[HBR] Blog post about HBR receivers

[Walt Hutchens]

Pete said:

> I subscribe to the belief that the HBRs were intended to be a work in
> progress.

Yep. That's half the fun!

> For my own effort I'll be adding a few sets of the half-lattice
> xtal filters (1682kHz) recovered from three HR-10 Heathkit donors for a
> six-pole filter at the first IF.

The reason for the double conversion design was to provide
selectivity that the hams of the late 50's-early 60's couldn't afford
to get via a crystal filter.  If you're going put in those filters --
and the number used in the HR-10 would be ample -- I would do the
whole IF at that frequency.

Double conversion has its own problems -- another set of images to
manage, another mixer to give you distortion, another oscillator, an
extra tube and socket. If you don't need it, then who needs it?

Can you also steal IFTs from the HR-10?

> I'm also considering a 6GM6 RFA, 6ES8 Pullen mixer, and 6EJ7s in the
> IF stages.

I would use the 6EH7 in the RFA and two IF stages -- that'll be all
the gain you need.

The 6EH7 was designed for TV IF service at 40 Mcs. It has the best
crossmodulation specs of any tube I've seen, as well as a whole bunch
of gain. I've used a bunch of them in HF RF and IF stages and really
don't think it can be beaten for updating of a vintage vacuum tube
design.

Like others, I haven't seen anything indicating the Pullen mixer is
more than a good low-noise mixer.  But low noise isn't the issue for
an HF receiver that has an RF amplifier: Good strong-signal
performance is what's required.  A lower gain dual triode design --
cathodes in parallel, ditto the plates, grid resistors to ground,
signal on one grid, LO on the other will do fine.

NOTE HOWEVER that if you make any such change you will have to do
something to select the harmonic at the LO input grid for bands that
use harmonic injection. Ted's design (in which the LO was injected
into the signal grid of the mixer) used the mixer tuned circuit to do
this.

There are definite advantages to NOT trying to tweak such a
well-thought-out design. Of course the reason I know it's well thought
out is that I have tweaked it ... then spent a few days figuring out
why my 'improvements' didn't work as well as the original.

> It would be interesting to take the designs several years into the
> future, when still retaining most of the mechanical details outlined
> in the original series.

With receivers particularly it's important to be careful about
mechanical redesign.  Been there, learned caution in tweaking THAT,
too.  Perhaps someone here remembers my very painful (couple weeks?)
discovery that the cold ends of the HBR coils should return directly
to the respective rotor contacts on the tuning cap ...

Walt
KJ4KV