[HBR] Re: HBR -- Part 3

[Walt Hutchens]

Heathkit-terminology steps since last time:

Second mounting of parts -- output transformer, volume control, filter
choke, ON/OFF switch, speaker, and misc.
First wiring -- filaments and plate power supply
Second wiring -- 1st & 2nd Audio stages

Only made one mistake wiring the filaments this time! Pins 4 & 5 on 7
tube sockets; how, you wonder, could I get that wrong? Well ... I
wonder the same thing.

The audio section worked right away, though I had the interstage
coupling cap wrong, so had a bit too much hum. With that fixed, it is
silent at low to mid volume settings, only some hum at full volume.

The full volume hum comes from capacitive coupling between the chassis
(which is at power line ground voltage) and the neutral power wire
which is B- and the audio signal ground for all circuits.  This
voltage is of the order of one volt RMS and usually 'trashy.'  With a
750k volume control, just a few mmf of stray capacitance is enough to
produce an audible signal.

The audio detector has a much lower (than 750k) plate resistance; when
it's wired it will lower the impedance at this point, eliminating the
hum.

The audio stages have a high level of negative feedback and thus
relatively little gain -- I think a voltage gain of 3 from the volume
control to the voice coil.  But 100 mW of audio for a 4 ohm speaker is
about 0.65 volts RMS so with voltage gain 3, volume control set for
10% of max voltage gain, the detector output will only need to be
a bit over 2 volts and the 2nd IF output, in the milivolt range.

Sharp cutoff tubes are fine in an IF operating at that low level.  But
a sharp-cutoff tube in an RF stage is not ideal for ham band service.
Does anyone remember how unhappy Hallicrafters was when some people
substituted 6AK5's for the RF tube in one of their receivers?  Maybe
the HQ-129X?

This is not that bad, because the AGC voltages will be correct for the
tubes I'm using, but better front-end performance would result from
using 6EH7's in all three sockets. Then you'd have an 11 tube receiver
that might possibly be done as a 300 mA series string set ... a
different design!

'High fi' audio with frequency response shaped for communications
purposes makes signals a lot clearer and takes just a couple more
parts. Use of 1/2 of a 12AU7 for the audio output stage requires a
high efficiency speaker: the modern midget extended low frequency
response speakers used for computer and car audio will not deliver
enough volume.  Speakers from taxi and CB sets are fine.

Most of the circuitry of this set is copied from the earlier band
imaging set with six tubes, push-pull oscillator and mixer, 80/40
meters.  So I THINK all the problems with 'transformerless' power
supply have been resolved.  Yeah, that's what I think ...

I plan to wire the audio and AGC plate detector circuits next. These
are respectively the triode halves of the 19JN8 1st audio and 2nd IF
stages.

Following that, I'll wire the local oscillator, so I can start working
on the oscillator coil while wiring the other stages.

The Eddystone dial that I have is factory-calibrated for six ham
bands; I think it came as part of a kit of parts for the G3LOK
receiver design. (Think: HBR with bandswitching, using itty-bitty
coils ... NOT a great design!) And I'm using the tuning cap that came
with.

This dial is a nice (silk screen?) printing job on painted metal and
the scales are very close to linear so it makes sense to try to track
the oscillator to the dial, rather than recalibrating.  However this
sort of thing requires a bit of cut-and-try ... probably a week's
work, on and off, to get a decent result.

This will require a three-point fit. Typically the three parts to be
adjusted would be the coil inductance, the shunt (trimmer) capacitor,
and a capacitor in series with the tuning cap, called the padder
capacitor. These would adjust the low and high ends and the midband
point, respectively. The W6TC designs use a tap on the coil rather
than a series padder cap, probably because tapping the coil is cheaper
than another high quality capacitor. However, moving a tap to get
midpoint dial tracking right is tough.

On the other hand, changing a capacitor down inside a coil form with
an APC trimmer in the top may not be simpler ... I'll probably try
using the tap, first.  I bought 10' of PVC for $2.39 plus tax, and I
have a bunch of rusty octal tubes ...

Just one more thought: that capacitor tracked that dial with the coils
that are in that (busted up) front end assembly. Maybe my first effort
should be to duplicate the in-set coils and copy the oscillator
circuit parts. Hummm ...

The tube lineup has turned out to be surprisingly tidy:

19JN8 -- RF & 1/2 mixer
19JN8 -- ECO LO & 1/2 mixer

This looks messy, but the RF drives one half of the mixer and the LO
drives the other, thus each pentode is coupled to the triode in the
same envelope.  The two triode plates and cathodes are joined; the
plates go to the 1st IFT.

19JN8 -- 1st IF & AGC clamp (triode is diode-connected)
19JN8 -- 2nd IF & AGC plate detector
19JN8 -- 1st audio & audio plate detector
12AU7 -- 2nd audio & 3.5 Mcs marker oscillator
12AT7 -- BFO & bias rectifier (2nd triode is diode-connected)

117Z6 -- Plate supply rectifier

There's also a 150 mA 9-pin miniature with a triode and two diodes
with independent cathodes that could be substituted for the 12AT7 if
another diode is needed. The 14GT8 was designed for use in combination
ratio-detector/1st audio service in AC/DC AM/FM receivers.

Another report in another few days ...

Walt
KJ4KV