[HBR] HR-10 to HBR project

[Walt Hutchens]

To recap: The front panel has been replaced and the front end is in pretty
good shape, the 80M plug in coils are done except possibly for tweaking, the
RF stage, mixer and oscillator are working fine.

About the only exciting part of that work was that the 6EH7 RF stage
oscillated when first built.   I had put a plate between the antenna and
mixer coils but testing with a scrap of tin here and there around the sides
showed that the plate was too small.   A new plate about an inch larger each
way solved the problem.

The audio section has now been rebuilt and converted to the 'sort of hi fi'
circuit I use in receivers:  Feedback from the voice coil winding to the
cathode of the 1st audio stage.  This required exchanging the plate and HV
connections on the output transformer to get the right feedback polarity.
(Couldn't swap the secondary connections because the secondary is 0-8-500
ohms.)   I kept this function in one tube but substituted a 6LF8 for a
saving of 0.15 amp of filament power.

A plate detector circuit has replaced the former diode -- actually this is
the audio version of the push-push mixer used in the receiver front end, but
with a 12AX7.    The BFO was relocated from its former home with the 2nd IF
and is now a 6BH6 ECO.   I have a temporary (1N4148) diode providing AGC for
testing.

The second IF (originally a 6EA8) has been replaced with a 6LF8: This has a
semiremote cutoff pentode.  When I replaced the socket -- none of the
sockets in this set can be trusted to make good contact! -- I left out the
tube shield as my experience is they're never needed.  OOPS!  Make that
ALMOST never needed: this stage oscillated until I shielded it.

I suspect there's more to learn here as this stage is not pushed very hard.
The IF coils are high-Q as such things go -- pi wound with adjustable
powdered iron cores -- but the cathode resistor is 470 ohms and the screen
voltage isn't very high.  I did explore bypassing issues but nothing jumped
out.  ???

With this stage straightened out the receiver works decently, although the
gain is low and the AGC isn't much help.   There's very little drift as the
LO coil is in the former accessory calibrator socket (at the right rear
corner of the chassis) and doesn't warm up much at all and the 6BH6 ECO is
mounted 1/8" above a 9 pin socket hole so some air flows up around it,
minimizing heating of connected parts.

I'm working now on replacing the 6BA6 1st IF stage with another 6LF8.   When
that's done the triode in this tube will be diode-connected as a rectifier
to supply -100V or so for the AGC circuit and the (now unussed) triode in
the 2nd IF stage tube will become an AGC plate detector.   This will give
far better AGC, though I may have to raise the overall gain somewhat.

The advantage of this AGC system (compared to the usual diode) is that you
can run the signal in the 1 volt range at the IF output, rather than needing
10-20V for AGC.  That means better linearity (less distortion) in the IF
stages and ultimately, cleaner sounding signals.   The disadvantage is
slightly fussier design and the need to adjust what amounts to a DC
amplifier, namely that AGC detector.   Furthermore, that adjustment must be
redone whenever the line voltage changes significantly: This is not an ideal
design for portable service.

The adjustment could be eliminated by adding another IF stage driving ONLY
an AGC diode but that requires an additional IFT; there's really not room in
this project.  

Part of the reason the AGC is weak now is that the original design depended
on that:  With the 1940s SSB/CW detection scheme (BFO coupled into the IF)
it is necessary to use the RF gain control to control volume when the BFO is
on: It wouldn't work well to have good AGC fighting your volume control
efforts and/or the BFO driving the AGC!   Modernizing (by isolating the BFO
and coupling it in at the audio detector so the AGC can't see it) allows
better AGC.  

Credit where due:  The audio and AGC detector arrangement was largely stolen
from the Tempo ONE, aka Yaesu FT-200 transceiver, ca 1970.   This was surely
the best price performer of all the commercial vacuum tube sets and beat the
contemporary solid state rigs all ways from Sunday.

Lots more to do but this is going well so far.   It is easier repeating
established circuits and using a transformer-type power supply, no doubt
about it!

Walt 
KJ4KV