KT150 to KT120..or KT120 to KT150

As an owner of an amp designed for the KT150, the ARC Ref 150 SE, my only comment would be why even wade into the weeds as to which is better is any particular amp when New Sensor has terrible QC in general and most terribly with the KT150? For over two years now, New Sensor has failed to make any improvements in their QC while continuing to put out KT150's to the peril of the consumer. 
If you buy a quartet or octet of "matched" KT150's from virtually any source, you are doing so at your own risk. And in case I am not clear to you, I am saying that most vendors who claim to match tubes do not do so very reliably. They either lack the necessary equipment, the necessary know-how, or both. 
If your amp has fail safe features such that an arcing tube will not necessitate any repairs and your supplier is willing to give you 60 days, I could see venturing into the 150 waters. Otherwise, I would stay away unless you have money to burn AND don't mind the inconvenience of taking your amp in for repairs/sending it back to the manufacturer for repairs. 

The varying opinions of the KT150 tube seems to be primarily dependent on whether the amp was designed to utilize the KT150 in the first place. Since the KT150 needs a more stout transformer and has to run on a higher voltage, i would question if those who are not pleased with this tube are simply seeing the results of swapping in the KT150 into an inappropriate platform! 

@fsonicsmith   You are using the KT150's supplied from ARC? They do seem to have good QC, better than most. In the past, ARC has told us that rolling in tubes from other vendors is dicey, regardless of the tube type.

@fsonicsmith
Does your ARC 150 amp have the upgraded and lowered main circuit board mod for KT150s, and with more clearance for the top cover?

I remember reading some (not all) of the ARC tube amps around 2015 having issues with KT150s and main board changes, some failing resistors or something, among other things got redesigned. Bias issue and close matching tubes might not be as much of an issue after mod, I don't know. Also, comments about improved sonic quality and longevity.

Seeing redesigned amps by different brands with KT150s past two years, and suspecting maybe they figured out how to drive the tubes differently, fwiw. I wonder if ARC might offer any tweaks for them today?    

Found a helpful article for anyone comparing the jump from EL34 to KT150 output tube based amplifiers.    

My former amps were EL34 output tube based.  Liked them fairly well. Loved the midrange.  Now running other new mono tube amps, jumped through KT120s, on to KT150s, not letting myself forget the former, EL34s.. 

Initially, I missed my old amp and the magical EL34s, until the KT150s got past 150hrs burn-in. Also, found the right pair of small input/drive tubes to pair up, thus changing the sound of the KT150 result in a great way.  After burn-in, soon realized I did not miss the EL34s or KT120s any more.  

Article, Hifi-Advice:  
See the "From EL34 to KT150" or "From KT150 to EL34" paragraphs.  A helpful read to confirm what I've been hearing, even with completely different amps yet experiencing similar findings after making the jump.
https://www.hifi-advice.com/blog/review/analog-reviews/amplifier-reviews/primaluna-dialogue-premium-... 


   

1/8/2022:

KT150 > KT120s. Re-testing and giving it a lot more time this go-around with two mono amplifiers specifically designed to run KT120s and KT150s. Big transformers, higher plate voltage, 40-45ma per tube bias.

Picked up a quad of Tung Sol KT120s a week ago. Giving it another try after locating and reading back through threads I’d not seen before on the CJ Owners Forum and DIY builders, noting several members favoring KT120s over KT150s and KT88s. I kept seeing these comments, and realized it was time for a complete do-over.

Update;

At 48hrs burn-in on the KT120s, here is what I’m noticing in comparison to KT150s. After more time and paying close attention, I’m observing similar traits as others.

The KT120s vs. KT150s, are:

• A little more rolled off on top frequency.
• Less low-low-end bass extension.
• More musical midrange, tone, texture.
• Not as clear and extended, but still nice.
• Engaging. Enjoyable, a bit more vintage tone.
• Closer to KT88 sound than KT150s.

Adding to the thread, hope it helps others. I’ve come to appreciate the KT120s a lot more than I did the first time around.  They are a nice [go-between] alternative to KT88s or the more clear and linear sounding KT150s.