Optimizing the Prima Luna Premium Integrated amp

All 12AU7s is the manufacturers tube designation for the amp we are discussing. In that event the OP may want to try another direct sub the 5814. The 50s Hytron and later CBS Hytrons are held in high esteem but it is necesarry to them alone to get a feel for the tuybe type. It has what I call the "strong, clean signal" that I find desirable in certain situations.
The RCA 5814 versions are very good and as are many others, try em they shouldn't cost a lot. The other clean strong signal 12AU7 I have some experience with is the Brimar 13D5 which has been around for a few years, but not sure who is selling them now.
As I said earlier if you can find some good Telefunken ECC82s even used testing good you may want to try them. Lets face it most of what is called NOS is just OS and not new.
I don't have a great deal of faith in the RCA 12AU7 clear tops , from personal experience which I admit was a gift of some decent and some unusuable tubes I got in a batch. I don't know much about the others, try Brent Jessies Audiotubes web site, he has some details on the US types. I settled on 7316s a while ago as my 12AU7 of choice and have been hunting them until recently now that prices are really too high for most people. On E Betrade people list them with singles foe $100 and up to start for single tubes. I can't recommend anyone try them at that price.
Current production has gotten much better from all over. My old Pal Joe imports some of the Chinese tubes which he has great faith in. Others have admired the new Gold Lion smasll signal tubes with many other New Sensor tubes.
The 5814s when I last looked were still reasonable, I think the Brimar 13D(X) tubes have gotten a bit more expensive and Teles while Desirable are plentiful as pulls and hit and miss on price. IMHO YMMV etc. etc.

I like the Tubestore Chinese "Preferred Series" 7025s in my Jolida. I mention this because nobody else mentions them...I've become a mention mensch.

I've run both vintage Sylvania black plate 5814's and Raytheon black plate 12BH7A's in place of the the stock 12AU7's in my Primaluna Prologue Three preamp with great success. Changing out the stock Chinese 12AX7's to vintage Telefunkins made a big difference as well.

Al. Wise

I've continued tinkering and over the last few months of tube rolling repeated, varied A/B comparison has led me to 1 inescapable conclusion.
In the PL Premium Integrated while the GL KT-88's are wonderful tubes with tighter control than the stock PL EL-34's at the frequency extremes, they fail on voices in the crucial mid range, with a dry papery affect. This is not a judgement on the GL KT-88's as I suspect in a different amp the issue would not exist, only a conclusion that in the PL Premium Integrated they aren't the best choice.
Additionally after rolling various 12AU7's in the front sockets (Mullards, Siemens, GL, Groovetube, EH) and then trying other 12Axx varieties its become crystal clear that 12AU7's are not the best sonic choice for the front sockets. No matter what 12AU7 I rolled for the front sockets, the pre sounded compressed and sleepy compared to the CJ PV10A pre feeding the HT input. Rolling in 12AT7's gave immediate improvement in dynamics and "liveliness". Using 12AX7 was similiar but more prone to distort

Where I now get best sound is
Front pre sockets - Amperex 12AT7 (orange world logo)
rear pre sockets - Siemens ECC83 (12AU7)
power tubes - stock PL EL-34

The PL power tubes are not my final destination as they don't have the control of the KT-88's and combined with the NOS Mullard 12AU7's showed distortion artifacts which the Siemens did not.
What this configuration does well is create a lush detailed mid range, good mid bass, and a high frequency end where percussion has both hardness (blocks) and sheen (cymbals). Sound decay is good, not chopped off as it was with 12AU7's.
What this lacks is deep bass control and good "black space" around instruments. All of which I think will be improved with better EL-34's
On the fence at this point for future upgrade between between the SED Winged C EL-34's and NOS Siemens EL-34's.

NOS power tubes are frought with difficulties. They havent been produced, in the case of EL-34s, since the late 70s at best. Finding good ones is tough but I was able to buy some Philips XF3s I believe as a quad that did look new in box. The price exceeded $100/tube so be ready for sticker shock. I am aware that some of the big tube sellers have NOS outputs but rices are commensurate with thheir rarity.

Can you sub in the New Sensor Gold Lion KT77? That would be interesting. I am kind of surprised the KT-88 wasn't good in that amp. Another "super tube to consider is the "Tung Sol" KT-120 if the trannys can handle it, ask uncle Kevy?
Finally I would make a point of trying 2 current products that have worked out well for me. The Electroharmonix 6CA7 "big bottle" and the JJ blue glass E34l (note E34l not EL34 not a typo) very underated, don't try the JJ KT-77. JJs can be bought from a webstore US based called Eurotubes.com.
I will be interested in hearing the result. I noted that no 5751 made your preamp tube list, did you try them?