Quote:
Originally Posted by Boater Sam
. . . when tuned in to a signal, the AGC stabilized the valve and the howl stopped or reduced substantially.
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Correct me if I'm wrong, but the valve that was stabilized by the AGC action is the I.F. amplifier, yes?
In which case that strongly suggests that the fixed -ve bias on that valve is insufficient. That bias is usually set by its cathode resistor, known as 'automatic bias'. (I assume you've checked its Ohmic value). If the decoupling capacitor across that R is leaky, the net resistance will be reduced . . . thus increasing the gain of that stage . . . possibly leading to its instability.
Another possibility is that there is a leak of HT+ from 'something' placing a small +ve bias on that valve's control grid - that 'something' could be due to leakage from the IFT's primary winding to its secondary, albeit an unlikely scenario.
Just a couple of thoughts.
Al.