My svt-2 is acting really wierd. I'll being playing it and it will just go silent and the standby LED on the front will blink on and off REDgreenREDgreenREDgreen. PLus, the green LED for the right biasing screw on the back of unit won't come on. The red one works as it should, coming on when a signal is going through the amp, but the green one never comes on. Wha does it mean!should I get new tubes, or is it something else?