Grumpy The Mule wrote:


If the capacitance of the grid is negligible, then the
resonance is determined by the inductance from the
portion of the winding paralleled by the variable capacitor.

So what about the 'top' portion of the coil being effectively in series
with the tank? Wouldn't that affect the tank values?

Tnx?

Howdy,


Maybe not. Imagine the top portion disconnected, it would
have very little effect. It's no more than a small capacitor
swamped out by the tuning capacitance.

I think if the grid capacitance times the square of the turns
ratio is small compared to the tuning capacitance, it can be
ignored. Because there are other strays that will require
a bit of pruning in any case.

The most significant effect would be when the tuning capacitor
is at its minimum value. So it's here that it may effect the
design of the inductor, requiring a bit less inductance for the
desired upper band limit compared to the other tuned winding.

There are no padding capacitors on that schematic. So I suspect
there are seperate tuning capacitors for each tuned winding on
the transformer. Because of this I believe it's not that
critical. You could measure the grid capacitance with the
tube mounted in its socket on the chassis. Then add that
capacitance times the turns ratio to the value of the tuning
capacitor when you calculate the necessary inductance value.

Or build the coil so you can stretch it (slip a few windings
further apart) a little once the thing is running. Then secure
them with a bit of wax.


73,
Grumpy



exray wrote in :

Grumpy The Mule wrote:


If the capacitance of the grid is negligible, then the
resonance is determined by the inductance from the
portion of the winding paralleled by the variable capacitor.

So what about the 'top' portion of the coil being effectively in series
with the tank? Wouldn't that affect the tank values?

Tnx?

Howdy,

Heh! I didn't look at the url for that schematic, it's an SW3!
I love National Radio gear. I had two HRO-500's, sold one, and
there's an FRR-59 in my basment. I regret having sold my pristine
RBL receiver which beat the pants off either the HRO-500 with LF10
or the Racal RAL7 with RA237 for LF/VLF work. My highschool's club
had an NCX-1000 which I lusted after... er, but only in my heart.

But back to the SW3. I recall padder capacitors mounted on the
top of the RF stage coil former. I don't know the value of the
dual gang tuning capacitor but armed with that, Wheeler's formula
and these sites, you could calculate the effect of the grid circuit
to some extent.

http://www.io.com/~nielw/sw3coils.htm
http://www.antiqueradios.org/gazette/swevol.htm


73,
Grumpy

Grumpy The Mule wrote:

But back to the SW3. I recall padder capacitors mounted on the
top of the RF stage coil former. I don't know the value of the
dual gang tuning capacitor but armed with that, Wheeler's formula
and these sites, you could calculate the effect of the grid circuit
to some extent.

http://www.io.com/~nielw/sw3coils.htm
http://www.antiqueradios.org/gazette/swevol.htm

Only the bandspread coils have the trimmer on top and thats basically to
set the rough frequency. The main tuning then becomes a bandspread
tuning. I've forgotten all this stuff - I'll have to look back at my
stuff...maybe the trimmer falls in series with the main tuning caps.

Anyway, thats just an example that came to mind. I've seen others doing
similar tricks for main tune/bandspread tune using the same cap but with
a different coil configuration.

-Bill

Grumpy The Mule wrote:
Howdy,

Heh! I didn't look at the url for that schematic, it's an SW3!
I love National Radio gear. I had two HRO-500's, sold one, and
there's an FRR-59 in my basment. I regret having sold my pristine
RBL receiver which beat the pants off either the HRO-500 with LF10
or the Racal RAL7 with RA237 for LF/VLF work. My highschool's club
had an NCX-1000 which I lusted after... er, but only in my heart.

But back to the SW3. I recall padder capacitors mounted on the
top of the RF stage coil former. I don't know the value of the
dual gang tuning capacitor but armed with that, Wheeler's formula
and these sites, you could calculate the effect of the grid circuit
to some extent.

http://www.io.com/~nielw/sw3coils.htm
http://www.antiqueradios.org/gazette/swevol.htm


73,
Grumpy
IIRC the SW3 used a dual 100uuf capacitor. It's a weird one that
tunes through 270 degrees of rotation.