Originally Posted by amplifier

Sounds like it's gone microphonic. Unless you've got the gear to test valves the only way to really test it is to buy a new one and see if it fixes the problem. Don't waste your money on a tech it's easy. There should be 4 screws on the top holding the chasis up. Hold your hand on the bottom when you unscrew them so the chasis doesn't drop and get damaged. The tube should be near the input jack. I'm not sure if the older ones have a retainer holding the tube in but if it does move it to the side and cafefully and firmly start making small circular motions till the tube comes out. Use the same technique to put the new one in and put your chasis back on. Good luck!