Tung-Sol kt170 tube started glowing brighter

@kitegod , Yes, you can replace the one bad tube with one with similar numbers on it and then check / reset bias. If your amp isn't rated for the KT170 despite being told it should work it might be safest to revert to the tube types that are specifically compatible with the amp? I guess the other option is get the new KT170, install it, check bias and see what happens- it may have been an isolated incident with the one bad tube. 

@OP The tube is starting to red plate. Don't try to rebias the tube as it will only hasten it's demise.

With only 50 hours on the tubes why not try to get a warranty type replacement from seller?  It sounds like you were sold a bad tube.

+1 to what @dekay and @jetter both shared.

I'm pretty sure you are seeing an event called "Red Plating" which is the tube overheating. There are a few causes- and finding them can be a mystery. 

The tube could be shorting (doubtful given what you've described)

The bias is certainly off. How could I know that? All biases are off at least a little. Add time, other component deterioration, and mismatched components and it's a boas nightmare. 

I had a mystery red plating problem and it turned out to be a bad 12BH7 high current driver - which caused me to have to replace 2 6550s. OUCH!