The KT88's you have are the square hole version which identifies them as being made for McIntosh by SED (Svetlana in St. Petersburg, Russia).

Bob

__________________
Amps: MC2301 tube mono's/dedicated 20A breakers.
Preamp: C500C/C500T
Digital Source: MCD1100 CD/SACD
Analogue Sources: SME 20/2 w/SME V arm and Palo Santos Presentation Cartridge & Akai GX-400D Reel-to-Reel w/relapped heads by JRF Magnetic Sciences.
Vibration Control: Nordost "TC" Sort Kones under CD/SACD & C500 and "AC" under amps. Shakti stones on amps.
Speakers: Sonus Faber Stradivari
Power Isolation: Isoclean PT-3030G III 3KVA Isolation Transformer w/Isoclean Super Focus In/Out Cables.
Power Bar: Isoclean ICP 206 Non-Filtered Power Bar.
Power Cables: Wireworld Platinum Electra on amps, preamp, CD/SACD.
Power Outlets: Isoclean ICP-003 Power Wall Socket.
Speaker Cables: Wireworld Platinum Eclipse
Signal Cables: WireWorld Platinum Eclipse XLR's on MCD500 & C500T to MC2301's.
Headphones: Sennheiser HD-650's and Koss Pro IV AA.
Tube Test Gear: Mint late '60's/calibrated Heathkit TT-1A & current SOTA MaxiMatcher tube test sets.

How about the signal tubes and could you recommend where to buy them?
Much appreciated.

Your set up is beautiful, and I am so glad that you are loving the sound.

I, as many others here, can tell you that replacing the stock tubes will yield results beyond your imagination.

Best of Luck, ENJOY!

__________________
Shindo/DeVore Super 8's

One good thing about music: When it hits you, you feel no pain.

Bob Marley

Beauty!!! Those 275's are what I have been planning for a 2nd system. Time will tell.....................

__________________
MX136, MC501s, MC402, MCD500
RtR 300Ds, JL Fathom F112, Mirage OM-C3, JBL 2600s
Dual CS 606, Ortofon 2M Black
Tandberg TD20A, Harmon Kardon HK 400xM
PPP, Power Port, WW Ag Eclipse IC's & Speaker Cables, WW Ag Electra PC's

Beautiful MC 275's! Hope you are loving them!

Deb