Jon Noring wrote:

Bill (exray) wrote:

Jon Noring wrote:


Nice to hear from you again, Bill!

Indeed, hi again.

I think the ultimate explanation is the desire for the tube tuner to
remain a pure TRF design, for audio quality purposes -- John Byrns
has discussed this as well

Firstly, I'm not getting the WHY this (TRF idea) is of such great
import. Conceptually its a nice idea to not add 'unnecessary' stages
but if one harkens back to why this was (and still is) the panacea to
overcome the TRF ills then maybe they shouldn't be categorically
discarded as bad things.

As soon as one decides the tube tuner is to be a pure TRF, then one is
instantly confronted with the very difficult problem in how to get
optimal bandpass characteristics for all the frequencies from 500khz
to 1800khz. As I read the many messages on this from the Google
archive, it clearly borders on a nightmare to overcome when the only
degree of freedom the TRF designer has to work with is a variable
air capacitor. John Byrns is wrestling with this issue even as I
write, trying to find the magic formula.

I'm of the mind that going pure TRF is not necessarily the answer to
your original request. But we can run with that for the sake of
discussion. There may well be some magic combination of ganging
inductors and caps but upon finding that we'll still have to weigh in
the cost, complexity, repeatability, performance, etc compared to a
superhet. Radio folk haven't reached that point yet in 80 odd years so
there's no disagreement to be found :-) And don't assume that radio
minds are in a 'box'. The crystal radio fanatics beat this issue to
death on a daily at a very sophisticated level.

When confronted with an intractable problem in design, it is time to
think outside the box. It is obvious we need to have more degrees of
freedom in tuning, but for continuous tuning all this does is add more
knobs to tweak, not unlike the TRF designs of the 1920's. Do we want
to go in that direction?

I don't...at least not for the purpose of hooking up something to my
home stereo for e-z audiophile listening.

But since we observe the stations on the BCB are restricted to
specific frequencies, this means we don't *need* to have continuous
tuning, and from this paradigm shift the channel TRF idea springs
forth.

I disagree 180 degrees. If BCB channels could be counted on as
equivalent building blocks maybe this would apply but we are talking
three octaves of frequency range.

As I noted in a parallel message I just sent out, the channel TRF has
its problems for practical implementation, and it goes against the
almost 100 year paradigm of continuous tuning that is so ingrained in
BCB radio tuner design, but I think it solves that otherwise
intractable problem with TRF tube tuner design. But, if John Byrns
or someone else can discover the magic way to allow one degree of
freedom to give optimal enough bandpass design for a TRF tube tuner,
then that's the direction I'd recommend going, and not the channel
TRF approach, interesting as it is. (Of course, understandably many
still recommend super-het.)

I fully understand what you are suggesting and all I can say is that
we've been there and done that. When I stated that you could build a
nice hi-q BCB circuit that would yield 3kc bandwidth at 550 and 25 kc
width at 1600 I wasn't exaggerating. Intuitively one might think that
hey, I'll twist the LC combo somehow and come back to the same Q across
the band simply doesn't work...either in numbers or worse still in
practice. I'd like to say you can't obtain a sharp 3kc bandwidth at
1600 with a simple LC circuit but thats too open-ended. Suffice it to
say that it ain't easy.

One can visualize some scenarios of mechanical (or electrical) ganging
of components that might approach this goal but that visualization
typically falls in the ditch once one tries to transfer the idea from
the brain to an actual breadboarded version of the concept.

Going back to some of the earlier filter flatness discussion, well toss
that idea into the mix when you think in terms of TRF. Not only do you
want to achieve a specific width but you want it to be flat. My 3/25 kc
TRF scenario isn't flat at all. Its a big peak that just broadens out.
When we say a 'bandwidth'number we are relating to something specific
like 3 or 6 db down from the peak. Its still a peak in this context.
So whats happening at 20 db down? You guessed it, that 25kc number is
150 kc wide. I dunno how you could control the width AND the flatness
AND the skirts.

I'm a fairly recent convert to crystal radios. For the sake of
discussion there's little difference in xtal technology vs trf
technology in that both are non-superhet. I get absolutely glorious
quality audio from my xtal set when fed thru an amp. With 6 or 8 knobs
on the front panel and top notch components I can find a dead spot
between semi-locals on 680 and 690. With a local station on 1370 it
takes traps and VERY hi-q stuff to ferret out semi-locals on 1240, 1290
and 1480. Its as if it were a totally different radio from one end of
the band to the other and this has been the plague of TRF circuits since
day one. If I didn't have the local 1370 I could safely say, hey Jon,
this is the ticket, but there's scarce few of us who don't have a strong
undesired local station to bollox up the works.

Go superhet, my man.

-Bill M