Bill wrote:
Patrick Turner wrote:
(SNIP, SNIP, SNIP)
Now in the real world we find that we can't really build our perfect
series loaded tank circuits, and some shunt losses intrude, causing the
tank Q to not increase as much as we would like at the high frequencies,
which results in a somewhat wider bandwidth at the top of the dial. I
suspect that the designers of these sets made an effort to compensate
somewhat for this effect, by choosing Qs that made the bandwidth slightly
narrower than optimal at the low end of the band
True.
(more SNIP)
For TRF, I'd suggest trying a "loosely coupled" antenna input coil,
then 3 other identical LC coils so a 4 gang tuning cap is required, or a pair of
old
but identical twin gangs, with those large dia wheels attatched of the same dia,
allowing
the dial cord to be run around the two wheels.
Be advised that the old 1920's multi-gang tuning caps are generally
found nowadays in very leaky condition. A thorough cleaning and baking
is quite often called for in order to have them behave respectfully.
The last 1932 radio I serviced seemed to work as intended
without coil baking, ( or even frying or grilling ) , :-)
All coils can be wound at home using 1.5" cardboard tubes and 0.4mm dia wire.
The coils need screening, so don't be tempted to use old steel tins from
the kitchen, that will damage the Q. Cans must be 1" away from windings, and
done with Al or Cu sheet, but need only be quite thin material.
Cardboard tubes are desirable if one is looking for MINIMUM Q.
I was going to suggest old toilet paper roll inners, soaked in varnish
before winding, and waxed after.
One don't want an extremely high Q.
If your
humidity hovers above zero percent you can count on even lower Q...but
it won't be predictable :-) Nowadays a nice solid/consistent BCB
inductor can be made with an FT-82-61 toroid core with +/- 50 turns of
#26 enamelled wire approaching midband unloaded Q numbers of 300 or
better. I've yet to find any coil from an old BCB TRF set that comes
even close to this. Under 100 is not atypical.
Even with Q = 50, the BW at 550 kHz is 11 kHz, which is OK.
It only allows 5.5 kHz of audio, so hence you'd need two
LCs stagger tuned at the low end of the band.
An added advantage using toroids is that screening is not normally
required. Shoving a 1.5" solenoid coil into a box with only 1" of
spacing is a good way to kill the Q of the ckt.
Again I have to pay homage to my ancestors and say that many an ancient old
radio had sufficient Q with RF coils which were a 1" dia solenoid inside a 3" can.
No worries.
Using ferrite reduces the size of the coil, and hence the can.
But DIYers wanting a spectacular radio should keep all the coils and vari caps
nice and big alongside old 6U7 pentodes with top caps.
But, I've described a method of getting 3kc selectivity at the low end
of the band that will likely be 20-25kc (measured, not a guess) at the
high end in a 2-stage set. The point is only to illustrate why this
isn't as good an idea as superhetting.
I have been saying all along that TRFing is a pita because of the unfixed bw
at different F.
Its only good for locals well spaced apart.
In the real world my experience says a 2 or 3 or 4 stage set works great
on a 650kc station with another strong local present at 700kc. But it
(the same scheme) will NOT work for your 1450kc station with a strong
local at 1500kc.
Fortunately, no such situation occurs here in Oz for local strong metropolitan
stations.
But in regional areas, its all a bit different, and its where the superhet kills all
TRFs.
This is a shirt load of work to trim for equal performance along the band.
Hasn't yet been accomplished in 80+ years of radio...
The radio of mine has stagger tuning of two LC circuits simply cascaded, both before a
resistance loaded
cascoded triode amp, which feeds the mixer of a supehet.
The first LC has a loose coupled untuned primary winding for the antenna,
so the antenna capacitance/inductance does not much affect the secondary, which
is tuned by a gang cap.
Then the output from the top of this LC has a 39k 1 W carbon resistor to couple
to the next LC, tuned by a second gang cap.
The R cas the right amount of stray C and R to make a good couple.
Both LC coils are lowish Q, and the attenuation just outside the 22 kHz pass band
at anypoint of the BCB is steeper than having a single tuned LC,
and provides some initial selectivity to stop a strong station
cross modulating a weaker station in the mixer.
I could have soldered on to make similar set up with 2 or 3 twin gang caps
each with identical tuning caps.
I think it wopld have worked OK.
It'd have been harder to align.
Patrick Turner.
-BM
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