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.