I was checking out an Ampeg V-4BH all tube head today. 100 watts all tube. I noticed alittle "hum" when I turned the standby off and engaged the tubes. Is alittle hum normal from tube heads, or are they supposed to be silent? The hum was ever so slight, almost like normal operating noise, but only when the tubes were "on". Not in standby mode. This one was 2 yrs old , looked brand new and had groove tubes. Are GT's stock from Ampeg?