I have a couple of comments, I know from that darn guy in Texas again.
1st off, the Valiant is likely spraying magnetic fields beyond the cabinet. If the microphone is dynamic, the magnetic fields from the mighty V might induce a hum into the microphone dynamic element. Moving the microphone position, or distance from the transmitter will have an effect if magnetic coupling were the issue.
Here at my station, there is always a scope monitoring what goes to my tuner input. On AM I have 4 cm carrier, offset 1 cm down from center. This way I can easily see -100% and + 150% modulation. A low level hum is visible, and going to line sync will sync the trace if the hum is from the AC main frequency or 2X that. On my Flex 3000 station, I have another scope looking at the Flex output....so two scopes, one before the amplifier, and one after the amplifier. Sometimes on the Flex I need to boost the power 13.8V to 14.5V in order to get good positive peaks.
Jim
Wd5JKO
Jim
Wd5JKO