[HBR] HBR -- Part 12

[Walt Hutchens]

Another problem solved ...

In the last edition it appeared that the reason the mixer gain was
about -20dB (instead of maybe +6 dB) could be that the triodes I was
using were not suited to mixer service. Sure enough I was unable to
find a 0.15 amp filament triode-pentode with a triode that was better.
So the 19JN8 RF-1/2 mixer and 19JN8 osc-1/2 mixer became three tubes:
6BH6 RF, 12BZ6 osc., and 19J6 mixer.

This required punching another socket hole and making up three '7-pin
but fits in a 9-pin hole' sockets, with aluminum scraps and JB Weld.
Because these three sockets surround the tuning cap, I mounted them
about 1/4" above the chassis to allow airflow up from below to carry
away the heat. The pins project down through the socket hole just
enough to allow wiring in the usual way. The wiring rearrangement was
easier than I expected.

The 6BH6 -- a sharp cutoff pentode -- may seem a slightly odd choice
for the RF stage, but its cutoff voltage is close to that of the
pentode halves of the 19JN8's used for the IF stages. That means that
(as AGC voltage increases) the gain of all three stages is reduced more
or less equally so there's less chance of causing distortion due to a
big signal getting through the RF stage and hitting a nearly cut off
IF.

Sure enough, the 19J6 is a FAR better mixer tube than the triode
halves of the two 19JN8s.  I haven't measured the gain, but it's in
the range -- the radio copied the Saturday night traffic on 3885,
FB, with some S-meter readings way up the scale.

I'd sure like to know the history of the 6J6. It must have been
developed around 1939-40 but it is YEARS ahead of the surrounding
tubes on the nomenclature charts: think 6F6, 6G6, 6H6 ... 6K6 ... It
was (to the best of my knowledge) the first serious UHF 7-pin
miniature tube, being rated to operate as a mixer at 600 Mcs and
working fine as an oscillator in the same territory. I think it
appeared for the first time in the BC-788 radio (radar) altimeter.
That was in production in 1942; I don't know how much earlier.

I believe the BC-788 is a GE-developed set; whether they also designed
the tube, I don't know. I wouldn't expect RCA to have done anything
quite so innovative. I pulled out a BC-788 junker set to get the value
of the cathode resistor for the 19J6 mixer.

Anyway, the little HBR project is basically working: The overall gain
is in the ballpark, though probably still just a few dB low.
Furthermore there's no trace of regeneration or oscillation (with the
RF stage re-arranged from grounded-grid to the usual grid drive
circuit) meaning that moving the mixer to its own tube eliminated some
unknown feedback path.

There's just a little audio distortion, probably in the plate detector
stage. The next step there is looking at the detector output waveform
with a scope and (probably) tweaking the operating point.

The octal sockets I used for the coils hold them firmly but because
the pins don't fit tightly enough in the socket the coils flop around
some. That's fine for the RF and mixer coils but not so fine for the
oscillator: I've picked out a ceramic socket that doesn't have the
problem and will make a swap.

There's a little too much low frequency audio response. It drifts
about a kc downward during the first hour or so -- I've made no
attempt to do compensation.

The band edge marker and BFO stages aren't wired yet. It will need
coils for other bands.  In other words, lots more to do.

I've gone looking for the early photos. They were lost during some
update of our website but I'm told they can probably be found, when
Sharyn has time in a week or two.

Walt
KJ4KV