[Amps] Cathode input circuit

[Mike Lord]

A question for the group.  Some time back, ZL1AXB published a circuit for a single 813 grounded grid, using a tuned coil input circuit.  He referred to it as a "bifilar wound coil."   I built the amp and it works like a champ.  The only other reference to this tuned input I could find was in Orr's 18th edition where he used it on a 6 meter amp. (pp 747 -749).  I couldn't find any further discussion by Orr.  Pat's circuit covered 40 - 10 meters.  I would like to use this circuit on 80 and 160.  Is there a formula for winding the coil?  As anyone else had experience with this tuned input?  Thanks.  Mike K4ELV