[MrFixr55]

Hi Ed,

Couple of things re the 6K7s.  The original tubes on these are the "G" types, and require shielding.  In addition, the 6K7 1st Audio tube probably gives a lot of gain being a pentode, so lead dress is important.  If metal 6K7 tubes were substituted for the glass ones, it is important to ensure that pin 1 of the sockets is grounded.  Some Zenith tubes were advertised as being "metal glass" - having an internal shield (aquadag?) The schematic seems to show a grounded shield pin for the 6H6 detector, 6K7 1st IF and 6F6 Output tube but not for the RF and IF tubes.  If Pin 1 of the socket is grounded, the metal 6K6 tubes should be OK.  if not, use {G" or bakelite GT Tubes and ensure that the shield is in place.Try shielding the grid lead (and cap) of the 6F6 1st audio tube.  In addition, do the Phono mod; it is documented as a shop mod so is legit.  If the oscillation occurs using the chassis as audio only, then the issue is AF.  If clean audio when used as an amp but issues as a radio, that is the telling.

BTW, a really pretty job on the underside of the Chassis!!

Best Regards,

MrFixr55