Life of KT 150 Tubes

grossman616,

I think the original poster explained that after one channel failed, he moved the tubes to the other monobloc amp and suffered the same failure. This means a bad tube in the first amp.  Whether or not that amps themselves are damaged is not clear, because he did not report trying the good set of tubes in either amp.  In any case, this is a good example for the practice of NOT swapping tubes when an amp goes down--you could be exporting damage to the other amp or channel if the tube failure is also causing damage.  The original poster also mentioned a faint burnt smell which pretty much rules out the failure being limited to the tubes themselves.  That smell means that, at the very least, a resistor blew (common mode of failure, my friend says cathode resistors also act as fuses, and does not consider that failure to be much more of a bother than changing fuses).

Faint burnt smell means something inside the amp went bad, probably a resistor burnt up.  That could be caused by a tube going bad.  Moving the bad tube to the other amp would then cause the other amp to go bad.  I never do the switch tube around test for power tubes because that can damage the other amp (or channel in a stereo amp).  It is far better, when something goes wrong with a tube amp, to first have the tubes tested.  

The sort of good news is that is that the most common failure-a resistor going bad-should be relatively easy to fix by a competent technician, even if the technician doesn't have the schematic.  Perhaps you could persuade the technician to have a go even without the schematic.

The Amplitrex is so easy to use and it does not require much time at all to use.  It has a warm up cycle that is timed for the particular tube being tested so that the tube would be measured when reasonably warmed up.  I would not want a tesster to be "fast" if that means measuring before the tube has been warmed up.

A local dealer that exclusive deals in tube equipment often sees repeated early tube failure and the kind of failure described here where there is also damage to the amp from the same customers while other customers running the same model of amp do not have such problems.  Most often it is from excessive voltage coming into the customer's home.  He has equipped those customers with voltage regulation to drop the voltage to some more reasonable level.

I hope all goes well on your road trip to the dealer.  The good thing about tube gear is that pretty much everything is repairable, and because the circuit is relatively simple, the cost should not be very high.  The exception would be something that damages the output transformers (very unlikely) or the power transformer (more likely, but still not that likely--the fuse should prevent that outcome).  I don't know if the dealer can do a while you are there kind of repair; let's hope that is the case and you don't have to have the amps shipped or you don't need a return road trip.