Life of KT 150 Tubes

@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. 

A schematic is nice to have, but not really necessary for trouble-shooting this particular problem. Basically, if there is nothing amiss with ancillary components (capacitors, resistors, tube sockets, wiring, solder joints), then you have to believe it was tubes. By the way, I forgot to mention tube sockets. Check that all wires soldered to the tube pins on the bottom side are well and firmly making contact. But even that doesn’t always rule out a cold solder joint. Then also, the socket has to have a firm grip on all tube pins. Some of the modern Chinese-made sockets, while they often look really nice, can have a loose grip on tube pins.

You could swap a good tube into a socket where the previous tube blew.  Start the amplifier t up and watch for a red glow.  If it glows red (not orange), shut down the power immediately.  That proves you have a problem in the circuitry.  Sounds like from what others wrote who seem to know these amplifiers, you have a bias current meter on the chassis.  Is that correct? That should be very helpful.

After reading the manufacture state the valves last between 1-2000 hours, I think you have your answer. You were close to the 1000 NOT the 2000 mark. I find that as another minus, but the manufacture published the specs before you bought it, (I hope).

I find that rather disheartening when someone builds ANY power amp that the valves only last 1-2000 hours and are very $$$expensive$$$ valves.

If no one want’s to help, why are YOU afraid to open up YOUR amp?

I’m sorry it happened BUT it’s time to fix the amps. BE BOLD! It could be 5 dollars worth of parts and charging up the electric screwdriver.

I just open up my new (old) Cary SLP-05. I could care less what any manufacture says about warrantee if they put a fuse on the inside. It is there to be changed and it blew for the amps protection, NOT the house or domicile. The home has breakers/fuses for fire protection. The Amp has SELF protection. It has zip to do with fire safety and everything to do with AMP protection.

5 bucks worth of parts and some new valves. I have my fingers crossed for you.

Post some pics after you open them.. Lets get some trouble shooting going on..

I use a 40 watt light bulb, switch, and pigtails with alligator clips. I hook up to the clips to the cap, flip the switch a few time and on to the next cap I want to discharge.

When I first started discharging caps in the house, 30 years ago, my wife came out with a pistol, she actually thought someone had shot into the house. I quickly went to the light bulb discharge method.

Remember Granny Clampett? The Little Misses is close. LOL

In any case OP I wish you well, I'm in a close second with a newer GG MC275. I think I lost a transformer. I've never had a Mac transformer fail, I think this one did. 700-1000 for a transformer. It a the new transformer too. My 1964 MC275 won't break. That is one tough amp..