[HBR] The long, SLOW HBR project

[Walt Hutchens]

> Once I figure out where to mount the new parts permanently,  I think I can
> look at the mixer to crystal filter mismatch issue!

Other than an intermittent plate-to-?? short in the (recently replaced) 2nd
IF tube that took out the plate decoupling resistor, nothing else got in the
way.   One MORE new 19JN8 and one 680 ohm resistor later ...

I rewound the primary of the 1st IFT from 25 turns to 50 and that much
improved the mixer to crystal filter match issue.  Instead of two rather
widely separated peaks the crystal filter now has a slightly humped and
slightly narrow single peak.  It sounds MUCH better.

Probably taking off 5 turns will deliver the wider, slight-dip-in-the-center
peak that I want.   The 'wider' would be good because it's currently around
1 kc, and the dip in the center with a rise at the edges means more carrier
is passed when receiving AM.

Then I dug into the 20M 'no sensitivity' problem.   I won't go through all
the stuff I looked at that wasn't wrong.   The REAL trouble (the reason it
was all but dead) was that I had exchanged two connections on the mixer
coil.   This didn't bother 80M because on that band the reversed connections
are jumpered inside the coil but on 20, they caused big trouble.

Changing the wiring of the set to conform to the coil got it going.   Don't
ask me why I didn't do a diagram of the coil as I assembled it, as I have
for the others.  

I found this finally by using the GDO on the coils in the set.   There were
other clues but nothing that really focused me on the right part.

I did turn up another problem:  I had forgotten to replace the 12BZ6 LO tube
with a sharp cutoff tube when changing to using harmonics in the LO.   Using
a 12DK6 is definitely better.

There may be even better tubes (those frame grid pentodes?) but AFAIK none
of them have 150 mA heaters.  I also wonder about something tricky involving
a 12BN6 to get narrower pulses of plate current, hence a higher ratio of
harmonic to fundamental output.

The gain on 20 probably isn't adequate yet: The strongest signals are strong
enough (kick the meter up to S9 territory), but 'enough gain' is enough to
hear the receiver front end noise with ease with the volume turned up and
we're not there yet.   And the (4th harmonic of 3500 kcs) band edge marker
still doesn't move the needle -- that's not a good sign.

I need to actually measure the sensitivity with a signal generator.   I'm
betting that this will require another grand tour -- checking all the things
needed for adequate gain.

Certainly the LO output could be a bit low.   I'm running it at 85 volts; I
think the real HBRs run 'em at 150.   On the other hand, the real HBR's use
6BH6's for oscillators ... half or less the transconductance?

You could make a case for adding another 5651, regulating a negative voltage
too, and using that on the cathode of the oscillator.  However running
higher power to a LO is a last resort.

Walt
KJ4KV