Think about it this way: if the coils are in line (on axis) the coupling is in-phase (eg zero deg). And if they are side-by-side, the coupling is reversed (eg 180 deg).

So if they are diagonally opposed there is a line of positions and angles where the two couplings are equal and opposite (not necessarily at exactly 45 deg).

In a triode TRF you can tweak the angle to give a whisker of negative feed-back to cancel the capacitive feed-back inside the tube.

........ Zim

>>>>>>>>> cancel the capacitive feed-back inside the tube.

I thought the external neutrodyne capacitor did that.

.....................

In those days there was a minefield of patents covering the different ways to neutralise a triode.

The neutrodyne used one method, but there were various others.

...... Zim

Mr Gottleib describes the arrangement of coils in neutrodyne receivers. "This method arranges the coils at a slant with respect to the horizontal. The 'magic' angle used was 54.7 degees, the tangent of which just happens to be the square root of two."

Sounds interesting. I have a copy of the magazine coming.

Mr Gottleib describes the arrangement of coils in neutrodyne receivers. "This method arranges the coils at a slant with respect to the horizontal. The 'magic' angle used was 54.7 degees, the tangent of which just happens to be the square root of two." >>>>>

Sorry Zim but did you view the links I put up previously?

>>>>>>>>>>>>>>>>>>>>>
TRF angles Quote:
""These receiver designs also often had the coils angled at 54.7 degrees between the axis of each coil and a line connecting the centres of the coils to reduce undesirable inter-stage coupling, as in this FADA.""

>>>>>>>>>>>>>
Just checked a Fada 175-A that I have - coil angles are in line and ~54 degrees (not 45) to the horizontal.

>>>>>>>>>>>>>>
Here is the link to the quote in my previous post:
http://www3.telus.net/radiomuseum/projects/FadaC75A/index.html

Hazeltine Neutrodyne - see page 8
http://www.antiqueradios.com/neutrodyne/
>>>>>>>>>>>>>

The neutrodyne was invented by Hazeltine and manufacturing rights sold to 20 other companies, FADA being one of them

....................

Yes, of course. It was the Neutrodyne that I was referring to originally.

I was hoping that the W.W. article might add some further insight on the Neutrodyne geometry.

But the Neutrodyne wasn't the only design which used angled coils. They were quite common, but they were usually mounted flat on the base-board, rather than cocked up vertically.

........... Zim

Any links to pics? I have seen plenty with coils mounted in X,Y and Z planes. Can't recall any mounted as you suggest. (It would obviously work but might take up more space).

I am talking "American 3 dial TRF's".

...................

The thing about the Neutrodyne was that the manufacturers who paid the patent fees were obliged to copy the mechanical and electrical designs exactly.
I suspect that's why they all look the same.

So perhaps the "flat" versions were European or English where the patent situation was different. Or perhaps American, but hobby built.

---

Here's a bit about various neutralizing schemes...

"Fred Terman explains that the instability is caused by the inductive loading at the plate, which looks like a negative resistance at the input.

If the input grid circuit impedance is too low at on the low frequency inductive side of plate resonance, oscillations could occur.

Radio makers like Atwater-Kent in the model 42, added a few hundred Ohms of resistance at the grid input,

Hazeltine and Rice used external capacitance with positive feedback to cancel the internal Plate-to-Grid Miller negative feedback capacitance, over a wide range of frequencies. Hazeltine obtained the positive feedback with a signal inverting transformer or extra coil at the plate circuit. Rice injected the external capacitance into a positive input with an extra inverting transformer or extra inverting coild at the Grid circuit.

Others shunt resonated the Miller capacitance in fixed frequency circuits, like transmitters. Other variants of tuned circuits were also used, that affected plate impedance, like the Browning-Drake Phasitrol.

One important point is that the oscillations are not caused by the highest gain that occurs at exact resonance, but by the inductance at the plate, that appears below resonance, and, in turn appears like a negative resistance at the input."

-----------

Another I know of is Hartley neutralization, which was based on the same principle as the Hartley Oscillator.

BTW this has little to do with the angled coils. That's a different issue entirely.

........ Zim

and

http://members.wideband.net.au/gzimmer/New/RFstabilizing.pdf

My apologies to neazoi for hijacking his thread.

........... Zim

I would like your comments on this please >>>>>>

Ganging C1a and C1b may not be practical as it may be impossible to track both gangs accurately with all the variable taps, loadings (i/p and o/p) and wavetrap interactions?

Others might comment.