John Stewart wrote:

Patrick Turner wrote:

John Stewart wrote:

This one is in RDH4. I'm a bit surprised no one has referred to it,
although I've not watched the posts too carefully recently. I know
several
out there have a copy of RDH4. Please excuse if someone has already
referenced this circuit.

See it at ABSE & ABPR

Cheers, John Stewart

What page in RDH4 has the improved detector?
Many of us *do* have RDH4.

I can't see anything about it at ABSE or ABPR.

Patrick Turner.

It shows up on page 1074 of my copy of RDH4.
This is an original I bought around 1956 while I worked at the
research division of Ferranti Electric. The publish date is February
1954.

Cheers, John Stewart

Yes, I see that schematic OK in the Book.

The 6SJ7 driver tube ahead of a 6V6 output has NFB applied from
6V6 anode to 6SJ7 cathode via a 150k and 1 k divider, so the SJ7 cathode
signal
is nearly the same as the SJ7 grid signal, which comes from the
wiper on a volume control pot which is the current sinking R from the
detector caps.
The grid of the 6SJ7 is biased from the 1k cathode R, so
the input resistance into the 6SJ7 is perhaps at least several times 1M,
which thus presents a high AC coupled load.

I might add that in real circuits, the value of R2 is quite critical for
lowest
thd, and it should be estabished experimentally for lowest thd;
too high a value will have terrible cut off distortion on the positive
peaks of the audio,
and too low a value will dissallow AVC voltage to be made, allowing too
much IF amp
current, and there will be terrible distortions.

But this circuit still has the diode of the detector powered via a vigh
impedance
circuit of the secondary of the IFT,
and its still not a best possible outcome.

Better of course would be to have a CF tube to accept the IF envelope,
and the low impedance output from the CF can then power a crystal diode, or
a tube diode
in a variety of ways I have previously explained in post on the matter.


Patrick Turner.