[HBR] My approach to the HBR... Suggestions?

[Walt Hutchens]

I think Dan's comments on audio-derived AGC are spot on. It's simple
-- not a bad choice for an absolutely minimal receiver -- and that's
about the only advantage.

There is no really simple AGC circuit that is also really good.  The
common diode detector AGC of most WW-II +/- ham and communications
receivers works well, but unless a separate AGC amplifier stage is
used, requires high signal voltages in the final IF stage, meaning
you're likely to have distortion on strong signals.

My favorite AGC circuit uses a plate detector with almost any of the
common triodes with the cathode returned to a negative bias of 50
volts or more and the plate through a resistor of 3 meg or so to a
negative bias that is panel adjustable from 0 to whatever is required
to cut off your AGC'd stages. This panel control serves as an RF/IF
gain control for most purposes, although you may also want a
conventional variable cathode resistor in the RF stage for extremely
strong signal/crowded band conditions -- 75M on winter nights.

(I've never used that control with this circuit, but some people might
want it anyway.)

The plate of this AGC detector drives the AGC buss through the usual
sort of filtering.

The front panel control is basically an AGC threshold setting. At 0
volts bias the gain is controlled only by the signals. As bias is
increased, the no signal S-meter reading will move upscale and the
gain will be reduced just as it would by a signal of that strength.
However when the signal gets above that level, the AGC will take over
and the S-meter will indicate higher. S-meter readings remain valid
above the threshold.

On a noisy band with strong signals you set the threshold to control
the no-signal noise level. The only situation in which 'hang' AGC is
better is on an SSB round table when you have some signals just above
the noise and others that are WAY above: a 'hang' circuit will give
you better quieting when the really strong guys are talking slowly.
But during the pauses when the guy whose turn it is has gone for
coffee or the mobile that only two other guys can hear is talking, the
threshold control scheme will work better.

You do need a tapped final IF transformer but the tap on the command
set output units works fine or you can use a capacitive tap.

The signal detector must be comparably high gain.  For SSB the usual
triode, gated or switched beam tube detectors work fine.  What I've
done, however, is use another plate detector, adding a BFO signal for
SSB.

The circuit is stolen with no important changes from the Yaesu Tempo
ONE; the circuit probably can be found on the BAMA site.  I found it
was necessary to redraw the circuit to understand it.  Since the T-1
is a transceiver, there's a connection through a relay to the
transmitter bias line; once you figure that out, the necessary change
is obvious.

AGC attack time is governed only by the plate resistance of the
detector tube and any triode seems to work okay. (No clicks on the
first word of a sentence when on SSB, strong signal and quiet band!)
I've used the triode section of the final IF 6KZ8 in one receiver,
that of a 19JN8 in another; I believe the T-1 uses a 12AX7.

One added complication is the need to neutralize the grid-plate
capacitance of the detector tube in order to prevent the BFO from
affecting the AGC. This isn't difficult: You build the circuit without
the cap, see what you get for an S-meter reading when the BFO is
switched on, and fiddle with cap values until the reading goes away.
The T-1 uses a 'gimmick,' I've used up to about 5 mmf mica.

With a 6EH7 RF stage, any of the better detector circuits (Pullen is
low noise -- not 'better' for an HF receiver having an RF stage) and
two stages of 6EH7 IF this will give you about the best that's
possible in an HF multiband receiver.

If you use an RF stage, I don't think it's a good idea to use any gain
at the 1st IF. The 2nd mixer overload problems are considerably
increased by doing so. Three 6BJ6 IF stages at 85 kcs will give AMPLE
gain. I personally would go for two 6EH7 stages because of the amazing
overload tolerance (with high gain) of this tube, but using what you
have is a ham tradition and the three 6BJ6's will work fine.

With the AGC circuit described above you can even use sharp cutoff
tubes in the IF stages because the signal levels at the output are
never greater than the fraction of a volt range.

I didn't need to remove turns from 1415 kcs IFTs to use at 1665 kcs.
Reduce the cap values by something over 30%. This does take some
fiddling because of the very narrow tuning range and you must make the
measurements with the Tx in the can due to detuning effects. The
easiest way probably is to build the receiver as far back as the 1st
mixer, then use your signal (or sweep) generator to adjust one winding
at a time by trial and error. De-Q the other windings by shunting them
with 1000-ohm resistors or something like that.

Even matching the 85 kcs IFTs can make you fairly crazy if you don't
use something close to the original IF circuit because the tuning
range of is next to nothing.

It is logical to use a crystal controlled 2nd HFO, however if you do
you must be able to adjust the 1st IF accordingly because the 85 kcs
IFTs have very little tuning range.

The 1st IF should not be on a multiple of 10 kcs due to possible
broadcast station interference issues.  With it ending in '5,' the osc
will have to be a multiple of 10 kcs to get an 85 kcs 2nd IF.  It
might be better to put the 2nd HFO on the high side of the 1st IF in order
to avoid any possible 'issues' there, like it trying to pull into
synch with a strong local.

At the time the HBR series was designed, there were no BC stations at
or above 1600 kcs due to the Loran situation. Today, 1600 and even
higher are used for local public information stations; I've seen 1610
and 1620, I'm pretty sure. I'm sure someone here knows the current
rules.

Walt
KJ4KV