[HBR] HR-10 to HBR project

[Walt Hutchens]

About a month ago I posted:
> I've been stalled on HB-10 to HBR project as the result of (a) flu -- now
> gone -- and (b) a dead car that needed fixing before hard winter -- still
> dead, but maybe soon.  But this project is looking good -- I think it's
> going to be the easy way to get one's feet wet in the HBR game.

Since then I've made some more of that negative progress that is my most
frequent product.  

With pretty much everything else working, I set about hooking up my favorite
AGC circuit -- a plate detector with the cathode run from a negative supply
and the plate driving the AGC buss.   Amplified AGC not only gives you much
better AGC but also lowers the maximum signals through the IF strip meaning
less distortion.   

In other receivers I've been able to combine the AGC detector with the 2nd
IF in a triode-pentode and I expected to do that this time.   But no such
luck.  Evidently the Heath IFTs are high enough Q that detecting the 2nd IF
output signal in the same envelope cannot be done without instability.   I
tried a number of variations, different tube types, etc. without success.
With the input to the AGC detector disabled the set was fine -- at least, as
fine as possible without AGC -- but when I hooked it up, the 2nd IF stage
oscillated.

Since finding that rather tart lemon I have been search for a way to make
lemonade and I think I may now have it.    I used a 6BH6 as the (100 kcs)
calibration oscillator but by moving the socket and using a triode-pentode
-- 6KZ8, 6JN8, something like that -- I can put the AGC detector in with the
calibration oscillator.   The calibration oscillator isn't an ECO anyway
because the screen is used as the plate for a Colpitts-derived crystal
oscillator so not having a separate pin for G3 doesn't matter.

That allows the use of a 6EH7 as the 2nd IF:  this is the tube of choice for
HF RF/IF amplifiers due to its linearity over a wide range of grid bias
voltages.   And having nothing else in that envelope will eliminate this
feedback problem.

Humm ... the 1st IF (triode-pentode) still had the job of negative bias
rectifier: Could that be moved elsewhere?   Well ... yes.   The BFO was also
a 6BH6: this should be a real ECO circuit (for the best possible isolation
from the signal, meaning less distortion caused by pulling) and thus can't
be done with a triode-pentode because there aren't enough base pins to bring
G3 out separately.    However the triode 'rectifier' was diode connected:
What about a DIODE-pentode?   There are a couple of them, and the 6AM8
appears to have suitable characteristics.   This tube will replace the 6BH6
BFO and take over the bias rectifier job, allowing the use of a 6EH7 in the
1st IF socket as well.

SO ... The changes coming out of the instability of the 2nd IF have ended up
with the use of 6EH7's in all three key sockets -- RF, 1st IF, and 2nd IF.
That's just about the ideal setup for a ham receiver of this type,
especially when combined with a relatively low gain but 'strong' mixer
circuit -- here a 12AT7 in the partially balanced circuit recently
discussed.

Definitely lemonade, if it works.  There are two issues with this seemingly
superior configuration:

1.  Because the cal oscillator is laid out completely wrong for the new
setup the change requires a chassis redo.   I have a second junk HR-10B so
that's just 20 hours of work or something like that.

2. There could be unforeseen 'issues' with operating a 100 kcs oscillator in
the same envelope as the AGC detector.   I don't THINK so but ...

If there are problems they'll occur only in CAL mode.   Maybe enough leakage
of the 100kcs signal into the AGC detector to cause upset there?   This
shouldn't be a big effect, since the plate of the cal oscillator is
effectively tuned to the channel frequency by the connection to the antenna
jack and the plate of the AGC detector is grounded for RF right at the
socket.   Just birdies, I think ... not at all strong compared to the
calibration signals.

Well, that's what I 'assume,' anyway.  I'm about 2/3 done with the metalwork
on the new chassis.

Hey ... it's a HOBBY.

Walt Hutchens
KJ4KV