[HBR] Under construction HBR-??

[Bill Higgs]

Hello fellow solder junkies,

The HBR-?? ( who knows how many tubes this thing will end up with?) =
continues under construction both on paper and on metal. Having bowed to =
the sage advice of Walt and others, I have mounted the variable =
capacitor from my poor cannibalized BC-453 on the front panel. I was =
able to obtain a 454 cap from a fellow mad scientist (!), I compared the =
two side-by-side. Much to my surprise the plate shapes are identical, =
the caps only differing in the number of plates, and therefore the total =
capacity. With this in mind, I'm either going to go with a high c =
oscillator circuit, or put a series mica in line, and then live with =
whatever nonlinearity I encounter. After looking through one of the =
early "Single Sideband for the Radio Amateur" books, it seems that =
several of the fellas used the 454 cap for roughly a 5.2-6.2 Mc tuning =
range, and the pix showed a dial with reasonable linearity (no worse =
than about 2:1). I played around a bit with my 453 cap, a toroid, and an =
FET (shudder!), and was able to get about a 3:1 linearity (tightest =
100kc was about 1/3 the space of the widest 100kc) while tuning =
5050-5750. Output was a bit lower than expected, so the high C circuit =
might not work (either in tube or FET configuration). The final =
receiver, of course, will use a tube.

The mechanical work has gone smoothly. I found a pan-shaped aluminum =
piece (obtained surplus several years back; probably the front/rear part =
of an old Bud cabinet) which was just about the right size to cover the =
dial and the tuning knob. I drilled a roughly 2" hole for the knob, so =
it's recessed a bit, then made a plate of clear plexiglass with a =
clearance hole for the knob which serves to keep scratches off the front =
and also serves as the window to the readout dial. The whole assembly =
looks nice and serves to strengthen the mounting, keeping the chassis =
and the front panel somewhat more rigid. I'm tempted to bore a hole =
through the bottom of the capacitor frame and fasten to the chassis, =
giving even more rigidity. A few holes, a coat of dark green paint on =
the pan and flat black on the front and rear panels, and it's ready to =
label. I'll post a photo as soon as my daughter shows me how to work the =
digital camera . . .

A couple of questions to the group: First, has anyone tried to use a =
Vackar of Seiler oscillator with either the usual HBR mixer or with a =
beam deflection mixer? If so, your thoughts?? It looks as though I might =
be able to configure a bit more C in either of these circuits, thus =
improving possible dial linearity, and possibly keep the output stable =
over a somewhat broader (700kc) range.

Question two regards the gain controls on the RF, Mixer, and IF amps. =
I'm looking seriously at the hang AGC circuit, which appears to have =
provision for manual or concurrent gain control by feeding a negative =
voltage into the circuit at the recovery diode and/or the agc line (I'd =
isolate it through another diode, though . . .) The 5W wirewound =
resistors in the values W6TC specified are practically "unobtainium" =
nowadays, and most catalogs jump from 2W to 12W. Digi-Key advertises 5W =
CTS wirewounds at $2.50 each, but the values run 1K, 5K, 10K and 25K. =
Since the point of this pot is to adjust the cathode bias from zero to =
some positive voltage (thus biasing the control grid negative), I have =
also thought of developing a negative voltage from a doubler off the =
filament line, and using a higher value pot to control a negative =
voltage to the tubes via a diode (my thinking is that as the grid is a =
higher impedance point, less dissipation on the part of the pot. Other =
idea (not quite Kosher), is to run the cathodes off a power transistor =
(2N3055?) controlled by a smaller pot, since the cathodes are fully =
bypassed. This keeps the solid state out of the signal path, at least. =
But does put silicon inside the box, which sort of defeats where we're =
going, doesn't it?

Anybody have any ideas on this one? I have yet to cut too many holes in =
the chassis, so I am still open to suggestions at this point. By the =
way, I did go with an external power supply. Gives more room to mount =
stuff (see hang AGC above, audio filter, noise limiter, ??), and keeps =
the hum generator well away from hard-to-shield beam deflection tubes.

73 to all,


Bill Higgs, NT4C

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