[HBR] 80m receiver project

[Ian Wilson]

Not an HBR, but I thought I'd write up a modest receiver project I've
been working on.

The impetus for this was a homebrew front end that I picked up a couple of years
ago. Very nice homebrew, including a tuning scale covering 3.5 to 4MHz
in 10.5" width.
The internals were an eclectic mix including Command Set tuning capacitors and
National RDZ IF transformers. This is a 19" rackmount piece, 4" high.
Electrically:
  - 2 6AS6 RF stages (4 tuned circuits plus antenna trimmer) w/RF gain control
  - 6U8A oscillator/mixer stage
  - 6BA6 IF buffer (with provision for external AGC input)
  - 0A2 VR for the oscillator stage

Recently I decided to put this together with another piece from the
same builder.
This was a 455kHz IF amplifier/AM detector/microvoltmeter. This had
just the right
number of sockets, etc, to rebuild as follows:
  - 2 6BA6 IF stages, AGC controlled
  - 6BN6 product detector
  - 12AU7 BFO + buffer
  - 12AU7 AGC amp + cathode follower audio buffer

AGC is audio-derived, using a couple of solid-state diodes. There is
also a 455kHz
ceramic filter in there but it is too wide to have any noticeable
effect on SSB. The BFO
is set to 456.5kHz. It is stable and reproducable enough (and the
front end BW is so
side) that I don't even have a panel-accessible control to set its frequency.

Everything is powered from an old Freed-Eisemann PSU/amplifier that
originally took
2x 5U4 and 2x 6L6 (plus a 6J5 phase splitter). I toned this down to
use 2x 5V4 and
2x 6V6. (The 6V6's are running above their rated max plate voltage but
I set up the
biasing for only about 8w dissipation. Unfortunately I melted the
screen grid on one
of them before I realized that a previous modifier had removed the
screen dropper
resistor!).

The two 19" rack units are mounted in the front of a 22"x14"x15" Bud
cabinet that I
was unable to throw away. The PSU/amp at the moment is sitting on the
bottom shelf.

The radio is a pleasure to use. Sensitivity and stability are
excellent, and drift is almost
absent. Tuning SSB and CW is easy with the long tuning scale and
impressively little
backlash. A filter placed after the mixer would help selectivity a lot
(I considered this
but decided to leave the front end completely original).

Returning to my 'real' receiver project now. But it was a lot of fun
putting this whimsical
rig together (and then forcing myself to stop making modifications to it!).

73, ian