Solid State Preamp to replace tube unit

@decooney yep I initiated that thread. Jeff did not ask me the serial numbers and as far as I know there was only one version of P12. Unless you sent it back to the factory specifically for the Cap upgrade. He does not change out the transformer in that upgrade. In fact the guy I bought the amps from ran it with KT120's in it before he sold it to me!! For some reason I am a bit nervous about putting in KT120, my luck the unit will blow up lol

Different output tubes will certainly sound different but will not increase the amplifier's output. The same energy in produces the same output.  KT 120s can replace those tubes but as you get to the KT150 and especially the KT170 the electrical requirements of the tubes are enough different that unexpected things might happen in some circuits.  So change them only for the sound difference, not performance enhancements.

@coppy777 yea I dont need more power, it would be a sound quality difference

@jimbones doing some quick checking over AudioAsylum and TubesAsylum to see if others with CJ Premier 12 and/or 6550s had tried others such as some of the better KT88s.  Also, located some comments indicating the Premier 12s are 650v at the plate. There is some mention, not a lot, but possibility of running good KT88s in place of 6550s maybe in your Premier 12s, worth checking more to see its a candidate. Seems the older discussions of running the Sventlana 6550s, led to others jumping over to KT88s.  But again, you may need to find multiple sources there to know for sure that its safe in your amps.

On a related note, I helped a friend and we upgraded his older Jolida amp, it use to run 6550s, and a few local friends and I suggested to the try the latest PSVane KT88 TII MKII tube in it. The result has been quite pleasing, less of a glassy modern sound, and more of a lush body/warmth to the amp now. Can be nice if that's what you like. Find a replutable tube reseller of KT88s if you try it.   Note: I run TS KT120s in my own QS Monoblock tube amps, and while I like the KT120 too vs. KT150s, if my amps had lower plate voltage, I'd be running really good KT88s as an alternate.  Long story, I cannot, i have the big amps with extra larger transformers and big plate voltage, will burn them up prematurely.   In your case, though - might be worth a nice try, and worth checking with peer P12 owners, and maybe CJ would advise you on this, worth asking. There are some big fans out here who prefer good KT88s over most of the othe TS KT series tubes fwiw.     

@decooney Thank you so much. Yes I have heard others say they used kt88 as well. Some prefer 88's other prefer 6550's.  from what I know I can use the 88's without any problems.