04-08-2025, 10:03 AM
OOPS!!
if there is a "flag" running up the side of the tube that touches the shield, then yes, a shield belongs there. Could be that the set was designed for a wafer and metal based tube instead of a full bakelite based tube. the shield looks too "spread out. Is that shield on the 6H6 or is it on the 6SQ7?
n a previous post a question was raised whether the AVC was connected to the screen grid off the IF tube. I don't think that it does. In some early "Screen Grid" sets with tetrode RF or IF amp sand no AVC, "Local - Distant" switches or even volume controls were used to vary the screen voltage to vary the gain of a stage. In these "modern" sets (any superhet set since about 1935 so using diode detection), the AVC voltage is generated off the detector and is used to bias the IF amp and sometimes the Convertor and RF amps through the return of the IF or RF transformer secondaries. In other words, instead of being between the grid and ground, the secondary winding is between the grid and the AVC bus. There should be a fairly large size cap (0.05 - 0.1 uF) between this AVC bus and ground. If this cap is missing or bad, severe "motorboating" will occur.
On your set, the AVC is generated from the diode section of the 1st AF Amp and the Detector / discriminator circuit, is filtered by a 0.05uF cap and controls the grid bias of the 1st IF and Convertor stages, but not the 2nd IF stage
if there is a "flag" running up the side of the tube that touches the shield, then yes, a shield belongs there. Could be that the set was designed for a wafer and metal based tube instead of a full bakelite based tube. the shield looks too "spread out. Is that shield on the 6H6 or is it on the 6SQ7?
n a previous post a question was raised whether the AVC was connected to the screen grid off the IF tube. I don't think that it does. In some early "Screen Grid" sets with tetrode RF or IF amp sand no AVC, "Local - Distant" switches or even volume controls were used to vary the screen voltage to vary the gain of a stage. In these "modern" sets (any superhet set since about 1935 so using diode detection), the AVC voltage is generated off the detector and is used to bias the IF amp and sometimes the Convertor and RF amps through the return of the IF or RF transformer secondaries. In other words, instead of being between the grid and ground, the secondary winding is between the grid and the AVC bus. There should be a fairly large size cap (0.05 - 0.1 uF) between this AVC bus and ground. If this cap is missing or bad, severe "motorboating" will occur.
On your set, the AVC is generated from the diode section of the 1st AF Amp and the Detector / discriminator circuit, is filtered by a 0.05uF cap and controls the grid bias of the 1st IF and Convertor stages, but not the 2nd IF stage
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Best Regards,
MrFixr55