I finally got the revised Stromberg Carlson radio tuner section running about as
well as it ever will.
It includes the dual cathode follower stage for which I posted a schematic at abse and
abpr last week.
The only changes I made was to use IN914 diodes, and 200pF and 1M for the audio RC
peak&hold
network, as well as use a 12AT7 for the CF parts.
The idle DV at the cathode of V1 is at +48 v without a carrier, but
with a test signal carrier a little larger than the strongest station here,
The DV at the C after D1 was +103v, indicating a carrier of
55 pk volts, and the audio at 400 Hz at 90% modulation was 36 vrms, or
51v pk.
I measured the thd with a 1 kHz low distortion af signal,
and got 3% at this level of signal, which is about 2 dB short of total
overload if the IF amp.
At 30% modulation, thd was about 0.2%.
Then when I reduced the RF input by 30 dB, the audio output and carrier level
all fell by 10 dB, due to AGC action, and remeasured and got the same thd figures.
The distortion is so low in the receiver including the detector that its thd
cannot be measured because it is dominated by the thd in the RF test gene,
which measures similarly when I measured it alone.
The RF gene can achieve about 96% mod but the thd becomes quite high at about 7%,
because the pentode RF tube used does not cut off at a linear rate, and
I really should have used a pair of PP tubes with a NFB loop to make the RF modulated
signal
have far less thd in the AF envelope shape.
So the conclusion is that the radio I have just got running
does not produce the buckets of thd like so many other radios I have tested,
and anyone is welcome to use the design I had in my posted schematic.
It was of some importance to get the AGC application correct.
Too much directly applied AGC will virtually cut off the IF vari mu pentode IF amp,
so that with 28vrms of audio from as big a carrier which will support that,
you may only have 0.5 mA of anode current, and when you plot
the load line, it just isn't quite right.
Better to make sure that with extreme levels of carrier, the anode current is
over 1.5 mA, and thd I expect is a little lower.
For lower levels of carrier, tube current will increase to a max of 5 mA
at no carrier at all.
There should be some method of applying at least about -1.5 v to IF and mixer tube grids
because
such tubes go a bit beserko when biased close to 0V.
I have the AGC generated by a 33pF from the V1 cathode taken to a IN914 with its cathode
grounded.
The negative voltage generated at the anode of the diode goes to a 0.05 uF via 2.2M
for the IF amp G1, and then 1M to another 0.05 to the G1 of the mixer, then 2.2M to
the -1.5v from a back bias R in the PSU.
Anyway, it works OK, and lots of other value changes didn't seem to work as well.
Audio bw was 7 kHz.
Both IFTs had their coils moved closer together, just short of causing a twin peaked
response.
After moving them and testing them in their cans, and connecting
the right value of fixed capacitance to each coil to allow the easy adjustment of the
adjust caps for 455 kHz, I rewaxed the coils in a vat.
I really don't like this type of IFT, with a ceramic base bolted to the top of a 60mm
dia al can
which is difficult to quickly inspect and modify,
and which were a complete pita to work on, but my patience paid off,
and the set was easy and reliable to align, for a symetrical attenuation slope
each side of 455 kHz.
Each of the four LC circuit are loaded with 150k, and loading with 100k
would be quite acceptable, and probably give an audio bw of 8 kHz instead of the 7 kHz.
The second CF cathode has a 39 k load to 0V, with about 2.2 mA of Ik at high levels of
carrier, and perhaps 1.2 mA at low levels.
The higher the carrier, the higher the CF idle current, which assures their linear
operation
with increasing carrier and audio output signal.
The audio is fed from CF2 cathode via 0.1 uF cap to 100k which is in series with a 100k
log volume pot.
A 390 pF across the fixed 100k slightly compensates the audio bw.
A tone baxandal passive 100k linear tone control pot will be fitted to give about
a further 5 dB treble boost or 6 dB cut at 3.4 kHz, necessary, because nearly all the
radio stations have compressed and quite bright sounding programme material,
and the mobile phone interviews and over compressed programmes are too bright.
Its quite a nice sounding tuner when connected to a 25 watt UL amp and one of my monitor
speakers in the workshop, which are well revised old Kefs, but nevertheless quite
revealing.
Bass respons eof the tuner is down to around 8 Hz due to the 0.1 uF and 200k AC load on
the output
of the second CF, which is direct coupled to the first CF, since there is no AC couple
load.
The 1M plus 200 pF could be altered to to 1M and 50 pF, which would increase the
ripple voltage about 4 times, but it will still stay substantially the same value with
audio
output signal of say 10vrms, when a low length wire antenna is used.
But the discharge rate of the 50 pF will be four times faster and the onset of
slew rate limiting will occur at a higher F and output voltage, the cost being
slightly lower detector efficiency.
The low pass ripple RC filter of 100k and 39 pF following the peak and hold 200pF and 1M
has a pole above 40 kHz so the attenuation at 45 kHz is around 22 dB,
and the level of RF finding its way into the audio amp and to the speaker would be
negligible,
and not cause any problems.
But folks could use a pair of 100k to feed the second CF, and have a pair of 39 pF caps
arranged in a feedback filter on the CF giving twice the attenuation of RF ripple.
It would be possible to construct one's own IFTs, with separate coils to be distance
adjusted, but
to get the required Q without ferrite cores which are hard for the diyer to make and
fit,
seven core fine litz wire is needed, and when one buys any of that today I don't know.
Its a pita to work with, since tinning each fine strand is difficult.
And when winding a coil, the traversing has to be done back and forth with a special
guided winder,
which hardly anyone would have, so they must make a lathe to generate the final wind up
which gives low self capacitance, and they must also have a method of makng the wound
wires adhere
to each other as the winding is done, ie, some sort of varnish.
Temperatures have fallen to around -5C and nights are a bit chilly even with a heater in
the shed;
it merely generates a cool breeze.
I now have the SET audio amp to build, which I should be able to do while I am asleep.
I think I have a spare 6L6, which would be easier to drive than a 6CM5 in triode.
Patrick Turner.
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