Tube arch

tubegb

In general Audio Research recommends changing/replacing output tubes in their amplifiers every 2000 hours. Assuming you have the owner’s manual, you should be able to confirm same. Another suggestion would be to periodically check the individual output tube bias.

Audio Research recommends changing tubes every 2000 hours in their amplifiers that use KT120s and 6550WE (Ref 250 and Ref 75). With the newer amps (250SE, 75SE, 160M, 160S) running KT150s, the manual now recommends changing the KT150 every 3000 hours. The 2000 hour recommendation however still holds for the 250SE and 75SE that still have 6550WEs. The newer 160 series amps now have auto-biasing.

I’ll keep this short. So my last post was about a tube arc. Took out a resistor on one of my Audio Research Ref 750s. I replaced the resistor and put in a matching KT150. My question is how can I avoid this from happening again.

@tattooedtrackman I see what you did there a 'short' is why a tube arcs...

Anyway there is nothing you can do to actually avoid this but you do have good advice as to how to minimize it. The thing is, bad tubes gonna arc and age has nothing to do with it. Sometimes shipping is an influence and other times the tubes are simply bad and fail regardless. What I'm saying here is a tube might last 2 minutes or 2 years; there's not much way to know which, other than vetting the tube as best you can. 

ARC amps use a sacrificial resistor because if the resistor was rated to not fail, Bad Things would happen elsewhere in the amp that would be a lot more expensive to repair!

A fuse of course could solve the problem. They could be a lot easier to replace than a resistor. An LED could light to show the bad fuse...

@atmasphere  Thank you for your response. So actually what u are saying is that when a tube goes bad it will always ARC ? And will take out a resistor along with it ? Is this on all tube amps ? Or Audio research? Or just on Audio Research Ref 750s because of all the 18 KT150s on ea amp ? 

So actually what u are saying is that when a tube goes bad it will always ARC ? And will take out a resistor along with it ? Is this on all tube amps ? Or Audio research?

@tattooedtrackman Arcing is the primary failure mode of modern power tubes but they can fail without arcing. Older power tubes might just fade away. This has to do with the cathode coating of the tube which was a lot better in the old days- nowadays it starts flaking off like old paint and so the tube starts arcing. Since any KT- numbered tube above 88 is a newer production tube, I would expect arcing as a failure mode to be pretty common.

Whether it takes a resistor with it depends on the design. I don’t have direct experience with ARC in this regard, but this is the sort of thing I’ve heard about them for some decades now. We built our amps to hold together when a tube arcs, for the simple reason that a tube can fail inside the warranty period of the amp, and shipping it back on that account can cost a lot of money over time as well as increased frustration for all parties concerned.

There are a good number of tube amps, in particular those designed around tetrode or pentode power tubes, where a resistor can fail if the tube fails. This sort of thing has been going on a long time; for example the old and venerable HK Citation 2 would do this and that amp was built in 1959. I’m guessing that ARC, grounded in history so to speak, didn’t see this as a problem.