[HBR] Another one-week (okay, probably not!) HBR -- Part 1

[Peter Bertini]

Hi Walt!  How about some photos as the project progresses? You
could open an account on PhotoBucket, and post the links as you
progress with the project.  Photos would be neat!!

Pete


On Sun, Sep 28, 2008 at 7:53 PM, Walt Hutchens <waltah at earthlink.net> wrote:
> Long time listmates will remember when I tried to build an HBR
> generally along W6TC lines in a week.  It actually took about five
> weeks and even that didn't include making the wooden cabinet.
>
> Ever since then I've wanted to try again. Obviously one motivation was
> to 'get it right, this time,' but besides that, I figure that every
> ham ought to have a receiver with plug in coils for each stage and one
> using an Eddystone 898 dial. I had the dial; when I discovered that
> 1-1/4" low pressure PVC pipe fits perfectly on the bakelite base of a
> metal octal tube, the building bug bit.
>
> I don't really trust solid state devices, so I'm not going to use
> them. (He said, typing on his laptop ...) Oh yes, this process is
> entirely rational. I'm not sure why you question that?
>
> Furthermore, I've always liked 'transformerless' designs. With modern
> three-wire cords (chassis grounded) and high-quality parts they're as
> safe as the conventional approach and there's a big saving in weight
> as well as some in complexity and heat generation.  There are design
> issues (RF grounds return to the chassis but DC and audio grounds go
> to the B- buss, heater cathode leakage must be managed by the layout
> of the filament string) but once solved, they're over.
>
> Untold jillions of transformerless vacuum tube AM/FM radios and TVs
> were built.  An all-band ham set isn't THAT difficult.
>
> I considered the W6TC double conversion scheme but that would mean
> another mixer and oscillator: more parts and (because the sharp filter
> is farther from the antenna) more difficult overload issues. However
> with a high (single) IF, a crystal filter is necessary. After a couple
> of hours of junkbox diving, I found a 1700 kcs half-lattice pair of
> HC-6/U crystals left over from another project.
>
> With this IF, the oscillator tunes 5200-5700 kcs for 80, 5200-5700 for
> 6.9-7.4 Mcs coverage (same oscillator coil can be used on both bands),
> 6100-6350 for 13.9-14.4 Mcs, and so on, differing only in the details
> from the W6TC scheme.
>
> (The W6TC choice of a double conversion scheme probably was driven by
> the fact that the higher single IF requires much greater care to keep
> the BFO signal away from the IF input. Furthermore, it requires a very
> carefully designed and built LC oscillator for the BFO, plus
> adjustment of a crystal filter to get best results. Neither of these
> circuits can be 'wire and go' like a double conversion IF. The
> modestly experienced builder, however, need not be bound by these
> considerations.)
>
> The 'second draft' tube lineup was:
>
> 19JN8 (pentode-triode) RF + 1/2 mixer
> 19JN8 1st IF + 1/2 mixer
> (Mixer is a push-push double triode design.)
> 12AT7 LO + AGC clamp diode
> 19JN8 2nd IF + AGC plate detector
> 19JN8 1st Audio + Audio detector (BFO injected for SSB/CW)
> 12AU7 2nd Audio + BFO
> 117Z6 (dual rectifer diode) plate and bias rectifier.
>
> The main filament string adds to just 100 volts; that will require a
> series resistor. However the plate currents total over 100 mA and
> since the 117Z6 is rated at 60 mA/section, both of them will be
> needed. In addition, (oops!) the mixer plate and 1st IF plate are at
> the same frequency: They cannot be in the same envelope.
>
> The 'third draft' lineup is:
>
> 19JN8 (pentode-triode) RF + 3.5 Mcs band edge marker
> 12AT7 Mixer
> (Mixer is a push-push double triode design.)
> 19JN8 1st IF + LO
> 19JN8 2nd IF + AGC plate detector
> 12AT7 AGC clamp diode + BFO
> 19JN8 1st audio + audio detector (BFO injected for SSB/CW)
> 12AU7 2nd Audio + bias rectifier
> 117Z6 (dual rectifier diode) plate supply rectifier.
>
> That's one more tube, solves the problems noted above, gets W6TC's
> band edge marker in there, has just a couple of watts more power
> dissipation, and a tidier front end layout. With filaments totalling
> 112 volts a series resistor of 50 ohms, 5 watts, or so will be
> adequate.
>
> The use of sharp cutoff tubes in the RF and IF stages is questionable.
> Unfortunately there are no remote or semi-remote cutoff tubes (that
> I've been able to find) in a tetrode/triode or pentode/triode
> combination with 150 mA heater. With low IF signal levels and a
> crystal filter in the first mixer plate circuit the potential overload
> problem is confined to the RF stage where it can be managed by
> reducing antenna coupling.
>
> The Eddystone 898 is a good looking, very smooth, high ratio dial --
> about 55 turns of the knob to rotate the capacitor 1/2 turn, making it
> near ideal for tuning in SSB signals across a 500 kcs band and the
> flywheel makes it possible to QSY quickly. The knob drive is of the
> pinch type, so there's no precise ratio but the slide-rule type dial
> is gear/dial cord driven with about 7" tuning range. There's a logging
> scale with a calibrated dial having 500 divisions, end to end. The
> dial scale is sub-mounted about 1/2", allowing edge lighting. This
> can't have been a terribly expensive dial to build, when it was new --
> maybe 3x the price of the National ACN and similar designs?
>
> That's the end of the good news. The more obvious news is that this is
> a remarkably ugly dial with which to build a radio. The knob shaft is
> 2-3/8" below the output shaft which puts it too close to the table if
> the tuning cap is mounted on the top of a 3" chassis. But you can't
> put it there anyway, unless you cut away most of the front of the
> chassis to allow for the flywheel on the knob shaft AND the sub
> mounting of the dial scale. Alternatively, you can space the chassis
> back from the front panel and deal with the issues that creates for
> the mounting of the controls.
>
> By raising the capacitor shaft about 1-3/4" above the chassis, the
> problem is reduced to cutting a 3" wide x 1-1/2" deep x 1-3/4" high
> notch in the upper front edge of the chassis to clear the flywheel.
> That's the approach I'll take.
>
> That's not all the ugliness of this design, though. The flywheel must
> be removed in order to install one of the mounting bolts for the dial
> assembly, so an inch or more of clearance is required behind the
> flywheel under the chassis. The dimensions tend to be 'about ...'
> making layout more interesting.
>
> I missed the MG and Jaguar experience; building with an Eddystone dial
> will help me feel better about that.  Perhaps what I really mean is
> "remember WHY I missed the MG/Jag experience."  I don't wonder that
> W6TC designed around the American made planetary drive dials even
> though the tuning rate on the single-speed ones is much too fast.
>
> The no-name (possibly Electroniques?) three gang tuning cap that came
> with is of very good quality and about the right size, so I'll use it.
>
> I have some fairly small IFTs that will work and only three are
> needed. Time to lay out the chassis. 8 tubes and three coils should
> fit on 10" x 12", right? I need to find a 3" speaker and a REALLY
> small meter. With a 12" chassis, the panel will be 14" wide by say 8"
> high ... hummm ...
>
> And finally, a question: There's no important handedness in the parts,
> the most often used control on a receiver is the tuning knob, and most
> of us are right handed. Yet all the HBRs I looked at on the web site
> (WOW, are there some BEAUTIFUL examples there!) have the dial on the
> left, making it necessary to reach across in front of everything else
> to tune. Am I missing something?
>
> Walt
> KJ4KV
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