[Lowfer] S-51 Interesting Puzzle

[Dexter McIntyre w4dex]

Good to learn that Mike.  I'll pass this on to my Son who is restoring 
old radios.  I recently learned from him what is inside those flat wafer 
multi lead devices I use to wonder about years ago.  Maybe that was the 
first of the integrated circuits?

Dex

Michael Sapp wrote:
> 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...
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