@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 :)
