[Lowfer] S-51 Interesting Puzzle

[Michael Sapp]

Dex & All: Interesting time tonite checking the tubes in the S-51 and giving the sockets the deoxit treatment. The radio was down in gain some from my Heath VLF converter on the LF band, but I did not think that was much of an issue. There is a string of 6SS7 tubes used for RF preamp and IF amps. A few of those were on the weak side, so I should get some extra system gain with a new set of 6SS7s.

This radio also has a 6V6 in it which I assumed was the final audio amp. But I noticed the filament was not lit and I had plenty of audio output. Wierd. Check the 6V6 and the filament lights and I get a solid "good" range on the emission test. Put the tube back, fire the radio up. Plenty of audio but no lit 6V6 filament. I pull the 6V6 and the radio keeps working. This is entertaining !!

Check the schematic and it turns out the grid input to the 6V6 is also coupled over to the grid of a 35L6GT---a 35 volt version of the 6L6 tube I suppose. The audio output transformer is center tapped and one side of the transformer is fed from the 6V6 plate and the other the 35L6 plate....

Now I get it. For low voltage DC operation the S51 uses the 6V6 for it's 6V filament feature.  For 120 volt AC/DC operation, the filaments of the 35L6 and 35Z5 rectifier add in series for 70V of voltage drop on the filament string. It had me going there for a few minutes.....:))

When I tested the 6V6 it may have been the first time the filament was ever lit.  Explains why the emission test on it was so solid...

73,  Mike wa3tts