[Arran]

    Hi Fred.. that tuning system is as you said.. The light gets dim when you are on a station and brighter when you are off the station.. Some thing that most don't know about this system is that the bulb is a 24 volt 3.5 MA .
  At the moment I can't remember where I got mine .. I did buy it on line from a supplier. Right now my computor is down so I don't have access to  my favorites and that is where I have the name of the company I bought it from.
  If I remember correctly, the MA is quite critical for it to work properly.
      The rogers 6F7M tubes are hard to find , but  I  have a fellow in town here that says he has  a few.. If you like I can see what he wants for one.
   Dan in Calgary

Dan;
  From what I can tell whether you really need a 6F7M or a 6P7G depends on how the tube was originally used. In the case of the Rogers 10-12 and the Deforest Crosley Buckingham the two 6F7Ms had their pentode section used as IF amplifiers, the triode section of each 6F7M was hooked up as a diode with one for second detector, the other as an AVC diode. So there are three ways to handle this if you can't find some good 6F7Ms: (1) Rewire each 6F7M socket to work with a 6P7G (2) Make an adapter to use a 6F7 seven pin in place of 6F7M which is octal. (3) Rewire one socket to accept a boring old 6K7 and rewire the other to accept a 6B8 which is a dual diode pentode.
Regards
Arran