[HBR] HBR2K Chapter 12 -- More Details

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The Miller oscillator failed:  output voltage was higher but apparent 
stray capacitance was enough higher to require lower L and thus cut 
the output on the higher bands so I wound up with a much greater 
variation in output voltage.   I went back to the Pierce.   There's 
plenty of gain in any case.

(The crystal looks inductive in these circuits; with it connected to the 
plate in the Pierce I speculate that it partly cancels out the 
capacitance seen by the plate tank, allowing a larger L for the tank.)

Sensitivity by bands (10 db s/n) after proper alignment and 1st 
oscillator changes:

80 -- 0.5 uV
40 -- 0.28
20 -- 0.38
15 -- 0.5
10 -- 0.6

Good enough for now; I may test a cascode RF stage at a later time. 
Because of the requirement for trapping of strong HF stations on the 
1st IF, a cascode is a bit messier than cathode coupled.   In the next 
few days I'll do the two-tone dynamic range test to see about 
crossmod performance.

As expected there was another bad trimmer on 10 meters; took me 
three tries to get a good one in there.  These are tiny (0.3" diameter) 
disk trimmers with a rotating ceramic plate on a ceramic base.  The 
most common failure is that the solder joint between the rotating 
brass hub breaks loose from the plated-on electrode on the rotor.   

The audio quality was not as good as I thought it should be.  
Revisiting the output transformer roll-off cap I discovered that it was a 
bit large -- fixing that helped the highs.   I went through the audio with 
a scope and found that on SSB, there was some visible distortion in 
the plate detector -- positive peaks were definitely 'squashed' a bit.   I 
reduced the drive from the carrier oscillator (it was pushing the tube 
operating point too far toward cutoff) and fixed that; sure enough the 
audio got better.

But still not really good.   Although a sine wave looked okay all the 
way through, it sounded 'muddy' on music.  I suspected phase 
distortion -- different frequencies shifted in phase by different 
amounts.   Could have put in a square wave and scoped that, but I 
followed a hunch, instead.  

I never liked my audio section design.   I had settled on a 12AX7 in a 
paraphase inverter driving p-p 6AQ5's with feedback from the output 
transformer to the cathode of the 1st stage.   The paraphase circuit, 
however, puts two coupling caps inside the feedback loop and there 
isn't that much surplus gain anyhow, compared to a desired overall 
voltage gain of 20.   Great place for phase distortion.

I tried again.   First time around I had not been able to get a 
conventional phase splitter (equal resistors in plate and cathode of a 
triode) to work with 1/2 12AX7 because with only 140 volts I couldn't 
develop the needed 15 or so volts p-p to drive the 6AQ5s -- the 
12AX7 has too high a plate resistance.   The obvious answer would 
be a 12DW7 = 1/2 12AX7 + 1/2 12AU7 but this tube is much sought 
by hi-fi enthusiasts and sells for $15 up.   

Another lightbulb experience:  The last RCA tube handbooks show a 
series of hi-fi amps using a 7199 triode-pentode.   The pentode does 
the voltage amp job with a gain perhaps around 400; it's direct 
coupled to the triode which is used as a phase splitter.   The 7199 is 
another popular hi-fi tube, but I didn't need extreme low noise and 
hum; could a 6U8 do the job?   The triode is medium-mu; it should 
do better as a phase splitter.

A breadboard hookup showed that It could, and with the RCA circuit 
the audio quality improved dramatically.  On AM, it's now as good as 
you're going to hear from a communications receiver with a 4" 
speaker.   It's better too, on SSB, but still, I think not as good as it 
could be.  I'd characterize the problem as 'slight roughness instead of 
clear as a bell.'

Jim, N2EY said:

> You might consider some sort of buffer to prevent osc pulling. 

You don't see any obvious pulling but with SSB, even a few CPS -- 
maybe 5?  10?  -- occuring on voice peaks would be enough to 
cause audible distortion.   Today's project is to rebuild the VFO with 
a buffer.   I'll probably switch from the 6DZ4 to the more pedestrian 
12AT7 as there does not seem to be a suitable later model dual 
triode.  Not a big job because the tuned circuit parts of the oscillator  
won't change but it requires a new mounting plate for the tube and 
nothing requiring metalwork is trivial.  

The other recent job was the front panel, now drilled and filed into 
shape and temporarily installed.   Big holes and a tougher alloy 
(5052 I think?) than I should have used; I wouldn't want to do that 
every day.  Paint and control markings to come later when the odds 
and ends of mechanical alignment and fitting of various parts are 
done and the set is mostly not being beat around on the bench 
anymore.  The latest picture is at:

http://www.timbreblue.com/HBR2K5.JPG

Not a great panel layout but that's the price of recycling so much FT-
101.   Thought I was being clever using a hard-to-find dual concentric 
control for the RF/AF gain functions but using separate pots with the 
RF gain located midpanel on the right would definitely give a more 
balanced look.

There is a spare tube socket now -- the result of moving the 100 kcs 
calibrator in with the filter driver stage in a 6U8.   The spare will likely 
become a 20 kcs multivibrator locked to the 100 kcs oscillator -- 
wonder if that will work?   I really liked the 25 kcs calibrator steps on 
the FT-101.   Or how 'bout a 6:1 division, giving 16.667 kcs 
calibration points ... Just kidding.   This *has* been a long project, 
hasn't it?

Another FT-101 parts set arrived yesterday.   Sure enough, with a 
modest rearrangement of the chassis it would be possible to use the 
original internal speaker.   Didn't think of that when I did the first 
design but I'll get it in the drawing.

Also in the "well, duh..." department -- the FT-101 doesn't use the 
CW carrier crystal on receive.   The USB crystal (below the 3180 kcs 
2nd IF center because of conversions) is used; you tune in 'the right 
note' of about 800 cps, then switch in the CW filter which has a 
3179.3 kcs center frequency -- i.e. 800 cps above the 3178.5 kcs 
USB crystal.   Guess who wishes he had ever even *listened* to CW 
on an FT-101.   

But I believe there's a patch of slightly less darkness ahead of me in 
the tunnel.   'Nuther month or two.   The documentation is 
progressing.

Walt Hutchens
KJ4KV