NO. Do not use a wire not made for A/C in a power cable. The insulation is the thing.
If it is not rated for 120volts, it will fail. And most speaker cable does not have a good enough insulation for A/C use. The reverse is not true. You could use any A/C cable for speaker cable and be OK.
Most A/C wire insulation is 300 volts. So it can be used for 240 volt applications. Most speaker cable is way less, probably rated to 30 volts,(a guess, but somewhere around there) tops.
So do not do it.
AND; if the original cord was grounded you need to use a grounded three wire A/C cord. Tube amps have lethal voltages (300volts, big amps too.) in them and taking the ground off is plain stupid.
If it is not rated for 120volts, it will fail. And most speaker cable does not have a good enough insulation for A/C use. The reverse is not true. You could use any A/C cable for speaker cable and be OK.
Most A/C wire insulation is 300 volts. So it can be used for 240 volt applications. Most speaker cable is way less, probably rated to 30 volts,(a guess, but somewhere around there) tops.
So do not do it.
AND; if the original cord was grounded you need to use a grounded three wire A/C cord. Tube amps have lethal voltages (300volts, big amps too.) in them and taking the ground off is plain stupid.