Problem with Egnater Renegade head

[charvel2]

Hi gang, Here is my Issue. My Renegade has developed a low frequency hum that starts about 20-30 seconds after I switch it off standby when cold. After amp has been played then switched from on to standby than on again, is there
Immediately. Doesn't matters what channel, settings or volume. Stays constant and always there. Seems to still sound good. Forgot to mention that makes a little "pop" sound before hum starts. Any ideas? Thanks in advance for the input.

 

[Alabama Thunderpussy]

That could be any number of things. Swap tubes out first. It sounds like an issue that occurs after they warm up. Could be a bad preamp tube.