I have a problem with my deluxe. After about 2 min of playing it crackles and drops in volume. I can crank it to ten but i only get the power of it on 2 or 3. I play on chan 2 so i pluged it into chan one to see if it was my V1 12AX7. I have not switched the actual tubes. Coukd this be a power tube issue or preamp?