Tube rolling with Quicksilver Mono 60s -- from KT77 to KT88? KT 120? etc.

@hilde45, your small input/driver tubes from Brent J are fine may I suggest. I’d recommend you leave those for now and rotate the power tubes (in your specific case). The original stock KT88s were just okay, as aI recall. The KT77s you have now are giving that magical midrange and smooth highs with a little less drive on the low end. It’s a give/take situation. Really good KT88s are the safest bet in your Mono 60 amps. Upgrades to the small triode input/driver tubes is good (as already suggested, and you’ve already done that) with reputable tube provider.

And, of course, KT120 / KT150 is an option, and while the Mono 60s will accept them, may not be totally recommended in your amps. If you do go that route with 120/150s - the sound will tip away into different directions some. The KT120s are like a big KT88s, and they were a bit dry and less full sounding in my QS Mono 120s with the stock coupling caps. I jumped to the KT150s with a whole new sound top to bottom. More air on top, more bottom end punch and a yet very neutral midrange and not so forward or focused midrange like a good old EL34 has. Sound stage set back a little with KT150s in MY QS 120 amps. Reminder, the QS 120 amps were designed to run KT150s, and I cannot run KT88s, it will burn them up, not backward compatible. Yours will run them, why? TIP: I’d suggest NOT going with KT120s or KT150s in your Mono 60s, reason being is the Mono 120s were upgraded again with even larger transformers to get the most out of the higher $ KT150 tubes. 150s or the future KT170s are 15w more per tube, not ideal for your amps. Waste of $ IMO, for your amps. Can’t drive them properly.

The transformers and amp circuit itself has a lot to do with it, and each tube will sound different in different amplifiers/circuits. Yours, for KT88s.

Try not to judge how your amp sounded with the lower grade stock KT88s they came with new. There are much better KT88s out there than what you started with. Same as others, recommend you try really good KT88s. Some are having good luck with the Shuguang Black Treasure KT88s, or Gray Glass PSVANE KT88s, and of course new re-issue GL KT88s.   For reference, we removed KT150s from my friends older Jolida integrated amp (had smaller transformers, lower plate voltage.  It was not designed for KT150s. We installed a tested and well matched quad of PSVANE Gray glass KT88s with quite amazing success. Sounded like a completely different amplifier with really good KT88s. He’s had them over a year now. The did open up after 100hrs. Time will tell.  

And, if you win the lottery, try some ultra expensive EAT KT88s. Really good KT88s can make a difference too :)

@yogiboy
@decooney My friend has the QS 120 amps and he has used KT88 tubes with those amps. It states this in the QS M120 manual!
Mono 120 for web (quicksilveraudio.com)

@yogiboy
IF they are the latest Mono 120s with the large transformers, have your friend call or email Mike Sanders at Quicksilver asap, and ask again, directly. Believe me, I’ve pushed it with him multiple times, fwiw. Other info...

Originally, I tried to order my Mono 120s with GL KT88s. Nope. Talked to Mike 3 separate times pushing him to share more, he keeps telling me "NO", no KT88s in my Mono 120s. I took his advice, fwiw. Says they will burn up, prematurely.

Tuning with KT150s:
And, once I got the right mid-1960s vintage dual triode Mullard "blackburn" factory 12AU7 and 12AT7 input/driver tubes, the KT150s sing now. More like a smooth KT88 sound with more depth in the sound stage and more power, dynamics.  Also tried the latest PSVANE small tubes too, a bit more midrange forward compare to the vintage Mullards.  The new Mono 120s needed a better pair of small tubes up front to precede the KT150s. Now I would not go to KT88s anyhow for the latest Mono 120s amps, even if Mike would permit it. . Once I changed coupling caps to high end Mundorf Silver-Golds, the synergy was amazing with the KT150s and Mullard small tubes up front. Super nice now.

Preamp:
They deserve a really good preamp. Now running a 6SN7 full Triode Cary SLP-98 preamp in front of the Mono 120s.  The updates to the Mono 120s and a good preamp in front of them brings them to an entirely new level. 
With the updates, I'd now put them up against the $20K KT150 Conrad Johnson monos all day long.  

The new Mono 120s have huge transformers, look for the big transformers, larger than prior version QS amps. fyi.

hilde45
No one has said a word about KT90s....

Bleh.  Stay with GL KT77s or for your upgrade, go with GL KT88s or TS KT120s instead vs. KT90 IMO.   

It also depends on the amp circuit and what input/driver tubes you use.  

Since you know KT77s now and their signature in your amps, I think it's fair consideration, the highs could be a tad more tipped up (in your amps) particularly when compared to EL34s or KT77s. Always a tradeoff. 

@decooney
Thanks for the information. I wonder why the manual says that the KT88 can be used? BTW, I use the KT88 with my QS Silver 70 amps and Mike told me that can also use the KT 150 if I wanted to give them a try. This is from the M120 manual! Tube Complement 1 - 12AU7, 1 - 12AT7 2 - KT88 or KT150

@yogiboy
Not sure I’ve helped. Re-check with Mike for sure with your friend’s 120 amps. This is why I say to contact him, have your friend give him the serial# on the amps and ask again. I’m not sure if he tweaked the plate voltage up on mine at some point for the KT150s. Maybe your friend’s amps are okay with 88s. Yes, the "120" manual shows KT88s in two places, and I challenged him on that too. Note: check the front of the web page on the Mono 120s shows only this now "2 KT120 or KT150 outputs". KT88s are no longer listed there for Mono 120s. Manual needs updating, I don't know - better to ask mike on your buddy's version of the Mono 120s, mine are 2019 version. 

Yes, the older Silver 70, 80s, 100 amps seem to be fine to run the KT88s as I understand it, smaller transformers, likely lower plate voltage (supposedly). Same with hilde45s Mono 60s, ideal for KT88s, the sweet spot. I’d also buy a quad of KT88s right now if he’d say it’s okay for my specific mono 120 amps. I won't ask him a 4th time :)