Okay, so i bought an old cheapy single tube amp off ebay. Its supposed to have a 25eh5 in it. I hooked it up and it works, but i noticed the tube lit up in half a second and thought that was odd. Turns out its a 50eh5 in thats been put into it. What should i do?

Sorry for the newby question.

Is the tube heater in series with a phono motor or just a dropping resistor? If it is just a resistor, and you can't find a 25EH5, it would be easy to calculate the value for a resistor change, using ohms law (E=IR)

A 25EH5 draws 0.3A at 25V, so using ohms law to calculate a 100V drop from the normal 125V line implies a 333 ohm series resistor.

A 50EH5 draws only 0.15A, but at 50V, so a 75V drop is needed. A 500 ohm would be the value for the series resistor.

hope this helps,
jr