It creates an Automatic Volume Control voltage to keep volume steady.
use an Isolation Transformer when working on the signal flow with test equipment. How can I help you further with this project? AA5 Radio Signal Flow
While not directly in the "audio path," it is vital for flow. It converts AC house current into the DC voltage (B+) required by the other four tubes to function. ⚠️ Critical Safety Note It creates an Automatic Volume Control voltage to