[MrFixr55]

Hi Benny,
There is an article in the Phorum's Philco Library on rewinding coils, however, the 91 is not listed.  The 1st detector circuit is very similar to the (notorious) 89 / 19 circuits, but since the resistances are different, it is likely that the number of turns for the winding are different.  Philcos of this vintage are absolutely notorious for failures of the "Primary" Winding (usually wound over the secondary winding for RF and IF transformers).  For the 1st Detector (Oscillator - Mixer) coil, the winding that fails looks like the primary winding, but is actually the "Tickler" or "Oscillator" winding.  The failure is due to deterioration of the acetate insulator that that separates the primary or tickler from the winding under it.  As the acetate breaks down, causes corrosion of the wire wound over it (and possibly under it). 

There are 3 windings on this coil, Primary, Secondary and Tickler

The specs for the 89 Tickler winding are 25 turns of #39 wire.  This wire is readily available from various suppliers on various commerce sites. Sorry, I do not have the specs for the 91 coil, but this part of the circuit is not tuned and not critical. It is likely that the tickler coil is wound in the same direction as the one under it, but I am not sure.

I'm sorry, I do not have the pinouts for the coil, but this can be figured out easily by tracing wires from the tube. 

The "cathodyne" 1st detector circuit was relatively inexpensive, cheaper than separate oscillator and mixer tubes, but nowhere as stable as the Heptode ("Pentagrid") convertor introduced (by RCA?) in 1932. Philco did not start using the Pentagrid convertor circuit until the mid 1930s.

Below are several images.  The first, below is a section of the Philco 91 Schematic from our library, showing the 1st detector circuit.  The Cathode circuit and tickler are highlighted:
   
The 2nd image is a blow up of the circuit.  The source is Nostalgia Air, because I could not magnify the png of the schematic from our library.  I labeled the pins of the tube. Trace the wire from Pin 4 (the cathode to the coil. This is one leg of the tickler.  Trace the wire from the plate of the tube to Cap 18, then to the coil.  This is one leg of the primary.  The other leg of the primary connects to Caps #23 and 24, and one leg of the secondary.  The other leg goes to the Osc section of the tuning capacitor.
   

The 3rd image shows the location of Part Numbers 20 and 21, the 1st Detector cathode bias resistor and bypass cap.  One side of this Bakelite block is grounded.  The other side goes to the other connection of the tickler winding.
   

The last image is from the RCA Receiving Tube Manual RC14, ca 1940 showing the pinouts of the tube.  Note that the pins are labeled clockwise from the bottom, under chassis side ("worm's eye") view.
   

Armed with this info, you can trace and label the connections.  You can then "Ohm" the connections.  Make good diagrams and take pictures before removing the coil.

When repairing the coil, attempt to count turns and note the direction of winding as you unwind the tickler and remove the acetate insulator.  Inspect and, if necessary, reinsulate the winding underneath the tickler winding.  As to what to use as the insulating collar between the tickler and the other winding, most suggest the thin plastic that prepackaged sandwiches from the store or 7-11 come packaged in.  Alternately, mylar tape is a good substitute.  Most people strongly suggest baking the coil in the oven at 200 Deg F and overspraying the coil and form with spray varnish prior to rewinding.

Good luck with your project, keep us all posted and please do not hesitate to ask any questions.  We are happy to help.  This is not a fun circuit to work on, but you will be rewarded with a nice sounding 93 year old radio.  Consider adding a "Radio - Aux" switch and jack at the input to the volume control.  You can then pipe program material from a CD, MP# or computer directly into the radio for a geat big band sound.