[julie_m]

Don't necessarily jump to condemn the valve. A faulty capacitor across the cathode resistor can cause loss of gain without apparent disturbance to DC conditions. What happens is as the valve conducts better (on the crest of the input signal), more current flows through the cathode resistor and the voltage on the cathode rises. This makes the grid more negative, which causes the valve to conduct less. Basically it ends up fighting against itself. With a big capacitor across this resistor, any increased current through the valve flows into this capacitor first; the capacitance is sufficient to prevent the voltage across it from changing much. Exactly the opposite happens on the trough; the valve conducts less, but the capacitor maintains a steady cathode voltage.

This capacitor was nearly always 25V, 25µF in most of the EL84 and UL84/UY85 record players. 22µF is the nearest modern value, but 33 or even 47µF would work fine if that's nearer to hand.