BC Audio JMX 100 Review
A reinvented plexi with octal preamp tubes and true point-to-point wiring.
The Plexiglas panels on the front of those early 100-watt heads earned them the nickname “plexi,” although the official model names were 1959 and Super Lead. (Marshall also made 50-watt plexis.) While guitarists no longer need to summon such volume onstage thanks to modern sound reinforcement, for many, the 100-watt plexi remains the definitive hard rock machine. And to this day, the model tends to be the starting point for designers hoping to craft the ultimate high-wattage beast.
The Super Lead is also a jumping-off point for San Francisco-based builder Bruce Clement of BC Audio, but the guy jumps pretty darn far. While the basic topology of Clement’s 100-watt JMX 100 clearly descends from the Super Lead, by no stretch of the imagination is this amp a clone. The circuit includes many refinements and departures, but two are especially significant. Unlike vintage Marshalls, the JMX 100 is entirely point-to-point wired, with absolutely no circuit board, turret board, or terminal strip. Also, the amp eschews the usual 9-pin preamp tubes in favor of octal (8-pin) SSL7GT tubes, which are larger and longer than the 12AX7/ECC83 models found in most amps. As on the original 1959, the power section employs a quartet of EL34s, though the earliest 1959s used KT66 tubes.
I’m embarrassed to admit it, but the first time I peered inside a BC Audio amp, I was so spellbound that it took me a moment to realize the entire thing was wired point-to-point without turret board. (See interior view photo.) The immaculate rectilinear layout resembles a roadmap of some idealized future metropolis. Wire runs are kept to a bare minimum. This is simply magnificent work. (Like some other point-to-point builders, Clement insists that eliminating extraneous wire contributes to the quality and immediacy of an amp’s sound. I’m no electrical engineer, so I remain agnostic on this claim. After all, most of our beloved vintage amps were created with conventional turret board. On the other hand, many of my favorite amps of recent years have also been point-to-point.) The parts are top-shelf, including the ClassicTone transformers.
The exterior is equally lovely. The head cabinet is solid pine wrapped in black tolex with eye-catching red racing stripes. (BC Audio also sells cabinets with matching stripes, though I reviewed the JMX 100 through an old THD cabinet. The miked speaker in the demo clip is a 65-watt Celestion Creamback.)
Playing for Peanuts
Clement says he inherited an affection for octal tubes from his dad, a WWII pilot and passionate electronics hobbyist. “Octal preamp tubes were used in the early days of audio before ‘miniatures’ like the 12AX7 were developed in the 1950s,” he says. “My dad told me that when they first came out, people called them ‘peanut tubes.’” According to Clement, overdriven octal preamp tubes produce a sound closer to power-amp distortion relative to those “peanuts.”
The amp’s sound bears out that claim. Clean tones have more sparkle and snap than equivalent vintage Marshall tones. At times you’d swear you’re hearing a large format Fender. Note fundamentals are rock solid. Transients are crisp and definitive, transmitting every nuance of pick and finger. This is one articulate amp!
Gain of the Gods
There’s no shortage of fine distortion when you advance the gain, but it doesn’t have the ratty/fuzzy quality you encounter with many modern high-gain heads. At hot settings, notes and chords have a clear, clanging sonority that really does sound more like hard-working power tubes than overburdened preamp tubes. Maximum-gain sounds aren’t as filthy as you may be accustomed to hearing from Marshall-derived amps, but that clarity and headroom yield godlike saturation when you connect a great-sounding fuzz pedal. The octal preamp tubes are also uncommonly responsive to your guitar’s controls. Even at high-gain settings, you can roll back your guitar’s volume pot for chiseled-in-rock clean tones.
Plexi fans know that the real fun starts when you jumper the clean and bright channels together. That’s true here, though you don’t need a jumper cable. Above the gain controls for each channel is a 3-way toggle that lets you drive either or both channels. There’s only a single input jack, so you need this switch to change channels, unless you connect a footswitch to the rear-panel jack. The normal channel is far darker than the bright one, which means you can obtain a vast palette of tones solely by setting the toggle to “both” and varying the balance. Meanwhile, the 4-band tone controls have subtle, surgical ranges, more suited to fine-tuning tones than dramatically altering them. I suspect that many players will “set and forget” the tone controls and perform much of their tone shaping simply by adjusting the bright/normal blend.
It should go without saying that this amp can get absurdly loud. But unlike vintage plexis, the JMX 100 has a fine-sounding post-phase-invertor master volume circuit. In fact, I recorded the demo clips with the master volume only halfway up.
No one can deny that the JMX 100 boasts superb workmanship and a huge array of rich, articulate, and harmonically pleasing colors. Whether it’s the right high-gain head for you may hinge on whether your favorite tones rely on fizzy preamp-tube distortion, or whether you prefer authoritative power-amp distortion with sufficient headroom to slather on upstream fuzz and distortion pedals as needed. The $3K price is formidable, but bear in mind that this is an extremely labor-intensive build, and that every bit of that labor was executed by hand with supreme skill. The JMX 100 is a masterful piece of work.