KT88 vs 6550 tubes

Forgive me but, that was unwise. You need to ALWAYS adjust bias in a fixed bias circuit if you change tubes, let alone tube types. 6550 and KT88s are NOT swappable without doing this.

Did the KT88 plates that blew the fuse turn cherry red by any chance before it did?

Well, I don’t know man. The MC275 doesn’t even have a bias adjustment knob and manual says I can use both. No, no cherry red, just the golden glow like the color of a light bulb. 

I’ve had similar problems . The best buy , unless you want to spend 2K $ on NOS (which may not be same) Sylvania 6550’s is to to buy a matched quad from "the tubestore.com " This Canadian firm is honest
and does a good job .I got a Quad of KT 88 from them few months back to replace 6550’s only to see the sound difference . In My AES-25 Super Amp the 88’s are a bit clearer which I like .

The 6550’s ran for 4 years and are still fine . tubestore calls these quads "Preferred" , only few bucks more .

I probably have 50 or so 6550s or KT88s. Unless you want the NOS Tung Sol 6550s which are pricy, the Genelex Gold Lions are a pretty good choice. I've had the 275s and have been running MC60s for the last few years. The reissue Tung Sols have really good "shove" but are just slightly grainy. Sophia Electric very resolving but the bass isn't quite there. Good tubes though.
I do have a matched quad of NOS Tung Sol 6550s three hole grey plate which have never been unpackaged. I've never sold anything on Agon but would list them if you're interested. The bass, while not light, is just a bit tubey. Voices and instruments are by a good measure more real. Got them from Tube Depot I believe.
If you want the ultimate tube I just saw some NOS Black Plate Tung Sol on Tube Depot, the tube the original 275s were voiced with. $650 each (whew). But they are the best ever made, or so I've heard. If you are keeping the 275s for the long haul these may be of more value to you than may be apparent. Like the grey plates, but supposedly the bass is the best of all. I've been watching for 5 years and these are the first NOS Black Plates I've seen. 

@dentdog 

there are quite a few Tung Sol NOS black plates on eBay. Are they not the same? I mean there’s no way to fake those things based on what I’ve read. 

@dentdog actually there are also some original British Mullard KT88s. I’m not sure whether they are safe to buy because of my previous problem with the KT88s 

http://www.jacmusic.com/KT88/kt88.htm

Lots of info at jacmusic.

My experience is that the vintage NOS tubes are considerably more reliable.
As for Ebay while I have bought successfully there, when it comes to high dollar output tubes I am hesitant. 
I wil say that the tubes you use in these amps can make a big difference.
Input and driver tubes can influence things greatly. Siemens-Halske 12AT7s are quite nice. RCA 12AX7s seem to round things out. Good bass. Andy at Vintage Tubes provided my AX7s. 

@dentdog thats true. I didn’t like the EAT tubes at all as they sounded muddy and lacked bass, despite being priced at $450 each. Popped my RCA 6550s and unit came back to life lol. I’ll see if I can track down a quad of black plates 

Try Tube Depot and see what they have.

https://www.tubedepot.com/

you probably got a bad tube. I would contact your seller for a replacement I have been running Golden Lion reissue and Shuguang 50 year treasures in my Mcintosh 275 vi for about 5 years with no issues whatsoever 

Yep... you got a bad tube.  I've got a number of BAD tubes from ebay sellers stating them as new.  (I suspect the tubes are factory seconds.) The tubes from certain ebay sellers measure poorer than the ARC supplied tubes with 2000+ hours on them.  I only now purchase from ARC, Tube Depot or Upscale audio.  I've had multiple EH 6992 tubes blow after 30 or 40 hours, from a seller on ebay; I inserted the original ARC Sovtek 6992's (with 1000's of hours on them 2 years ago.) and everything is great.  

   So the “ KT” means kinked tetrode , this was a design change to circumvent the original patent. I have NOS Tung Sol 6550 ( 3 hole ) and SED Winged C 6550’s . I have NOS GEC KT-88’s and KT-77’s (smoked glass ). New production GL KT88’s and KT-77’s. Also new production Psvane, JJ , Tung Sol . Over the last 2 years , I’ve had 1 SED 6550 go bang ( bottle split ) with no amp damage . And one almost new current production GL KT-77 go bad . The latter looked fine but the amp produced a huge pop through my speaker and a wisp of smoke from my amp . It’s not a MAC , but it is “ Self Bias “ . I sent it in for repair . I was told by Mr. Had , that when he tested the tube it was bad and had caused a run away plate voltage issue that toasted the self bias circuit . He also told me on a previous conversation that if I wanted to run NOS 6550’s in my amp to get the “ 3 Hole “ version and not the solid plate version. I’ve run most of the tubes in my Dynakits too . FWIW I like the SED Winged C 6550’s in the Dynakits and the NEW Tungsol KT 120’s in the Had. For late night quiet listening I run either of the Nos GEC ‘s and a 5U4G to drop the voltage . If you don’t want to pay crazy prices for NOS , and want a honest seller ( don’t we all ), try Johnny at 3M Audio in Houston . I use him and I live in California. BTW he’s a high volume Mac dealer . Check his web site . He also lists on Agon and EBay . Another source of info is Chip ( RumDog ) on EBay . I bought a box of SED’s from him . He runs Twin MAC 275’s bi amped with SS on vintage JBL’s ( the big stuff) he has a lifetime of colllecting JBL and is into MAC’s . Cheers , Mike B. 

I’m sorry 3MA Audio , Houston, TX 

I’m a fan of the current production Gold Lion KT-88, just make sure you buy from a reputable seller. I like using McShane. 
Good luck!