[HBR] Yet Another HBR Concept

[hbr@mailman.qth.net]

Okay ... let's try to put it all together in an idea for Yet Another HBR.

1. No RF stage.   Not needed for sensitivity below at least 10 meters, 
useful as variable (AGC-controlled) attenuator but let's keep this 
simple.

I have done a no-RF-stage receiver before -- they work okay if you 
have enough IF gain and use a 2nd detector with gain.

2. Antenna tuned circuit is push-pull, drives grids of 6ES8 (or 6AQ8?) 
mixer in push-pull.  That ought to give excellent IFDR ... like around 
100 db.  This idea is from the G2DAF Mk II receiver except that the 
push-pull RF stage is omitted.

3.  IF is 6 Mcs.  VFO tunes 4.25-4.75 Mcs.   Premixed with multiple 
of 1.75 Mcs to get f+6 Mcs, for example 4.25-4.75 + 5.25 = 9.5-10 
Mcs to tune 3.5-4 Mcs.   I *believe* this conversion scheme has no 
spurs in the low four bands -- does anyone see something I've 
missed? 

Hummm ... 2nd harmonic of the VFO comes very close to the output 
frequency on this band.  Might be well to move the IF down a little 
and reduce the VFO freqs -- maybe 5.75 Mcs and 4.0-4.5 Mcs or 
something close to that.   And/or might balance the premixer to 
reject the VFO.   For example push-pull VFO premixer input, crystal 
controlled signal injected on cathode, single ended premixer output.  
(Unusual form of mixer -- might not work!  Need to check ...)  A push-
pull VFO has very low 2nd harmonic output.  It would require a two-
gang VFO capacitor but you get some help with stability because 
tube capacitances are effectively halved.     But stability for a VFO at 
< 5 Mcs is easy anyhow -- might not even bother with a 'unit VFO' 
design.

The oscillator chain output is ganged with the mixer grid tuning -- 
requires a three gang cap.   Three coils (center tapped mixer grid, 
crystal osc output, oscillator chain output coil) must be 
bandswitched -- counting the antenna winding on the grid coil, a total 
of five contacts.   No two coils on the same frequency.

4.  The multiple of 1.75 Mcs is produced by a Hahnel oscillator  
which is basically an oscillator locked on multiples of a crystal 
frequency.   They were investigated by the military in the 50's as a 
possible way to do bunches of crystal controlled channels.   The 
thing will do amazingly high multiples -- in the 100's -- with 30db or 
so purity.  It isn't too complicated -- can be one tube with at most 
two tuned circuits -- although the circuit parameters have to be 
carefully adjusted.   And output must be link coupled from the 
multiple tank circuit since RF voltage at both the crystal frequency 
and the multiple is found throughout the circuit.

The military never went anywhere with it because solid state was 
about to happen and synthesizers and phase-locked loops (both 
capable of better purity at the cost of more complexity) took over. 
But the circuit remains potentially useful in simple equipment -- here, 
for example, to get all the ham bands from a single crystal.   

5.  Drive to mixer is on cathode -- single ended.   Mixer output push-
pull via transformer to 6 Mcs ladder filter.   The matching is not too 
hard via a capacitive voltage divider.   I need to check what strong 
SWBC stations are on or very near 6 Mcs -- with only one premixer 
tuned circuit, one wouldn't want to be spot on one of those religious 
megastations.

An IF trap will be needed in the antenna circuit in any case.

6. Two stages 6EH7 IF @ 6 Mcs.   The high gain means great care 
in shielding and bypassing.

7.  BFO is 'rubber crystal.'   At 6 Mcs you can get plenty of swing to 
receive both sidebands.

8.  Usual 12AX7 AGC/audio plate detectors and 6U8->2x6AQ5 audio.

That's all.   I wish someone else would build this thing first.

Walt
KJ4KV