Groundbreaking and undeniably collectible guitar amplifiers have made frequent appearances in this space over the years, but so have prototypes, limited runs, rare, or unusual examples that hold a fascination above the “standard.”

Often, these rarities shined a light on the evolution of a deserving as a window into the thinking of their designers while simultaneously charting a course through the needs and desires of the players who commissioned them.

This month, we’ll re-examine five prototypes and would-be pathfinders from among our profiles of the past 15 years. Several are classics while one was doomed by overkill, but all afford an interesting examination of where some gifted minds thought the field was headed at the time of their conception.

1961 Ampeg Jet Reverb Prototype
Ampeg is often credited as the first major maker to provide onboard reverb in its combo amps, or at the very least the first to do it well, and their spring-loaded offerings of the early to mid ’60s have come to define a lush, rich, watery form of the beloved short-reflection delay effect that stands out from others to hit the market soon after.

The first production model to carry reverb, the Reverberocket debuted at the summer NAMM show in July of 1961, but Ampeg was clearly working up to the release for some time before and the original vehicle for the effect had a slightly different platform; as early as that April, Ampeg experimented with an add-on reverb chassis in the bottom of its standard Jet model, a modification apparently made at the factory concurrent to the assembly of the amp itself.

Not only is this a notable chunk of Ampeg history (profiled in the May ’16 issue), but its owner, VG reader Mac Lore, stumbled upon it in one of those fortuitous vintage-amp discoveries.

“I bought it in 1997, from an old ham-radio guy in Riverton, Vermont,” Lore said. “It was in his carriage barn, and he thought it was a small PA with reverb. He’d acquired it at a ham swap meet in Connecticut 15 or 20 years before.”

The investigative trail eventually connected Lore to Ampeg vice president and designer Jess Oliver, who remembered it being put together as a prototype of the would-be Reverberocket. History aside, it’s also a great-sounding vintage combo on its own.

“Turned all the way up, it screams like Bluesbreaker-era Eric Clapton,” Lore recalled. “The reverb adjusts from subtle to gigantic.”

1963 Fender Twin Reverb Prototype
Any declaration of, “Hey, I’ve got a rare one here!” needs to be taken with a pinch of salt. Such was our attitude approaching this believed-to-be prototype of Fender’s iconic Twin Reverb combo made in early ’63, which washed up at a music store on the outskirts of Boston.

The amp was purchased by Craig Jones, proprietor of Bay State Music, after it had first been brought into the shop some 30 years earlier by a student at Boston’s Berklee College of Music. Hailing from California, the guy bought it at a sale on the sidewalk in front of Columbia Records in L.A. after they’d cleared unclaimed gear from storage rooms behind the studios.

The closer we looked for our January ’21 feature, the more the “prototype” claim seemed to line up. To name but a few salient details, the circuit – as confirmed by Bay State’s amp tech (Craig’s son, Aaron) – was a brown Vibroverb preamp plus harmonic vibrato, married to a blond Twin output stage. It carried a full eight preamp tubes (when no other Fender amp of the era had more than six) and was clearly rendered with meticulous workmanship arguably even above the usual immaculate Fender standard. It was all housed in a cabinet covered in brown Tolex that had been painted black, and the rear wall of the chassis – which appeared never to have carried a printed panel – was hand-etched with the prototype serial number, P398087.

Just after the issue was published, guitarist and noted luthier Michael DeTemple called to say he knew the amp and its backstory. Leo Fender had indeed built it as a prototype and given it to his pal, Red Rhodes, to test. Rhodes was an avid inventor himself, but also a first-call pedal-steel player working the L.A. studio scene. According to DeTemple, Rhodes often talked to him about that amp and wondered what happened to it – likely, it turns out, left at a session at Columbia Studios, then spirited into storage for many years.

1973 Mesa/Boogie combo
The rolling of the decades from 1950s to ’60s and into the ’70s didn’t mean the end of formative guitar-amp designs, and one of the most-significant to come to fruition was Randall Smith’s Mesa/Boogie. The screaming, eternal-sustain-generating Boogie circuit famously originated around 1969 as a joke of sorts, when Smith cobbled a high-gain circuit into a Fender Princeton that was dropped at his Bay Area repair shop by Barry Melton of Country Joe and the Fish. He topped the prank by wedging in a 12″ EV speaker, and the resultant “Boogie” hotrods (so dubbed by Carlos Santana after he wailed on one) were built into similar Princeton combos for the first few years of the design’s existence. Then, demand soared and doner amps ran short… and Smith embraced the need for a ground-up build.

One of the earliest is this ’73 Mesa/Boogie prototype owned by Peter Beka (March ’20), who recognized it as something special almost immediately upon picking it up from a seller in Chilliwack, British Columbia.

In addition to the DIY look of the walnut cabinet and indications of a very early face plate and logo plate, Smith himself described several circuit “tells” that confirm the amp’s prototype status. One involves the positioning of the extra gain stage at the very end of the preamp stage, rather than at the beginning as on subsequent Mark Series Boogies.

“As it was always ‘high-gain’ and difficult to dial a good clean sound, I soon moved the extra stage to the front of the preamp chain with a separate jack to enable a typical Fender-style clean,” he told us. “I still remember the moment I figured out the solution.

“That became the Mark I, so named only after the foot-switching Mark II came out and earned my first patent.”

On top of everything else, even the rough-looking cab of this pre-Mark I Boogie was made by Smith himself.

1975 Delta Labs Concept 1
For some makers, the “prototyping” process was more about hitting the ground running, with early incarnations of a design fated to record meager production numbers heading straight into the hands of players rather than languishing in beta-testing purgatory.

Such was the brief life and short run of the Delta Labs Concept 1, a would-be world-beating arena amp designed by music industry legend Harvey Gerst, in collaboration with former colleague Hank Walcott.

After playing Carnegie Hall and Woodstock as a musician in his own right, then logging groundbreaking designs for JBL speakers and Acoustic Control amplifiers, Gerst began developing the Concept 1 in 1974, at a time when stadium-sized venues meant major rock bands required a comparably mammoth amplifier – and mammoth it was.

The Concept 1 used 19 tubes, generating its whopping 300-watt output from eight oddball 33JV6 “horizontal output tubes” that were typically used in televisions.

“They were dirt cheap, but they were great, great tubes,” Gerst told VG in the February ’15 issue. “They also had very low power consumption.”

Other standout features included a pushbutton array at the center of the front control panel labeled “Delta/Tele/Gib/Fend/Mar” which accessed different voicing networks to emulate sounds considered archetypes at the time, a footswitchable tube-driven distortion stage, and the piece de resistance – built-in tape echo. Rather surprisingly given all this firepower, the Concept 1 carried no Master Volume control. But Gerst says it just wasn’t needed. “You don’t have to crank it up, the thing sounds incredible at any volume. It sounds just like a recording.”

This example, owned by Duke Kelso, is one of only about 20 made before the venture fell by the wayside due to internal strife among the design and management team. Before the model was deleted, though, Jeff “Skunk” Baxter of the Doobie Brothers bought two, former Doors guitarist Robby Krieger played one, and Frank Zappa put in an order that was ultimately never fulfilled.

1994 Bogner Ecstasy 101B
Sometimes, a creative engineer develops a prototype without even knowing it. Such was the case with Reinhold Bogner, a young German amp hotrodder who packed up his soldering iron and headed to California in the late ’80s to join the high-gain revolution.

While devising his own sizzling, shred-worthy circuits as modifications, built Frankenstein-like into existing amp carcasses, Bogner unwittingly paved the road to full production models and a high-end amp business that would thrive in the rock market. When session guitarist Matte Henderson sought him out in ’94, though, he was still weaving his magic using the heads and chassis of larger second-hand Fender amps.

For all intents and purposes, the amp in this gutted ’66 Showman is a Bogner Ecstasy 101B, though the circuit is meticulously hand-wired across a one-off custom layout rather than rendered in the high-quality PCB style Bogner would later use for production models.
Like the production Ecstasy models of a few years later, this head has a clean channel with independent Bass, Middle, and Treble controls, two overdrive channels (usually referred to as “blue” and “red”) with their own Gain and Volume controls, gain structure, and gain boost switches, a shared three-knob tone stack, and Master Volume and Presence controls.

“I wanted a huge clean channel, and this delivered it in spades,” Henderson told VG in the February ’12 issue. “The classic Fender Bright switch was retained, and the amp puts out 130 clean watts before distortion. True to the 100B spec, there is no boost on the clean channel. The blue channel can be a considerably more-aggressive take on the 100B… with great cleanup from the guitar Volume. The engaged boost on the blue channel offers ample gain for liquid single notes. The red channel is over the top – infinite sustain with controlled feedback on any note. It’s the ultimate ‘violin tone.’”

This article originally appeared in VG’s January 2024 issue. All copyrights are by the author and Vintage Guitar magazine. Unauthorized replication or use is strictly prohibited.