Ok, just put new preamp tubes. It was working fine before and after the preamp tube change. I didn't use dexoit but used a quick drying elect contact cleaner - sprayed on all preamp tubes inserted and removed a couple times. when I take amp off stand by I hear some static and then a hum starts coming from the speakers and its a hum that gets a little louder and stays steady. I can tell it's not a good sound. I cleaned speaker input cables and yet it's still doing it. I cleaned the footswtich cable and the plug. I cleaned the instrument cable and plug. Thoughts on what this could be? maybe a power tube going out?? I did not change the power amp tubes.