Bill (exray) wrote:
Jon Noring wrote:

Nice to hear from you again, Bill! I'm still in the process of
restoring the Philco 37-670 console, and will need your advice on a
couple of issues, such as how to replace the rubber insulators on
the RF chassis and on the back end of the tuning capacitor, which
are all disintegrating due to the radio being exposed to the LA
smog for decades.


For a one-channel receiver it makes perfect sense. Beyond that any
advantage is lost.

Why would I say that? You can create a perfectly acceptable single
IF filter with not so much ado. Lets use 455kc as the example. It's
considerably easier to build a single 'custom' IF filter at 455kc to
do what you want to do than it is a bunch of modules at three or
four times that frequency. Yes, you could do as you suggest but I
see no advantage in doing so. It would be more critical, more
expensive and probably not yield as good a result as a nice 455
filter.

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 (yes, we've hammered to death the poor
quality of most AM broadcasts, but that's been covered elsewhere.)

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.

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?

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.

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.)

Jon