I'm thinking on it. Let us know if you come across a good deal on a couple of quads.
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The KT77 was the MO Valve version of the 6CA7, or the EL34.
The KT90 is a tube that was developed by the Ei tube company in Yugoslavia in the late 1990s to be a more Powerful Octal based Beam Pentode.
The B+ for the 6CA7/EL34/KT77 is considerably lower than for the KT90, and while the KT90 has the same Pin-out, the operating conditions are different.
Hope that this helps,
Jimborjrib ,- all in the for what it is worth context - I have a Cronus Magnum with the KT90s. I haven't tried the KT77s in this amp or EL34s for that matter,had another amp that accepted EL34s and although I prefer a somewhat warmer sound the EL34 wasn't to my liking. While you didn't ask this, for comparison purposes the old Lion KT88s I use in my Ayon Spirit 2 are a warmer tube than the KT90.
What 12ax7 and 12au7s are you using? My Cronus came with Sylvanias, after awhile I switched in RFT 12au7s and Tungsol 5751s which to me gave a warmer touch.
The center tube is a "Rogue" picked upgrade Mullard tube. One thing I want to don't want to illude to is that the KT90's did not sound great, because they did, I was just shocked at how good the EL34's sounded especially with my relatively hard to drive Martin Logan's. I'm still leaning more towards the KT90's, but taking my time. Thanks for all the input!