John Byrns wrote:

In article , Robert Casey
wrote:



Pat, I just ran a simulation of the use of two cathode followers (I used
12AU7's) and
a 6AL5 diode ddetector. I got very good results, using a 1KHz
audio sine wave, modulated onto a 95% modulated 455KHz carrier, got 55dB
down
of the 2nd and 3rd harmonic. I also threw in a little bit of positive
bias to the detector
diode to partly get it up above the "Knee". About 300mV worth of bias.
Of course
this requires that the AM signal be at least 2Vp-p and that the
modulation index be
about 95% (leaving 5% carrier at the valleys). As I'm pushing a bit
into the "other
side" of the AM signal. IOW, I moved the zero crossing voltage a bit to
make "fake
exaulted carrier".

See a screen capture of my simulation schematic and results in ABPR




Robert,

Your simulated detector seems quite different than Patrick's, mainly
because your bias network tends to turn the diode off, while Patrick's
bias turns the diode on, compensating for his poor AC/DC load ratio, at
least at one fixed carrier level.


The diagram is a bit confusing, but the simulation "power supply" which
has its normally
positive end tied to ground I made it a negative voltage. (I was lazy,
easier to change the
value and make it negative than to redraw the wires...). In any event,
I'm biasing the diode a bit more "on".
This to get up out of the curved portion of the vacuum tube diode
characteristic.



I still have to actually try this circuit (with or without bias) in
hardware. I have a Heathkit
AM tuner I could try it in. I'll use submini tubes with wire leads,
smaller and no sockets to
mount.