bendix: Bendix 847S "Facto Meter" Tube substitution

Hi All,

I was restoring a "tubeless" Bendix 847S Facto Meter and discovered an undocumented manufacturing change. The FM RF tube was not a 7-pin device (6AG5 as listed in every piece of documentation I could find) it was a 9-pin socket which isn't documented on the tube chart, schematic or any other place I can find. I thought about replacing the socket but of course it's buried under the gang switch and almost impossible to inspect how it's connected not to mention how it's connected.

I asked the good people on the Antique Radio Forum and Jim Mueller responded, "It's probably a production change for the tube that didn't make it into the labeling. Since it is the RF amplifier, it might be a dual triode like a 6BQ7 in a cascode circuit."  So I check everything I could get to and it appeared to be at least wired for the 6BQ7 so I gave it a try.  I am happy to report that it works great.  My thanks to Mr. Mueller and I hope this proves helpful for others.

Cheers, Scott

  

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