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RE: Rogue Cronus Magnum II - long

Posted by classfolkphile (A) on July 28, 2019 at 16:00:41

In Reply to: RE: Rogue Cronus Magnum II posted by stereo5 on July 27, 2019 at 11:38:14:

Okay, so I've posted a few times about tubes in the Cronus and Cronus Magnum (1). Even wrote a review of the amp years ago here. I bought the original Cronus and had it upgraded a few years later to the Magnum edition. A couple of years after that I then sent it back to Rogue to have the output coupling caps changed to Mundorf Evo Silver/Gold/Oil and the bypass caps removed (the latter produce a treble that arrives before the midrange). I also added Synergistic "Red" Fuses. These modifications resulted in a lower noise floor and a smoother, more resolved sound. I still have the amp. The tubes I have used over my years of ownership of the various configurations of this amp include the following power tubes (and yes, I had the tube sockets replaced on one of the trips back to Rogue ;)):

Sovtek EL 34
SED EL34
Tesla EL34
Black Treasure 6CA7
RCA 6L6GC Black-plates
Sylvania 6L6GC
Tung Sol 6L6STR
Reflector 6P3S-E
SED 6550
GE 6550A
Gold Lion KT66
Gold Lion KT77
Gold Lion KT88
Penta KT88SC
EH KT90
Tung Sol KT120

In the CM, IMHO, the EL34 family (as in most - but not all - amps I've heard them in: exception, Dodd Mono 120s) are truncated at the frequency extremes. The KT88s (as in most - but not all - amps I've heard them in: exception some VAC amps) have an overemphasized bass (with the SCs sounding more refined than the GLs however). The KT90 sounded lean, the KT77, though refined and airy, seemed to have discontinuities in the frequency ranges (apparently not endemic to the tube itself: I wonder if it was the effect of the bypass caps that were then in place).

The KT120 through its extension and power appealed for a long time but eventually - in comparison to my favorites - began to sound grainy, with a thickness to the bass (though not as much as the KT88). (Interestingly, the Tung Sol 6L6STR, 6550, and KT120 have all sounded thick in the bass to me: the 6L6 and 120 in the CM, the 6550 in a Quicksilver and a CJ). Full disclosure: I don't recall hearing either graininess or thick bass with the KT120s in Mark O'Brien's system that he was kind enough to let a reviewer friend and I listen to several years ago. This was comprised of a Hera pre-amp, Apollo mono-blocks and Wilson Alexandria speakers. It was a treat.

I've always favored the 6L6/KT66/6550 family in the amps I've owned and heard a lot, and it holds for the CM. They are balanced, extended and refined. While I still have the GL KT66 and NOS Sylvania 6L6GC, my favorite is the Reflector 6P3S-E (probably unavailable now). I wish, however, given the changes to my amp and to other of my equipment, that I had not sold the GE 6550As, SED 6550s and RCA 6L6GCs and could again compare them. And I'm thinking it may be worthwhile to revisit the KT77s.

As I've posted previously, all of the small signal tubes make a significant sound difference in the CM (with, yes, the pre-amp tube the greatest contributor).

In new production 12AU7 and 12AX7 I've tried Mullard, Tung Sol, EH, JJ Tesla, Shuguang, Groove Tubes, and Genalex. The Genalex were the best of these.

In NOS, I've tried various Mullard, Amperex, GE, Sylvania, Tung-Sol, Tungsram, and Telefunken.

In all, I have always preferred the Radiotechnique 6189/12AU7 in the pre-amp position and the Mullard CV4003/12AU7 in the driver slots (NOS Mullards go noisy in the pre-amp section). In the phase splitter positions the Amperex or Mullard 12AX7s are my favorite, with the RCA or Raytheon 12AX7 black-plates perhaps as good, if different (a tighter, more focused sound).

Tungsram 12AU/AX7s are good, if thicker sounding than Amperex/Mullard (but more affordable - see Tubemonger) but I find the RCA/Raytheon 12AU/AX7 black-plates are better, and cheaper yet. Mixing Tungsram and RCA/Raytheons, with either NOS GE long-plates or Genalex works if NOS Radiotechnique, Mullard and Amperex are unavailable/unaffordable. I'd suggest the GE or Genalex in the phase splitter positions, the Tungsram in the driver slots and the RCA/Raytheon in the pre-amp position.

Mark O'Brien likes the Telefunken in the pre-amp slot but I found it bright.

As always, YMMV.

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