[HBR] HR-10 to HBR project

[Walt Hutchens]

In the previous episode the problem of having the AGC plate detector share
an envelope with the 2nd IF stage causing the stage to oscillate led to a
plan to redo the chassis moving the AGC detector in with the calibration
oscillator and the AGC bias rectifier from sharing the 1st IF stage to
sharing the BFO.   The 'lemonade' from this almost-starting-over was the
potential for 6EH7's in all three appropriate sockets -- RF, 1st & 2nd IFs.

Also the uncommon 6LM8 triode-semiremote cutoff pentodes were eliminated.

Reworking the second HR-10 chassis for the HBR layout is done and the power
section and everything from the 2nd IF to the 2nd audio is hooked up and
tested.   There were just a couple of glitches:  I actually woke up one
night thinking about switching transients from the bias rectifier sharing an
envelope with the BFO so when I got a buzzy hum that was plainly audible at
full volume, I was ready with the bypass caps.   There was still a bit of
hum: That was greatly reduced by dressing the BFO output lead away from the
diode power pin (210VAC) of the tube.   Hum is now inaudible unless you put
your ear near the speaker.

The other problem was oscillation of the 2nd IF stage.   That was cured when
I rediscovered the need for a parasitic suppressing resistor in the grid
lead.

The 6EH7 is very hard to operate at  its full gain in a communications
receiver:  These tubes were designed (and have the gain for) fairly
broadband (TV) IF stages operating at frequencies in the several Mcs range:
In a narrow band application with a higher Q IFT the cathode resistor will
need to be increased.   That's not a problem: First, they'll achieve as much
gain as any other conventional tube and second the big reason to use them is
that they have very low crossmodulation (good linearity) even with high grid
bias voltages.   I have a 330 ohm cathode resistor in the 2nd IF stage and
it's rock stable; I'll probably experiment with lower values later.

The 100 kcs calibration oscillator sharing an envelope with the AGC plate
detector seems fine: No trace of anything on the S-meter and nothing heard
when it is on.   Since the RF stage, mixer (and so on) aren't there yet,
that's the right answer.

With nothing ahead of the 2nd IF, the receiver can hear a GDO (tuned through
the 1681 kcs IF) nearby on the table and  holding the GDO near the grid
circuit kicks up the S-meter.   That's good enough for now:  I'll go on
wiring up earlier stages in backward order.  The 1st IF is the only one of
those that changes at all (moving the bias rectifier out and changing the
tube type) so once that's working it should be straight going, mostly just
transferring wired sockets from the first build.

Maybe another week to have it all hooked up?  IF there are no big surprises.

Walt
KJ4KV