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 Western Electric 717A pentode sub for 12SK7 in tube 'AA5' radio
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 Return to top of page · Post #: 16 · Written at 10:29:40 AM on 24 June 2023.
Marcc's avatar
 Marcc
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5483

In the older handbooks that is covered under "Automatic Volume Control". With some reference in "Frequency Conversion" and the tubes themselves.

Delayed AGC gives better rectification efficiency and a small gain in voltage. The reason its fed from the secondary is to get less voltage and it also decreases the shunting of the detector diode load.

Not done without rhyme, nor reason.
 
 Return to top of page · Post #: 17 · Written at 10:49:57 AM on 24 June 2023.
Marcc's avatar
 Marcc
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5483

Just on Pentagrids. It is worth looking at the construction there are similarities claimed in relation to 6L7-G and 6SA7. However to increase Plate resistance 6SA7 has a suppressor grid & to be different a shell on the outside of the plate, tied to the suppressor. It is also has a curved electron path such that no electrons can get to grid 1, which is intended to be the oscillator grid.
 
 Return to top of page · Post #: 18 · Written at 4:31:06 PM on 24 June 2023.
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 STC830
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1313

RDH 4th Edition Ch 27 Sect 1 p1074 for those interested. But no doubt STC had many considerations when building thousands of radios a week.
 
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