WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Delighted to say, I rescued one from under a workbench in college today, has been kicked around for years. I think, all we need is to replace the valves (college has mounds of them), check the circuitry and off we go
Assuming you're serious (I can't tell whether you're kidding or not!), please take care. Look for signs of damage or burning and be prepared to replace components affected.
Be aware that there's a more than even chance that some capacitors, especially electrolytics, will not perform well. Some resistors can also have drifted out of spec. Electromechanical contacs (switchs, plugs and sockets, tube holders) may need cleaning. Transformer windings should be checked for continuity.
I'm sure other people can suggest plenty more cautions before assuming that it will work.
I've worked on a number of Leak amplifiers and you can be assured that most components will have drifted out of spec by this point in time, and I would not power it up at all without first replacing all of the electrolytics and coupling caps - I'd check all resistors as well at this point and replace any that are off.