[DaveWM]

one more thing, if the lin pot has been hacked up, make sure the cathode bypass cap (100uf electro) is NOT being cut of of the circuit when the lin pot is turned to min, esp since that 950 fixed resistor is in there. Worst case you have a 950 unbypassed cathode resistor with the lin pot turned in such a way as to take the cap completely out of the circuit. IIRC there is something different between how the schmatic is drawn and how the lin pot and cap are actually done, but I may just be remembering that wrong. If it was me I would for grins just put a 500 ohm fixed resistor and the cap in there and see how that works (disconnecting the hacked up lin pot/resistor combo), as is sure seem like its a gain issue on that tube, assuming of course the grid bias and drive are correct.