G'day Jim B,
Not sure why no one has bothered to reply... anyway, apologies for the lateness. Here goes;
Been using CJ gear since I was about 15, ran my own dealership network through Spore and Sth East Asian region since 98 to 2004, and CJ was our top marquee brand in tube gear, apart from the few others we also represented, VTL, ARC and Manley Labs.
The Premier 12's are outstanding amplifiers. They are quite old though and may require recapping, along with those nice upgraded metal foil resistors from Vishay. Teflon caps are the way to go, and with these two upgrades done withTeflons and Vishays, the Premier series gear operates at a higher level of refinement.
With small signal tubes (input and driver/phase inverter tubes), we just stuck with the original type required. However, with the output tubes, we changed over to KT120's from Tungsol. To me, they are the most accurate, well balanced and natural sounding, compared to the KT150 or KT170's, which not designed optimally for that power amp circuit, can sound over-blown and way off in terns of musicality. In terns of power, dynamics and so on, yes they do have a blast but nothing compared to a well designed KT120 push-pull based linear pentode design. Of course CJ's newest offerings, the ART150 and ART300's both use the KT150 Tungsols and they're well designed circuits utilising the KT150 output tube topology. CJ hasn't yet decided on the KT170, perhaps more testing is required... I don't know.
However, they've put out a brand new monoblock version called the ART108A's, which are 160w Class A, now that's what I call finesse! However, in this case they're not using either of the KT120, 150 or 170, rather they've gone old school, back to the popular KT88. Genalex Gold Lions reissue I think, and they should perform mighty fine. Need to get to an audition asap!
So, if you're considering output tubes once again for your Prem 12's, it would be either the original 6550C type or the new KT120's from Tungsol will work fine. Either tube will bring those Premier monoblocks to life, it's just a matter of personal preference when it comes to output tubes.
Hopefully you find something excellent. Cheers, and enjoy those fine tunes !
RJ