Why the KT120 & KT150?

Considering an amp that would benefit from this upgrade and looking up prices for matched pairs and quads. Couple sets pay a good part of what a half decent solid state amp with similar to more power and none of the maintenance headaches go for, and asking myself why go there?
A certain cultish motorcycle mfgr's campaign once ran something like "if you have to ask, you don't understand", or some such. I'm afraid I do, and I don't on a rational level when it's my money as a power amp junkie going down the rabbit hole.
Still, I cannot look away.
Can certainly understand your point of view since I shared it for over 40 years. I always thought why screw around with tube equipment when one can get much more power and far better specs with solid state equipment? About 7 years ago I thought "what the hell I'll just try a tube buffer and hear what if anything it does" Much to my surprise I could actually hear a little tiny difference (probably from 2nd order distortion) but I liked what I heard so I bought a tube preamp and then I really liked what I heard. Then I just had to have a tube power amp. So my system has been all tube for 5 years now and I'm not going back to SS. But YMMV.
 
I've a mix of tubes, bipolar, and FET gear. I like fets for splitting the difference, but they each have their appeal. The Marantz amp uses a minimum of reasonably priced tubes.
 
These big bottles (KT120 & KT150) also suffer from poor quality control:

http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=5813 & http://www.diyaudio.com/forums/tubes-va ... ost4975833

I have 2 pair of 120's, and a single pair of 150's. None can be run past 70% without starting to show red in the seam of the plates.
My amp is SE UL, but others are PP, and both are subject to this problem.

http://audiokarma.org/forums/index.php?threads/kt120-150-se-ul-build.734130/
I put in a pair of SPDT switches on the bias circuit to run both tubes.
that being said, I feel and like the 120's much better than the 150's. Is it because I can't push them harder?

My 2 cents. Stu
 
Ok,just for reference,I removed a KT120 tube from my newly acquired Audio Research VS110,and took the best photo I could manage with my crappy-cam.They are all in similar visual condition.I did not cherry pick the worst.I haven't had it very long,and I have used it minimally as it required power supply work before being put into service.

The previous owner (lucky bugger has now moved on to a Reference Series) had purchased these replacement KT120's from a local ARC dealer,represented as having been supplied by Audio Research.The receipt is dated last December,and he also re-tubed his preamp (which has that wonderful hours-of-use counter) at the same time.When I purchased the unit,the tube usage was indicated at 800 hours.He is very dilligent about checking his bias,which he claimed was always stable and never really varied.He was also a very ''moderate'' listener as regards output levels.

Each tube has indicated (hand written with a Sharpie?) on the metal base ring the socket it is to be installed in,and each tube also has two additional three-number sets.

My own testing and matching procedures have revealed that these tubes are far from what I would consider ''matched'' under any circumstances,and I ended up assembling two better-than-nothing quads by selecting from the eight tubes on hand.

Apologies for the lousy photography:

DSCF2593.JPG DSCF2596.JPG
 
I have noticed that many of the KT120's that I see have ''tired'' getters.Almost always the two side getters,but sometimes the top one is fading away as well. According to information provided by the owners,the amount of use (estimated hours,ARC time counter or time in service from date purchased) does not,in my mind, explain or warrant this level of degradation.

In my experience,only the oldest,hardest run tubes that I have encountered have shown this behaviour.Makes me wonder about the longevity of this tube type.Thoughts?

I'm using Brice's rebuilt Allen Organ 75s with the KT-120s. Here's some photos taken but I don't see any issue myself as they are approaching 400 hours. Other than they sound tremendous matched to this awesome rebuild, they seem to be fine. The bias is solid and balance hasn't moved hardly at all through the entire usage. Looked at them versus another set being used on Brice's rebuilt ULIIs but not sure there's any real difference

They sound very good and are a very good performance value versus KT-88s for these 75 watt amplifier mono blocks. The luminosity appears in use maybe a bit subdued but not different than the smaller similarly aged tubes with them. KT 120s 2.jpgKT 120s 2.jpgKT 120s 2.jpg
 

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