Slight variations in the normal conversions of the 274N series receivers have been found to improve the operation of the receivers. In the plate circuit of the 6SR7, second detector-first audio, it is found best to use a 250,000-ohm resistor from plate to B+. A 100-uuf. condenser is connected to ground from the plate.
Generally the BFO coil and allied parts are removed in order that the triode section of the 6SR7 may be used as the first audio tube. In our latest conversion of the BC-454, the BFO coil is left in but disconnected from the 6SR7. The dynamotor mounting studs are removed from the rear of the receiver chassis. A seven-prong miniature socket is installed in one of the dynamotor stud holes for a 6C4 tube. The BFO coil is then wired into the 6C4 socket. A switch from the CW shutoff line to ground completes the installation.
Robert R. Jessup, AF8FAG
from MARS Bulletin, July 1953, page 14