[HBR] The long, SLOW HBR project

[Walt Hutchens]

> 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.

That was almost a week ago.   A few hours of work since then have produced
mostly negatively useful results: I've got a pretty good idea of the reasons
for the low 20M sensitivity.

Basically the problem comes from low oscillator injection.   If I use my
little solid state GDO to inject an LO frequency signal things get a lot
better.   The big difference is probably the oscillator circuit.   The use
of tickler feedback oscillators (in the HBRs) favors harmonic generation
because the impedance seen by the screen grid (which serves as the plate for
the oscillator section of the tube) rises with frequency.

The Colpitts-derived circuit I'm using is likely to be the poorest for
harmonic generation because both grid and place circuits look into
substantial capacitance.

Additionally the HBR-series sets have much more gain in other places so
somewhat low 1st mixer gain could be compensated by turning up other
controls.   I don't think those sets are run with all manual controls wide
open in any situation.    My little project is designed to run with the AGC
threshold control full up, with the AGC controlling the actual gain and
there are no other manual controls.  It has PLENTY of gain -- under good
conditions on 80M it'll copy strong SSB signals without the antenna
connected! -- but there are no controls to crank in even more on other
bands.   

A contributing factor is the difference in oscillator plate supply voltage:
The real HBRs run them at 150 volts, I have 85.

So, there are two ways to go here:  One is to redo the oscillator as a
tickler feedback circuit.   That has the appeal of a tested circuit with
already-designed coils and I already have the right tubes in place.  There's
still the issue of low plate voltage but the voltage could be raised if
necessary.    

The other approach would be an oscillator specifically designed to generate
a second harmonic.   You can do that with a push-pull oscillator if you take
the signal from a common cathode.   Push-pull has other advantages: The use
of two tubes allows even looser coupling to the tank circuit and since the
tube capacitances are effectively in series you get another factor of two
help in that department.

Push-pull oscillators are usually symmetrical in design but with such narrow
bands a slightly unbalanced circuit (tuning cap on only one side) should
work okay.   

A push-pull circuit delivers the fundamental and second harmonic at
different points so you have to switch per band.   That can be done in the
plug in coils and gives the added advantage that you don't have a large
trash signal (the fundamental when on the higher bands and harmonics when on
40M and below) bringing new forms of noise and distortion to the mixer.

(The high Q mixer coil probably takes care of this, but  it's better not to
inject an unwanted signal at all than to inject and then mostly kill it.)

At the moment I don't know.   But I won't have time to work on it today,
anyhow.   

Walt
KJ4KV