[HBR] Another Receiver Project

[waltah at earthlink.net]

The list has been fairly quiet the last couple of months ... summer 
and growing grass, I suppose.  

I decided that the recent general coverage receiver project was 
simply too many tubes for current conditions in the KJ4KV 
household -- growing grass and various other things.   That project 
has been set aside and I've started on something a lot simpler, 
namely an attempt at a better ham-band only set under the name 
'HBR-4' for 2004.   That series of names will be good until it starts 
overlapping Ted Crosby's numbers and six years is a long time.

I wanted a single-conversion premixed design along the lines of the 
W1OMX set and the Tempo One transceiver.   That eliminated the 
3.18 Mcs IF of the FT-101 because the VFO frequency comes out 
too low when that conversion scheme is switched to single 
conversion.  The Tempo One used a 9 mcs IF with crystals 14 Mcs 
above the upper end of each band and a VFO tuning 5 - 5.5 Mcs 
and subtracting the VFO from the crystal frequency and I had a 
couple of parts sets, including an intact VFO, crystal filters, and 
more.  

I wanted either the push-pull front end of the G2DAF receiver or a 
beam tube front end, probably without an RF stage, along the lines 
of the JMD and others.   I settled on the beam tube because of the 
difficulty of tuning and bandswitching three sets of balanced coils 
and because I wanted to try a beam mixer.  

I'm using a 6JH8 beam tube -- much cheaper *and* later than the 
7360.  Plus one of the SSB For The Radio Amateur books gives a 
few design numbers and some discussion of that tube.

Only the SS-1R of all the sets I found used a beam tube correctly --
that is, with push-pull excitation of the deflection plates and push-
pull output to the IF.   But that was a double conversion design, 
with beam tubes for both first and second mixers, leaving me to 
work out a scheme for doing the premixing that would have 
balanced output.

I settled on a pair of 6J6's, each tube operating as a push-push 
mixer, #1 grids excited in parallel from the VFO and #2 grids push-
pull from the crystal oscillator, probably through a suitable 
transformer.   (No push-pull oscillator circuits that I know of for 
series-mode crystals.)  The plates of each 6J6 are in parallel and 
the two tubes operated push-pull to drive the beam tube deflection 
plates.  

That arrangement is balanced against the VFO which is the only 
oscillator below the signal frequency.   W1OMX had some trouble 
with birdies (1 per band except 6 on 15M); I'm hoping to at least 
keep the amplitudes down.

The VFO will be a 6KT6 pentode.  

Three IF's because there's no RF stage for AGC.   There's space 
for a multivibrator for 25 Kcs marks.   The T-1 IFs are basically 
unusable because they were intended for PC board mounting; I'll 
wind my own using toroids.

I used an FT-101 chassis and planned the compartment layout to 
further reduce spurious signal propagation.   The oscillators are in 
one compartment, the front end coils (planned with no RF stage) in 
another with no other wiring except the crystal calibrator.  The 
mixer is located at the partition between the compartments.   

The mixer output goes through a 9 Mcs crystal filter which hops 
over the partition separating the front end area from the rest of the 
set.   Everything coming into the front end goes through feed 
through caps; the IF/mixer filament supply is daisy-chained through 
chokes and is on one leg of the 6-0-6 VAC filament transformer 
with the oscillators on the other.   

I'm sure I've missed something but it's the best layout I've done.  

The Tempo One VFO parts mix and match pretty well with the FT-
101 and what I've got is the best of both -- the T-1's excellent 
planetary ball final drive and VFO box with the FT-101's dial plate 
which is correct for my conversion scheme which tunes in the 
same direction on all bands.   (The T-1 uses no crystals for 80 and 
20 so its dial is calibrated 0-500 and 500-0 with nothing that's really 
right for 80.)  A few adjustments are requred to shaft lengths to 
make everything fit.

The alternative would have been to use the FT-101 VFO as a  
foundation but that one tunes 8.7-9.2 Mcs so I'd have had to 
replace the coil and start at square one getting dial tracking.   

The tank circuit parts are all in the VFO box; I used glass-metal 
feed throughs to get the two connections to the tube socket so as 
to sink the heat coming up the wires.   

There are a few other novel points but I'll wait until they work (or 
fail!) to describe them.   

After about a month I have the chassis -- new top plate on FT-101 
frame -- and panel done.   Most parts are mounted and wiring of the 
power supply and audio is done; starting on the second detectors 
and AGC stages.   

Hoping to get over 100 db dynamic range with decent sensitivity 
with this scheme but -- we'll see.   

Walt Hutchens
KJ4KV