John Byrns wrote:

In article , Patrick Turner
wrote:

The diode loading of the IFT does cause some non linearity in the IF amp,
which is detected, and comes out as audio thd/imd.
In my radio there is no AVC applied to the IF amp which is a sharp cut
off type
6BX6,
with an unbypassed Rk, and with the lower than usual IFT but linear R load
only, there is a fair current change in the IF tube, so a fair amount of
current
fb from Rk,
so the IF tube amplifies the enevelope with far better linearity than
most other

sets I have examined.

Now this seems like a reasonable concern about the effect of the diode
detector on the IF amplifier, unlike your concern about the current pulses
into the diode which are rendered irrelevant by the filtering/flywheel
action of the IFT.

The load varies during thre audio cycle if the ripple voltage changes.
So the IF amp gain changes, ie, there is intermodulation distortion caused by
the
detector. My concerns are not irrelevant.



When the signal level is low, or the negative modulation high, the non
linearity of the diode detector causes a lighter loading on the secondary
of the IFT. This of course is going to create envelope distortion at the
secondary of the IFT, and the plate of the IF amplifier tube, and many
authorities claim this distortion is a good thing because it implies that
given the finite source impedance of the IFT secondary, the drive voltage
to the diode detector will increase at these points, partially offsetting
the distortion of the envelope detector under these conditions.

I don't know what authorities say that distortion is a "good thing"

A cathode
follower would negate this effect.

You've been trying to tell me a CF isn't needed all along, but here you say its
OK.

One thing that I haven't seen
mentioned is how the IFT plays into this since it has a sort of impedance
inverting characteristic, as we discussed earlier, if the secondary is
shorted, the impedance of the primary increases, just the opposite of what
we would expect.

I found that loading the secondary reduced the gain of the IF amp, ie,
the load is reflected to the primary.


I suppose this effect may not hold over the range of

impedances we are actually talking about here, but if it does, it would
seem that it could negate this claimed advantage of driving the diode
directly from the IFT secondary without an intervening cathode follower.
I am curious now and will have to crunch a few numbers, see what really
happens with this, and see if the experts are really correct.

Regards,

John Byrns

Happy number crunching, but I prefer late nights with a soldering iron & cro.

Patrick Turner.



Surf my web pages at, http://users.rcn.com/jbyrns/