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.

Patrick Turner wrote:
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%.

Don't forget to do a measurement at 5KHz, as various distrotion
products can go up for higher audio frequencies at the demodulator.
Namely tangent distortion, which may not show at 1KHz but may show
at 5KHz.


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.

Modulation Rise it's called in RDH4. If you have an RF amp stage, use
a variable mu tube there (signals are still small) and change the IF
tube from say a 6BA6 to a 6AU6 or such sharp cutoff tube. Another
solution is to use only a fraction of the AVC voltage on the IF tube.
Voltage divider is the usual method.

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.

The local oscillator (usually a 6BE6) develops a fair amount of negative
bias that you could tap. Connect a 10 or so megohm resistor physically
near the oscillator (to lessen added stray capacitence) G1 and connect
the other end to the AVC line.

Another method is to add more resistance to the cathode resistor on the
IF tube and bypass it to ground. This will make the G1 look to have
more negative bias on it as seen by the cathode. And reduce the gain
some. Additional shielding and careful lead dress might help tame that
tube.

Robert Casey wrote:

Patrick Turner wrote:
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%.

Don't forget to do a measurement at 5KHz, as various distrotion
products can go up for higher audio frequencies at the demodulator.
Namely tangent distortion, which may not show at 1KHz but may show
at 5KHz.

At 35 vrms of undistorted output at 400 Hz, the detector seems fine, but at 2 khz the
tangential distortion starts.
But with 10vrms output of audio this distortion occurs first at a much higher F.
Reducing the value of the peak and hold cap from 200pF to 100 pF would improve the undistorted
bw.




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.

Modulation Rise it's called in RDH4. If you have an RF amp stage, use
a variable mu tube there (signals are still small) and change the IF
tube from say a 6BA6 to a 6AU6 or such sharp cutoff tube.

In this radio I wanted to have a vary U octal tube, and the 6G8 was chosen because I had it.
In another radio, I use a 6BX6, with fixed bias.

Another
solution is to use only a fraction of the AVC voltage on the IF tube.
Voltage divider is the usual method.

That is indeed what I am doing, but it takes awhile to get the divider values 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.

The local oscillator (usually a 6BE6) develops a fair amount of negative
bias that you could tap. Connect a 10 or so megohm resistor physically
near the oscillator (to lessen added stray capacitence) G1 and connect
the other end to the AVC line.

The ECC35 is a triode hexode, and the arrangements I have made for it are fine, and a bit
different to
6BE6, which I quite like.

Another method is to add more resistance to the cathode resistor on the
IF tube and bypass it to ground. This will make the G1 look to have
more negative bias on it as seen by the cathode. And reduce the gain
some. Additional shielding and careful lead dress might help tame that
tube.

In triode hexodes, the cathode should be grounded, lest the oscillator cathode current
be injected into the hexode cathode circuit; the grid of the triode oscillator
feeds into a grid of the hexode, and that's all that is wanted.

Patrick Turner.