Audio Research d79 blows plate fuse

Output tubes. When were they replaced last?

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.

Thanks for the response.I'm gonna get a new set of tubes and hopefully that is all it is.

Thanks for your response.I'm gonna get a new set of tubes and hopefully that will take care of 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.