[HBR] Mixer questions and other tech questions..

[Walt Hutchens]

It's good to see the list alive again!  First, does anyone have any idea 
why all my messages to the hbr list are rejected as spam if sent directly
by email but are okay if sent via the webmail interface?  For the last 
year or two?  

The web interface is a pain, so I don't post much.  And this responds to 
several posts.

The W6TC HBR designs yield a receiver that is simple enough
mechanically and electronically that nearly anyone with reasonable
proficiency with hand tools can make one work well. However this
results in some limitations. This is a double conversion receiver with
a tunable 1st oscillator, that is, an oscillator that must cover (at
least on harmonics) the entire tuning range of the receiver.

That means that the 1st mixer almost has to be singled ended, since a
balanced oscillator output over this wide range is a very difficult
job.  On 10, for example, stray capacitances will greatly affect the
amplitude and phase of the mixer drive.  Trying to make this work with
conventional plug in coils would be a real challenge.  You COULD try
doing it by building your own balanced plug-in coils like the ones
used in transmitter push-pull stages.  Maybe use miniductor and tube
bases?

The advantages of a beam tube (6JH8, 7360, etc.) 1st mixer are very
high gain if the signal is placed on the control grid and there is
ample LO injection. However, 'ample' is at least several volts p-p and
10 or more would be better. The LO signal isn't rejected at the
(push-pull) mixer output. That's not too big a deal since the HBR LO
would be well above the IF and well above or below the signal
frequency, but it will mean a couple or possibly more spurs to think
about -- the basic HBR design is almost entirely free of this issue.

These receivers have an RF amplifier.  They don't need a high gain
mixer.  If you reduce the oscillator drive to the point where the gain
with a beam mixer is no more than needed, then you've got pretty much
what you'd have with the original HBR design.

On the lower bands -- say through 20 meters -- the biggest issue in HF
receiver front ends isn't gain or noise, anyway, but performance when
there are multiple very strong signals within the passband of the RF
tuned circuits.   If any stage is driven into non-linearity even by a
signal you're not actually hearing (because it is eliminated by the
IFs) the result will be distortion of the signal you're listening to.
This is 'crossmodulation.'

One tweak that will help in the front end is a 6EH7 as an RF amplifier.
This tube gives far better dynamic range than any of the tubes used in 
the original design, plus higher gain if you want -- the HBR's have a
separate variable cathode resistor RF gain control. Combine the 6EH7
with a 12AT7 or (better) a 6922 or 6DJ8 1st mixer in a push-push
circuit. (Cathodes are in parallel with about a 470 ohm UNBYPASSED
cathode resistor, one grid driven by signal, other by the LO, plates
in parallel. Grids return to ground through say 100k each.) This mixer
is very resistant to crossmodulation AND is semi-balanced against both
oscillator and signal appearing in the plate circuit. While the gain
is somewhat lower than other circuits, the RF amp will more than make
up, especially with the 6EH7.

This is no harder to get working than the original HBR circuit,
although the greater care you use in wiring the 6EH7 RF stage, the
more of the potential gain you'll be able to use on 20 and 10 meters
where high gain is sometimes useful. Both cathode terminals must be
bypassed to ground directly at the socket, all grounds return to one
point, leads must be kept very short, a shield across the socket is a
good idea, and so on.

If you don't do these things the maximum usable gain will be no lower
than for any other tube and you'll still have the wide dynamic range,
meaning less crossmodulation trouble.

> I was wondering about cascading two 85kc R-23 IFTs for a better
> shape factor. My plan was to use two transformers after the last
> mixer in cascade.

85 kc R-23 IFTs can be cascaded for better selectivity and shape
factor. If you're only going to cascade one pair, yes, put them in the
second mixer plate circuit, since that protects the rest of the IF
chain from out-of-passband signals that can cause crossmodulation.
There's no advantage to common inductance coupling. Adjust downward
from 100 mmf or so until you get distinctly lower gain, then check the
passband shape. I wound up with 68 mmf in my set.

I wouldn't try putting a T-notch between IFTs since tuning the notch
will change the tuning of the transformers as well. What about a
select-o-ject tunable audio frequency circuit? Or go with conventional
placement of the T-notch.

> I had intended to use a pair or even 3 6EH7's in my IF strip, and running
> cascaded pairs of these transformers at the first 2 coupling points. That 
> would be 6 transformers total. But now I'm thinking that a strip with 3 or 4 
> 6BJ6s would be easier to tame. I'll breadboard both and run some tests, I 
> suppose.

The 6EH7 is a superior tube because it was designed for IF service at
up to 40 Mcs in premium TV's where extremely large signals have to be
handled without crossmodulation. It's really the only pentode that I'm
aware of developed for wide-range signal handling ability at HF/low
VHF. Two is plenty for an HBR IF section. Three 6BJ7's might be a bit
simpler but short leads, careful orientation of the sockets, and good
bypassing isn't rocket science, especially at 85 kcs. You DO need
larger bypass caps than you might expect -- .05 mfd or more -- at
these low frequencies.

Just like in the command sets.  Fancy that.

Finally, my favorite simple receiver design uses a 12AT7 push-pull
mixer driven by a 12AU7 p-p oscillator; the osc. tunes 1400 kcs to
1900 kcs range giving coverage of the 80 and 40 meter bands LSB/CW/AM
by band imaging with an IF of 5400 kcs. The two IF stages are 19JN8
triode-pentodes; the first triode section is wired as a diode and used
as a bias rectifier for the AGC; the second is used as an AGC plate
detector with the cathode biased negatively. Manual control of the AGC
is by setting a minimum bias level, so the S-meter readings remain
valid. There's a conventional audio gain control.

Another 19JN8 serves as a crystal controlled (fixed-tuned) BFO and
pentode 1st audio, driving half of a 12AU7 2nd audio. The other half
of the 12AU7 is the audio plate detector. A 35W4 rectifier supplies
B+; filaments are in series. A 3" loudspeaker is built in; there's no
provision for headphone use. There's audio negative feedback from the
transformer voice coil winding to the cathode of the 1st audio.

Oscillator drive is to the mixer cathodes, signal to the grids, both
push-pull with single ended output from the paralleled plates to a
half-lattice crystal filter.  The unbalanced to balanced conversion is
by a capacitive tap on the 1st IFT secondary.

With no RF amp and a push-pull mixer, the thing has no crossmodulation
problems. With the mixer and both IF stages on the AGC line it has
excellent AGC and the oscillator stays within 200 cps or so from a
cold turn on; AGC causes no oscillator pulling. About the only special
parts I used were mica caps in the LO circuit. The IF coils are home
brew, using APS-13 forms and cans. The RF and LO must be separately
tuned because they tune in opposite directions on 80M; both use two
section receiving-type caps. The RF tuning cap has a built-in
reduction; the LO has a JB 6/36:1 dial.

There's a tweet in each band from the conversion scheme -- 3600 and
7200 kcs.

Adding a crystal controlled converter and reducing the tuning range to
get rid of the spurs should make a good all-band receiver. I started a
general coverage project along those lines a couple of years ago, but
life intervened; I'm hoping to return to it this winter.

> There are days I really feel that I started this project about 40
> years too late. :(

The good news is that we have to live an extra 40 years to make up for
it!

Walt Hutchens
KJ4KV