[Patrick Turner]

On Jul 19, 7:15*am, John Byrns wrote:
In article ,
*Patrick Turner wrote:

On Jul 18, 5:51*am, John L Stewart John.L.Stewart.
wrote:
Turns out we discussed this one & others back in 2004. So here it is
again from p1074 of RDH4.

All very well, but it uses a twin diode tube. If I am to put an extra
tube into an existing radio it'd be for a 12AU7 cathode follower
detector. Probably works better than the boffin method in RDH4.

I don't imagine that they intended you would use an extra tube, only an extra
resistor and capacitor. *I assume that this circuit dates from the era when 6H6
detectors were popular so the diodes were already there. *this circuit could be
implemented in radios using a 6AV6 detector and 1st audio tube by replacing the
6AV6 with a 6T8.

All the old radios I have seen contain NO separate double diode tube.
The only signal diodes are grounded cathode types within the vari µ
pentodes for IF. The 27.4 schematic shows a tube diode 0.1uF and
variable R5 between cathode and 0V, and this would not be able to be
implemented using a pentode tube whose two diodes have a common
cathode.

The question is, does this circuit really do anything useful, and if so what?

Its very simple John. The D2 generates a positive going voltage at its
cathode from 100pF feed from IF amp anode. This is supposed to counter
the cut off distortion one sees with RC coupling in detectors, and if
RDH4 says the ducks can fly, then they will, and you need to be
careful looking up when thousands of ducts fly over.
I suggest you don your duck observation gear and venture to the swamp.
Beware of duck hunters with shotguns.

Notice there is a 1M taken from D2 anode to feed AVC, and this is
necessary to get the + voltage at the D2 cathode.

D1 in the 27.3 circuit could be a diode in a pentode, an D2 could be a
Ge type because its distortion production does not matter, and D2
cathode is bypassed to 0V via 0.1uF. D2 is merely there to make a Vdc
voltage.

But like many things in RDH4, I have never ever seen this circuit in
any old radio where performance is so often quite appalling in 101
different ways.

Patrick Turner.