Agree with Hifigeek1 - Output tube may have shorted, taking out a resistor. Needs to be checked by a qualified tech. Doubt just putting in new tubes will solve the problem.
Not so fast. You may have damaged a resistor near the problem output tube if it is indeed an output tube failure. If you replace the output tube and a resistor(s) failed, the tube may not light or the new tube may be damaged. You should have the amp looked at before installing new tubes.
Hopefully; it's simply a tube. Another possibility is a leaky or shorted coupling capacitor(especially based on the age of the amp). If so; it will either be the 2uF or .22uF(bypass)/600V, mylars.
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