What causes cathode resistor to blow?

I had this issue happen before on my ST120 when a tube redplated. Cause? Effect? I couldn't say. But one of the cathode resistors had blown. Replacing it and putting in new tubes (a matched quad of TungSol 6550), the problem hasn't returned.
Until today! I installed a brand new quad of Gold Lion KT88 tubes, because I missed their fluorescent glow. Seconds after switching the amp on: POP! Another cathode resistor blown.
What can cause this so immediately after installing new tubes? Could I have a bad tube? Or is it something else?

Is the resistor that's blown on the same socket as before or another one?

Same side, different socket.

Well, I think I would replace the resistor and put the Tungsol's back in or try the Gold Lions again on opposite channels to see if the problem is in the tube that blows a resistor if it happens again..

ChetJettison wrote:I had this issue happen before on my ST120 when a tube redplated. Cause? Effect? I couldn't say. But one of the cathode resistors had blown. Replacing it and putting in new tubes (a matched quad of TungSol 6550), the problem hasn't returned.
Until today! I installed a brand new quad of Gold Lion KT88 tubes, because I missed their fluorescent glow. Seconds after switching the amp on: POP! Another cathode resistor blown.
What can cause this so immediately after installing new tubes? Could I have a bad tube? Or is it something else?

Did you turn down the bias adjustment to minimum bias before changing tubes ? If not
the new tubes might have drawn excessive bias and thus blows the resistor.

Any powertube changes need bias adjustment, and should start with bias at the minimum.

Chet, also clean and retention the sockets pins. Different tubes do have small differences in pin diameter. Dale