KT120 back to 6550

Worth bumping this thread again as this was a revelation to me and I think gbmcleod articulated what I heard very well.  I run Rogue M180 monoblocks with KT120 tubes and have a mature and highly resolving system, but was still lacking natural tone and transparency to vocals in particular.  Very detailed, layered and resolving, but not smooth, magical and perfectly clean and clear.  

Tested a matched set of new KT120s recently and while they were bigger, clearer and more layered, they still sounded somewhat the same.  Just newer and better.

Last night a friend stopped by as we have been discussing tube options (no surprise now) and he had picked up a set of 16 Svetlana winged C 6550 tubes for his ARC Classic 120s.  We dropped 8 into my M180s after level setting 3 album sides and were really impressed.  Lower noise floor and just such a perfect natural tone to vocals.  Nothing felt forced or had any edge.  More surprising was the bass just felt tighter as well.  Nothing bloomy like with the KT120s.

While certainly not a fair test of well used KT120s to NOS winged Cs, I did recently enough try new and matched KT120s to know I did not get the same transparent and involving experience, only better and similar to what I already had.  Makes sense.

Went back briefly to the KT120s last night before we all agreed, nope, put the winged Cs in for the rest of the night.  The pure transparency and natural tone of the winged Cs in my system was truly impressive.  One of the last albums I put on was the 45rpm 10 year year edition of Nicholas Jarr's Space is only Noise and it was pretty incredible.

Got about 6 hours of listening in before we had to pull them, but impressed enough to email my local tube guy at midnight and get a set of 8 6550 winged Cs ordered for me.  6550 tubes on my M180s?  I'm in. Who knew...

p.s. - To the OP, yes, I believe ARC switched to the KT120s when they ran out of said Svetlana winged C 6550 tubes specifically.  In fact, I think they had bought the bulk of them once they stopped being produced so they could run with them for as long as possible.   

@kingpin75s  I have heard the same thing as you on several amps, the 6550 sounded better. However, these amps were originally designed to run on 6550's and then modded to work with KT120's. Of the amps that were originally designed to work with KT120's, that is usually the best sounding tube for them. Was your amp originally designed ( circuit wise) to run 6550's?

@daveyf  Your observation makes sense.

Regarding:  Was your amp originally designed ( circuit wise) to run 6550's?

I would expect so as the original M120 monoblocks were based on 6550/EL34/KT88 tubes. 

@daveyf  you are talking one point of the whole story.

Which is the design of  amplifier for specific tube.

The other point we need know is about the feature of individual tube.

6550 is better for vocal for sure than kt150.

Kt150 might be good at both end but not middle.

Kt120 is in-between kt88 and kt150.

@runwell  "6550 is better for vocal for sure than KT150"--where do you get that from?? "KT150 might be good at both end but not middle"---Huh???