for such a sensitive front
end.

One of my all-time favorite receivers is a seven-tube series filament
band imaging 80-40 set: 35W4 rectifier, 12AT7 mixer (signal push-pull,
injection push-pull, output single end) to a half lattice crystal
filter at 5500 kcs with a 12AU7 push-pull (Colpitts-derived)
oscillator driving the 12AT7 cathodes.

IF is two stages of 19JN8 sharp cutoff pentodes. Two plate detectors
deliver the audio and AGC signals. AGC is applied to the mixer as well
as the two IF stages; because of the low LO frequency this doesn't
pull the LO frequency significantly.

Note that there's very little gain -- just a triode mixer -- ahead of
the crystal 'knothole.' And that the push-pull mixer tends to cancel
distortion products so it's more linear on the signal path than a
single triode would be.

The antenna/mixer coil is a 1" toroid wound full of #24.

The gain that isn't there in the front end is made up in the (plate)
detector so the IF signals stay small -- fraction of a volt. All you
want there is enough to be sure of overcoming 1st IF noise at the
input and driving the AGC at the output.

In your set, the 6EH7 -- designed for TV IF service -- gives you a lot
of flexibility.

Ham signals on 40 were easily copied (at 8AM local) on this set with
no antenna -- the unshielded antenna coil and wiring were enough and
touching the high-z antenna terminal gave room-filling volume from a
3" speaker.

Gain is a non-issue for an HF set, at least below 20M. Low noise is
unimportant below 10M because there's so much noise coming with the
signal from the antenna. The challenge through 40M is the hatchet work
of getting rid of the off-channel signals before they drive something
into non-linearity. The mixer is the most likely victim and in my
opinion the Pullen has a kick me sign on it.

A great advantage of the W6TC designs is very high Q RF-mixer tuned
circuits -- maybe 150-200. These much reduce the amount of garbage
reaching the mixer. Those tiny HR-10 coils might have a Q of 50. The
greater the front end bandwidth the tougher the mixer has to be.

I once saw a speculation from some receiver designing ham about the
characteristics of an 811A as a mixer. He was kidding, but the point
was that the first requirement of an HF front end is robustness.

Things get a little harder for an all band receiver because 20-10 do
need some front end gain. The excellent G2DAF design uses a 6ES8
push-pull triode RF stage operating at unity gain but with gain
REDUCED when needed by the AGC so it's really just an AGC controlled
attenuator. The mixer is a 6DJ8 in push-pull -- there's your front end
gain. That's one way to do it.

For all-band service you pretty well have to gain control the RF stage
-- either manual or AGC. There's just too much change in the
conditions between 160 -> 40 and the higher bands.

I would rethink that front end.

Walt Hutchens
KJ4KV