Should I upgrade my Tubes

Assuming you’ve checked your bias, have more than one source and they all exhibit the same result into the Cronus: I’d replace the middle 12AU7 (preamp tube), first.      Generally speaking; tubes lose their juice more slowly (not a sudden event, in both channels).      If the loss was really that sudden; you may want to contact Rogue for guidance (570-992-9901).     Tube rolling/tasting can be a lot of fun.      If you desire the best sound: contact NOS purveyors (ie: Brent Jessee Recording, Jim at McShane Design, Kevin Deal/Upscale Audio, Andy Bouwman/Vintage Tube Serv), for advice given according to your listening preferences/tastes.    That can get expensive, as well as addictive.

If your amp can accept 6550's Svetlana , go with those,, I am running the Svet 6550 in my Defy7, That tube has a  solid reputation, and I concur lives up to the hype.
The new and old are IDENTICAL,, no such thing as NOS Svet are superior to the new, thats snakeoil, and some folks are getting 2x's the price. 

The 6550 will deliver all the same/or very close (= a superior balance bass to mids) bass as the 120’s, but also give you high res mids
What i am saying is
the 6550 (kt88 = same tube, but I perfer the Svet vs all other 88 labs) is perhaps the finest power tube ever made. 
and I perfer the Svet over all other 6550's

I used NOS octets of Sylvania 6550s for a few years, purchased from Upscale, until they ran out of stock.      Kevin Deal recommended the Winged ’C’, Svetlanas, as his number one choice for KT88/6550 types, and they were close enough in performance to satisfy.      When the Svetlana plant in St Petersburg burned (2012), production of that (very nice sounding and reliable) tube ceased forever.      I then found a supplier for octets of NOS GE 6550As, which were superior in every way.      My new favorite 6550/KT88 tube is the Gold Lion KT88 repop, from Russia.    A conversation with either Kevin Deal or Brent Jessee would be invaluable, far as the straight scoop on the sound and reliability of the current, "best", far as new production KT88/6550 types and what became of Svetlana’s version.      Some history, regarding the Svetlana plant and 6550 (Winged ’C’), here: https://valvetubeguitaramps.com/svetlana-winged-c-valves/      Lots of opinions, here (note the dates): https://db.audioasylum.com/cgi/search.mpl?searchtext=winged+c+6550&SelectForumtubes=tubes     Of course; everyone’s ears and systems vary.      A couple tube purveyor observations: https://www.thetubestore.com/winged-c-sed-6550c      and (bottom of page): https://www.tubesforamps.com/6550-tube-selection-guide

@rodman99999
"...My new favorite 6550/KT88 tube is the Gold Lion KT88 repop, from Russia."

During some of my own "KT88" replacement chats with a few colleagues and some tube resellers, the new Genelex Gold Lyon KT77 is mentioned quite often as well.

Ever try these and compare KT77 to others noted above?

I had the same amp. Found that KT90s sounded better than KT120s, especially on voice.

Btw, not to go in another direction on this thread, but —
Replacing the fuses had more of an effect in this amp than anywhere else in my systems. My guess is because the fuses are DIRECTLY in the audio signal path in this amp. Just trying to be helpful. 

Call me picky, but I have some great 6550s Tung sol (GEs) they make the MC60s just sound wonderful, NOS 1964s I think.. A set of 8 NOS would be 2k easy more like 3-4k.  

KT88s sound NOTHING like a 6550, 6550 is US made KT88 is across the pond stuff.  The old Golden lyon is another great valve, but to be honest higher failure rates, than I'm use to.. (the new stuff anyways).

A lot of the newer KT88 stock is real close in sonics, to me. But a 
1957-68 or so GE 6550. Just one step beyond better...

PSVANE is a real good valve, you might want to look into that one to..
Very positive reviews from some pretty snobbish valve guys...

Regards

I'm concerned that you report a sudden loss of power.  For me this suggests something else that just a bunch of tubes gradually wearing out.  After checking bias you need to check the tubes individually, starting, as already indicated, with the one nearest the front, that is shared by both channels.

Did you re-bias the KT120 tubes?  In my Cronus Magnum 1, the factory supplied KT120 tubes had 3 years on them and I played the amp every single day.  They still sounded great.  Check the 4 fuses where you bias the tubes.  1 or more may be blown. If that isn’t the case, check the preamp tube, it may be dead. 

Rodman Gave very sensible advice

GL KT77s are great tubes, although currently I'm using a pair (I have a quad of these) of unused Sovtek KT88s I've had for a while...somehow they sound great.

@decooney - Never did try anything with less potential power than the KT88/6550, being an SPL lover and trying to squeeze as much as possible, with the lowest distortion, from my modded SLM-100s.      A relatively small, highly treated room with 93dB/2.83V speakers (w/active biamping and SS, below 80Hz, 60dB/oct) helps, but.....                                            The best presentation I’ve had, was provided by the GEs.     Glad I never owned an octet of the original Genelex or Tung-Sols.      I’d have probably been ruined for life.      Would have been nice, had the GL KT77 been available when I was modding/updating ST-70s.

I fixed it, my bad. I am totally digital streaming and I have a Codex which I guess I accidentally put in Preamp mode. Changed the setting and everything worked. Cost me 2 days of lost pleasure though.

I may punish myself (sic) by spending $ on another set of power tubes (as a back up) and enter the world of tube rolling. Any solid suggestions for listening to blues guitar, jazz and chicks singing ?

Also, do I need to be concerned about the 5 preamp tubes ?

You have one preamp tube (the middle 12AU7).      The rest of the nine-pin miniatures, comprise the front end of the Rogue’s power amp section.      If it’s sounding good; no (no worries).      Those will typically last much longer than the output tubes.       KUDOS, on finding the issue.

Check them at least, ay. Do you have a way, or know someone?

It will also tell you how much is still there, and when you might need to change them, or at least be looking for some good replacements.

Panic buying is no fun, over a valve. A spare or two is never a bad idea,
and you really don't want to let them FAIL, before replacing...
It can cause more than a valve to fail.. ok...

The Tubes, what a crazy band.... :-)

Regards

An easy, more noticeable (generally) and less expensive way to enter the tube-tasting whirlpool, than swapping power tubes: a change of the preamp tube.      There are a number of nicer 12AU7/ECC82 tubes, that will change the Cronus’ presentation.        Mention your tastes to any of the tube mongers I’ve listed.      They can easily guide you to sonic bliss (kinda flowery, but- yeah).      Some opinions, here: http://hktubeaudio.homestead.com/files/best_of_the_best_12au7.htm      One nice thing; many of the better NOS, nine-pins are rated for 10K hours.       That tube will need to be tested/certified for low noise.      Any good seller will know that and provide such.

After some research and calling Nick at Rogue Audio, he told me that the biggest impact will be by tube rolling the center (of 5) preamp tube 12AU7. He mentioned the Mullard 12AU7 NOS made in England for the military. Anyone have experience with it ?

http://www.audiotubes.com/12au7.htm

readheads,

I agree with Nick at Rogue that tube rolling the center with a Mullard 12AU7 NOS will give you the biggest impact since that is what I’m using on my Magnum 3. Great detail, bass and quite smooth without any edgy brightness. Call Andy at Vintage tubes and he’ll be more than helpful plus he tests every tube he sells. The 12AU7 should cost around $120.


I also purchased my Magnum 3 with Golden Lion KT77s instead of the standard KT120. The KT77s are a great all around tube and will cost around $200 for the quad. The KT77s also need to be biased at 25 to 30 instead of 35 for the KT120. I found no sound difference setting it at 25 which makes the tubes last longer.