Life of KT 150 Tubes

How did you test the KT150’s? I am not saying that they are not damaged and that they did not damage your amp when they blew, BUT the KT150’s need 600 volts on the line to test and very few tube testers can actually supply that.

@vishu  Always had bias set between  55mu-60mu as recommended  by Auris. I spoke to Tung-sol they said life is between 1000- 2000 hours. Thats is a big variation!!!!!

Opinions will vary - and while it may not be related, for reference I run my KT150s at 45-50ma, designer says I can go up to 60ma with no issue yet I have tried them as low as 40ma and they still sound pretty good. Currently running KT120s at 50ma too, good enough, no need to burn out tubes prematurely if you don't need to.   

You'll see manufacturers reporting different life values between 2000-3000hrs for KT150s, and half the for KT120s and yet that can change for the better if you are not running the bias up too much. Ask your amp mfg/builder again on bias too.   

@decooney- I have spoken with the engineer at Auris and he specifically said to adjust bias 55-60 mA.I have tried 45 mA no difference to my ears. Never tried KT 120, engineer Said not to. Thanks for your input

If this is the first and only set of tubes you had, my first instinct would be a faulty production batch from the factory. I wouldn't suspect the amplifiers unless the problem happened with one amp. Moreover, a bias current of 55 to 60mA means the amp is running in Class AB with a B+ voltage around 550V for a 100 watt rated output. At this condition the KT150's are dissipating only 50% of their rated power. This should make the tube last way longer than the 1,000 hour life quoted by Tung Sol.

That said, there are a couple of things a competent tech can do without opening the hood. Power up the amps without the tubes and measure the voltage on pin 5 on each KT150 socket to check the grid voltage. All four grids should have the same voltage within a few hundred millivolts. The pcb pictures of this amp show a time delay to turn on the B+, it is critical -- critical -- that the bias voltage appears on the grid before the B+ turns on and the B+ voltage drops faster than the bias voltage when powered off. Having a substantial voltage on the plate without a bias voltage drives the tube into high current and shortens the tube life considerably.

My suggestion would be to buy a set of tubes from Upscale Audio where they claim to weed out weak factory batches and run the tubes hard prior to sale. 

I guess don't buy nuttin' without schematics. Or from a source you can ship to for service.

From here on.