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.
Why do you want to replace the plug with a three-pronged kind? If it was designed to be safe with a two prong plug and that plug was damaged, doesn't it make sense to simply replace it with what it had before?
Anyway, how is this "Tube" related... Doesn't it belong in the "Electronics" section?
Probably you're talking about old tube equipment. Yes, it is safer to use 3-prong mains cord, and it is a must. Just connect the safety ground wire (the yellow/green one) near the mains wire entry point, with a good connection to the chassis.
And also, if you've got the "polarity" switch (like old guitar amps) just remove anything that conects to it (sometimes there's a cap to the mains wires to the chassis, it's called "death cap" because you know what happens when it gets leaky or short ), remove it please, with the 3 prong connections the mains wires shouldn't have any connection with the chassis at all).