I bought a Crate GT-50 combo for clubs gigs for $100. It seems to function fine, bit I measured the filaments and one side is 4 plus volts and the other, 1.8 volts. Is there anything i can do to correct this? Also, the highest the bias voltage will go is 31 volts, which seems a little low. (4X6V6's) I'd appreciate any help I can get. Thanks.