EMRFD Message Archive 2285

Message Date From Subject
2285 2008-10-19 08:58:05 bkopski AN ANTIQUE RADIO TECHNIQUE QUERY
I'm seeking info regarding a technique employed in an antique radio
that I vaguely recall described in an article some decades ago.

Specifically, as best I remember, the article described its amplifier
chain of 2 or 3 stages wherein the interstage transformers were
physically tilted (leaning over) so as to "minimize coupling" between
them for stability reasons (I think). I believe they were all in a row
and leaning in the same direction at the same angle. I do not know if
this was a TRF or superhet configuration, but the accompanying photo –
as I recall it – looked more like an i.f. strip (?). And of course I
don't recall the name of this product / design – that would make it

My curiosity lies mostly in "why" and "how" this tilted-transformer
technique works (promotes stability) – assuming it does.

Thanks everyone, in advance,

Bob, K3NHI
2286 2008-10-19 09:12:13 Graham Re: AN ANTIQUE RADIO TECHNIQUE QUERY
Were the interstage transformers shielded, or open coils?
2287 2008-10-19 16:01:15 Allison Parent Re: AN ANTIQUE RADIO TECHNIQUE QUERY
Antique radio it was open and relied on positi
2289 2008-10-20 06:38:12 bkopski Re: AN ANTIQUE RADIO TECHNIQUE QUERY
2290 2008-10-20 08:01:30 Graham Re: AN ANTIQUE RADIO TECHNIQUE QUERY

Well, unshielded coils will transfer energy between them, and if one is
in the input of a tube, and the other is on the output, then you could
have an
oscillator or negative feedback, or something in between. If coils are
rotated, then you can change the relative coupling, and even the phase
of the coupled signal. If they are at 90 degree angles, then there
can be close to zero coupling.

So, it sounds like some smart engineer used these concepts to
control the feedback in the amplifier stage design. So, whether he used
the tilt to stop oscillation, or to add a little negative feedback for
or a little extra positive feedback for regenerative gain, we will probably
never know.

They usually use shields these days, rather than tilt the coils.

I still see factories "knifing" coils and streatching and compressing and
twisting coils in super cheap RF electronics, such as TV tuners and
Set top box tuners.

2291 2008-10-20 10:24:30 mirroromatic Re: AN ANTIQUE RADIO TECHNIQUE QUERY
I think you're describing a "neutrodyne", a three stage TRF (tuned
radio frequency" receiver.

They had three separate tuning knobs and caps, with three coils, all
tilted at a certain angle, or mounted at right angles to each other.

See http://www.antiqueradios.com/neutrodyne/pages/00.htm

Since they were tuned grid/tuned plate amps (in effect), they need to
keep the coupling down between stages to keep they from oscillating.
They also used a neutralizati
2296 2008-10-20 12:12:41 bobtbobbo Re: AN ANTIQUE RADIO TECHNIQUE QUERY
--I think those coils might have been called "variometers"

Bob, K1AO
2297 2008-10-20 12:12:43 Alan Melia Re: AN ANTIQUE RADIO TECHNIQUE QUERY
Hi Bob if it was Wireless World I guess it must have been a pre-WWII issue.
I have a pile of these mags since about 1960 only. I am not sure how easily
it would be to search indexes.

It occurs to me that inclining them the same way would do little to reduce
the coil-to-coil magnetic coupling....so it perhaps it might be that the
coils were inclined to move the unscreened coil further away from the
electic field of the plate (anode in the UK
:-)) ) whilst still keeping a compact layout. ....?? just a thought !

Alan G3NYK
2299 2008-10-20 12:29:54 bkopski Re: AN ANTIQUE RADIO TECHNIQUE QUERY
2300 2008-10-20 13:01:29 victorkoren Re: AN ANTIQUE RADIO TECHNIQUE QUERY
I have found the answer to the question in the book "Behind the front
panel - The design & development of 1920's radios" by David Rutland.
A radio with such an arrangement is the Freed-Eiseman NR-6 which is a
two stage TRF (a two stage TRF had three tuned circuits).
The reason for the tilting of the coils is:
"Hazeltine developed a mathematical theory of electromagnetic
coupling between RF transformers. He found that the coupling was
minimum when they were placed with their axes aligned at a certain
angle to a common center line. He calculated the proper angle to be
the one whose tangent is the square root of 2, that is 54.7 degrees."
Victor - 4Z4mE

2305 2008-10-22 06:18:07 bkopski Re: AN ANTIQUE RADIO TECHNIQUE QUERY