Help me, Amp guys!

[Masterof1angel]

Question... What would cause my Mark IIC+ to hum when you push in the bass pull shift on the gain channel but not when you pull it out?

 

[Boogiebabies]

Signal to noise ratio, or most likely the preamp tube cathode assigned to that function is reacting to the increase in gain/voicing from the shift. Sometimes a slight boost in gain, even though clean or just a few dB's, can decrease noise.