[HBR] What would W6TC do?

[Shoppa, Tim]

> 1. I think the first conversion to an IF in the 2 Mcs range and use of
> plug-in coils in the front end would continue. Too much of the 'you
> can build it' and 'it will be a top performer' lies in these
> fundamental choices.

I disagree about keeping the IF at 2 MHz... move it to circa 9 MHz.

But yes, plug-in coils would continue.

> 2. WARC and SWL band coils would be part of the plan for those that
> want them. The use of an AM-FM three gang capacitor with jumpers in
> the coils to connect the larger sections would be an option for those
> wanting general coverage -- say 2-6-18-30 Mcs.

I disagree here. I would make the radio would even more ham band centric.
Eliminate AM detector and make it be strictly SSB and CW. If anything
the tuning ranges would be narrowed, not broadened. And yes, additional
coil sets for anyone who wants WARC or SWL bands, but they would all
be specific to each band and not general coverage.

It's even likely that instead of trying to get tracking in the front
end, we would just have pluggable bandpass filters. Many (most?) ham band
receivers by the 1960's had abandoned tracking front ends.

Tracking front ends and single-knob tuning were a bit of a
holy grail in the ham homebrew community
for those who wore their "my homebrew receiver is as good as or better
than the commercial receivers" chip but this led to creeping featurititis. The
best homebrew ham receivers should be ham receivers... not copies of every undesirable
or unneeded feature in commercial receivers.

> 3. The second conversion to a low fixed IF (100 kcs or 85 kcs
> according to the variant) would be eliminated. In the 1960 time frame
> this was an easy and relatively inexpensive way to obtain a 3 kcs (or
> so) bandwidth as the necessary IF transformers were open stock or
> available from junk 'command' receivers, but not anymore.
> There are two easier ways to go now: A half lattice filter in this
> frequency range, or use of a filter salvaged from one of the less
> expensive ham sets of the 1960-70 time frame.  I suspect that a
> popular salvage item might set the frequency with the option of a
> half lattice design using a pair of custom crystals.  HOWEVER junk
> vacuum tube ham sets are often cheaper than a pair of made-to-order
> crystals and almost any ham set filter will have better skirt
> performance than a half lattice filter.

I agree that getting rid of the second IF and having all selectivity in
the crystal filter would be key points. BUT... I suspect the filter would
be based around cheap readily available microprocessor crystals. Remember
W6TC's ubiquitous goal of using commonly available parts.

> 4. The mixer might be changed to a dual triode.
> Pentode mixers generally are the worst performers in a strong signal
> environment

Seems likely. Would we use a broadband centertapped coil for mixer
balance or make that plug-in? Either way could work but both have some
complications. The move to a balanced mixer would have huge advantages
that far outweigh the small effort required.

> 9. Instead of the conventional multi-winding power transformer, a 15
> or so watt isolation transformer would be used with a solid state full
> wave bridge rectifier to deliver 130 volts for plate service and a 25
> watt 12.6 VAC transformer would power the filaments. Almost nobody
> makes multi-winding transformers for vacuum tube equipment anymore,
> the usually-available plate voltages are much higher than desirable
> (higher voltage means more heat and thus more thermal drift), and what
> you can buy is almighty expensive. Two small competitive items are
> cheaper than one that's nearly custom made.

I disagree with the unavailability of transformers. Right off the top of
my head, Hammond 263, 269 series transformers fit the bill and are
readily available. If you think the plate voltage is too high, that's because
you're using a bridge rectifier instead of noticing that the center tap
is still there :-).

> In concept at least there is another way to go for power: Series
> filaments and 'transformerless' high voltage. This further reduces the
> total dissipation (small transformers aren't very efficient), expense,
> and weight at the cost of some additional design and construction
> wrinkles. If all the circuits in a large color TV can be made to work
> without a power transformer, certainly a short wave receiver can.

Cheap commercial SW receivers did use series filament strings and hot or
isolated chassis... and I think at least a couple Knight-kit radios did too...

but correct me if I'm wrong, the ARRL/QST never
published designs for radios that didn't have a filament transformer or
that ran the plate without a transformer. (I know of a few non-radio
accessories, like CPO's, that they showed without transformers.)

> this happens one circuit at a time and I'm still working on it. You
> don't build a ship in a bottle because you want a model ship and I
> wouldn't recommend a transformerless HBR as the way to go because you
> want a receiver.

It's just anecdotal but my feeling is that ARRL/QST would never
publish a transformerless radio design.

It is a win, to decouple the power supply from the radio. At some point
we get bored with building more power supplies and just plug into the
ubiquitous HP-23A, and put our effort into making a radio :-).

Tim N3QE