Delta Labs Concept 1

Delta Labs Concept 1
Amp and photos courtesy of Duke Kelso.
Preamp tubes: Two 6U8A, two 12FQ7, in addition to more-common types
Output tubes: eight 33JV6 horizontal-output tubes
Rectifier: solidstate
Controls: Volume, Treble, Mid, Bass, Resonance, Distortion; Echo effect: Mix, Repeat, Delay; Tremolo: Speed, Intensity. Pushbuttons for voicing.
Output: 300 watts RMS +/-
Speaker: two 12″ and four 10″ Eminence speakers

You’d be forgiven for assuming Harvey Gerst’s resumé was a script for the music-biz adaptation of Forrest Gump. “Plays Carnegie Hall with folk group the Villagers then redesigns failure-prone JBL speaker to create the legendary F series, 1963; writes two gold-record songs for The Byrds, 1964; band Sweetwater plays Woodstock, 1969; gives Jim Morrison a lift home from the Whiskey A Go-Go, grumbles only mildly when rock star throws up in his car, 1970; designs Acoustic Control 260 guitar amp, 360 bass amp, and Black Widow guitar, 1972…” The list goes on.

But, what is among the achievements Gerst gushes over most? The creation of the Delta Labs Concept 1 amplifier, arguably the most advanced and fully-featured guitar amp of its day. You need 300 tube-powered watts with built-in tape echo, tremolo, distortion, and push-button analog tone modeling? The Concept 1 is your machine… if you can find one.

The Concept 1 was developed by Delta Labs in 1974 and manufactured in ’75 in the company’s facility in Sun Valley, California. Designed to fulfill the quest for the biggest and most versatile all-tube guitar amp of the ’70s, the venture had a lot of support from Jeff “Skunk” Baxter, who bought two he used extensively with the Doobie Brothers. Robby Krieger of The Doors was also a fan, and Frank Zappa purportedly wanted one but bad, though Delta never got the chance to deliver.

“Professional” was the watchword with this, and everything was intended to be top-notch. The amp’s circuit was laid out by Hank Walcott, Gerst’s former engineering colleague at JBL, and it used 19 tubes to get the job done, including some real oddballs. In fact, the tubes responsible for the brunt of that 300 watts were among the oddest ever used in a guitar amp; 33JV6 “horizontal output tubes” – eight of ’em – which were usually used in televisions. “They were dirt cheap,” Gerst relates from the Texas-based Indian Trail Recording Studios, which he runs with his son, Alex. “But, they were great, great tubes. They also had very low power consumption. The power for the output tubes came basically straight out of the wall. When you cranked that thing up, it would dim the lights in the room.” In order to suit the amp to smaller rooms (with or without lights dimmed) the amp was also given a switch to cut six tubes from the circuit, leaving two that helped produce approximately 30 watts in what was labeled the “Studio” setting.

Delta Labs Concept 1 The push buttons at the center of the front control panel labeled “Delta/Tele/Gib/Fend/Mar” access a voicing network designed to emulate sounds optimized for different guitar types – or emulate different breeds of amplifier. When he started Delta, Gerst set up a test lab and found a way to chart the frequency responses of different guitars and amplifiers, the results of which went into these preamp switches. Interestingly, though, you can push one selection, or push several to combine them. The Distortion effect is triggered by one of the footswitches bundled into the octal socket on the amp’s back panel, bringing in a tube stage to add extra gain, while the Resonance governs the negative-feedback loop around the output stage – but going beyond most such loops, runs from negative to positive – to take the amp’s interaction with the speaker cabinets from very loose to very tight.

And speaking of speaker cabinets, the Concept 1 was equipped with a couple of doozies, designed not only for what they did to pump the amp’s signal back into the air, but also for how they interacted with its output stage. The bottom was loaded with a pair of high-wattage 8-ohm/12″ Eminence units wired in parallel in a ported cab for a 4-ohm load. The top was four 10″/ 8-ohm Eminences in an open-back cab also wired in parallel, for a 2-ohm total load. This top cab resonated at 80 to 90 cycles, Gerst says, “So, when the top cabinet started to go up in resonance, the impedance would rise. The bottom cab was sitting there, flat [resonance-wise], and would pull the other one down, so it was pretty much a constant 2-ohm load.”

What all that means is the Concept 1 was able to deliver a constant output at all power levels, something that’s surprisingly unusual in the guitar-amp world.

The built-in tape delay is also an impressive feature, and something not seen in a guitar amp since the one Ray Butts built into the Echo-Sonic amps used by Scotty Moore, Chet Atkins, and a few others in the ’50s. The Concept 1’s echo used a standard 8-track tape cartridge for its loop, run by a variable-speed motor that offered a range of delay times via the Delay knob. Add a rich, pulsing tremolo and a comprehensive three-knob EQ stage, and there’s a lot of tone crafting available in the Concept 1. Surprisingly – for the era and given the amp’s massive output – there’s no Master Volume. The objective, though, was power onstage (which still demanded a lot from guitar amps themselves at the time), and there was of course the built-in Distortion, so perhaps it all makes sense. In any case, Gerst tells us, “You don’t have to crank it up; the thing sounds incredible at any volume. It sounds like a recording.”

With all this going for it, why didn’t Delta’s Concept 1 become a household name?

“We had real problems from differences of opinion, internal politics,” said Gerst. Hedged by their astronomical cost, the venture was cut short after only about 20 production examples were made in addition to a couple of prototypes. Gerst has one, but had to drive from Texas to Illinois to acquire a pair of original cabs for it. That said, examples of the Concept 1 pop up now and then, so keep your eyes peeled if you want an amp that just about does it all, and does it loud when necessary.

Delta Labs Concept 1


This article originally appeared in VG February 2015 issue. All copyrights are by the author and Vintage Guitar magazine. Unauthorized replication or use is strictly prohibited.