Tube Power Amp Reviews

Then Kevin Deal, of Upscale Audio, sent me a pair of early-1980s Mullard CV4004 tubes (in their own little Royal Air Force box) to replace the Alpha's two stock 12AX7s. Talk about a dramatic, cost-effective tweak. Not only did the Mullards enhance the focus, accuracy, and nuance of the Alpha's bass, but the harmonic detailing, layering, dimensionality, and clarity they elicited from the upper-midrange/lower-treble region was so magical that the CALs' resolution now approached the Sony's. The Delta-Alpha's performance is now so musical and involving that I remain mystified why this classic combo is not a mainstay of Stereophile's "Recommended Components." [Because California Audio Labs informs us that neither product is still in production.—Ed.]

Setup
Much as there was to love about the build quality and sound of the Magnum M-120s, they did run hot. During their initial break-in period, I found myself wishing for a vial of eau de transformer—I could plainly smell all the electronic components burning in. However, after about a week the smells disappeared and the amps settled into a more than acceptable level of long-term performance.

A curious aspect of the M-120's design is Mark O'Brien's tendency to sequester all manner of features, switches, and fuses deep inside the chassis. Not that it was so bloody difficult, when I was trouble-shooting the source of some audible distortions late in my auditioning, to loosen 10 screws and remove the cover to inspect the four slow-blow fuses. (I replaced three fuses and three tubes during my time with the two M-120s.) Nor was it terribly complicated to disconnect the ground wire that screwed down to the chassis when I encountered ground-loop hum during initial setup. Still, I had to don oven mitts to remove the hot 6550s so that I could access the triode/ultralinear switches. As we go to press, Rogue informs us that all new M-120s leaving the factory will be equipped with external switches for the ground lift and the triode/ultralinear selector.

Sound
I was captivated straightaway by the sheer drive of the Magnum M-120. Fast, muscular, and accurate, with fantastic rhythm and pacing, the Rogue had all the intimacy, refinement, and dimensionality I expect of triode, but with a dynamic thrust and transient snap I could feel in the small of my back. The Magnum M-120 delivered an honest 60W into 8 ohms, and believe you me, its dynamic headroom belied its rated output specs. And compared to other ultralinear amps I've heard, the bump in power you got from the M-120 in ultralinear mode didn't result in a brasher, brighter presentation (as I got with the VAC Avatar integrated amplifier), but a plumper, more fleshed-out style of resolution. The listening perspective was comparably more forward in the upper bass and lower midrange—rather than sitting toward the back of the room, I found myself right up on the lip of the virtual stage.

No one wanting to really boogie should pass up an opportunity to audition a pair of Magnum M-120s in ultralinear mode with some classic rock recordings. The transient snap, crackle, and pop of Stewart Copeland's bass-drum beater against a Mylar drumhead was portrayed with thrilling immediacy on the Police's "Voices Inside My Head" and "Bombs Away" (from Zenyatta Mondatta, A&M 75021 3720-2), without neglecting the ambient shimmer and stereo spread of Andy Summers' Telecaster or the subtle layering and dimensionality of the mix.

In listening to the cathartic performance of Johnny "Rotten" Lydon, Keith Levine, Jah Wobble, and Martin Levine on their eponymous single, "Public Image Limited" (from PIL's The Greatest Hits So Far, Virgin 91581-2), the M-120 delivered all the tubby depth and gargantuan weight of Wobble's Ampeg SVT-assisted bass tone, while successfully reproducing the wealth of dub-like details that pepper the mix. Better yet, I could actually make out the lyrics in Lydon's gnarly vocals.

Which mode sounded better: ultralinear or triode? Each represented a very different style of sound, and a lot will depend on what styles of music you favor and the size of your room, but mostly I'd say it will be a function of the interaction between the Magnum's output transformer and your speaker's crossover. Even so ballsy an amp as the Rogue sounded a touch underpowered in triode mode when trying to drive the older version of the Joseph RM33si Signature speakers (though these problems ended with the implementation of the Joseph's newest crossover design).