NOTE: Unfortunately, I do not own any of the instruments discussed or pictured here.
This list was tricky to compile as many of these instruments were used widely by other genres besides prog & some were used by only a few prog musicians who had a big impact on the genre. Consequently, I tried to weight my choices based on their proliferation in recordings as well as their contribution to the unique sounds associated with the early days of prog(1969-1974). Absent are the wildly-popular instruments introduced later, even though they became prog mainstays as well(my apologies to fans of Korg, Oberheim, Roland, Sequential Circuits, Yamaha, etc ...also great keyboards, but beyond the scope of this list).
#8. ARP 2600 You know the 2600 if you've ever heard Edgar Winter's, "Frankenstein", & who hasn't? ARP combined the infinite flexibility of a large modular synthesizer without requiring the use of lots of messy & intimidating patch cables by internally hardwiring the most common connections, but allowing these to be rerouted as desired by simply inserting a patchcord & choosing a new destination for that signal. It also had a pair of built-in speakers & a spring reverb. During its production, from '71-'80, ARP made many design changes resulting in at least nine different versions. There were changes to its cabinet & cosmetics, electronic components, filters, reverb tank, etc, etc. However, one of the most functional upgrades was the addition of the model 3620 keyboard controller which added a Low Frequency Oscillator(LFO) for use primarily as a vibrato generator so that none of the main unit's three oscillators would have to be dedicated to this simple, commonly required task. Additionally, the 3620 provided note latching & an auxiliary set of control outputs allowing "duophonic" playing in which one oscillator could be assigned to the lowest note played, another to the highest, & both sounding together when a single key was pressed; useful for some things, however it resulted in a normal thick sound on single notes, but a thin, single oscillator sound per note when harmonies were played.
The 2600 was used in the prog world by Tony Banks, Pete Townshend, Patrick Moraz, Joe Zawinul, Dennis DeYoung, Kansas, Starcastle, & a host of others including most all genres to present(&, of course, the voice of Star Wars' robot R2D2 ...bleep, bloop, bleeeep ;-).
#7. Eminent Solina String Ensemble Originally designed & built by Dutch organ manufacturer, Eminent, it was soon imported by ARP who initially simply placed stickers over the "Solina" & "Eminent" logos & declared it the "ARP String Ensemble SE-IV", the name by which it became widely known (soon, all similar instruments would be referred to generically as "string machines"). It utilized "top octave divide-down" organ technology to produce a fully polyphonic instrument without having to have a separate oscillator for every key, thus making it ARP's only instrument at the time capable of playing full chords. It went through many revisions, mainly involving the chorus circuit which imparted the unique sound & rich character that made it so popular. One clever revision added gate & trigger outputs for interface with another ARP synthesizer, such as an Axxe, Odyssey, 2500, or 2600, allowing the String Ensemble to control that synth's envelopes as the SE's output was routed through the slave synth's filter section thus creating a single-trigger polyphonic synthesizer (Pete Townshend may have used this arrangement on The Who's Quadrophenia). While limited to simple, simulated "string" sounds on its own, it was marketed by ARP as a sort of "poor man's Mellotron", yet far more reliable, capable of infinite sustain, & smooth, automated attacks & decays. It can be heard on innumerable songs across all genres, usually as a high, sustained string section part & was an essential element of many a prog band's keyboard rig.
#6. EMS The Putney (VCS3) First produced in 1969 by the English company, Electronic Music Studios Ltd., this unusual-looking synth, while not a "keyboard" per se, had rear interface jacks for connecting one. However, in its original form, its pitch instability made it better suited for use as the ultimate sound effects & processing machine. It is most famously heard in "On the Run" from Pink Floyd's Dark Side of the Moon & served as their primary sound effects generator as well as that of English space rock band, Hawkwind. Other users were The Who(Who's Next), King Crimson, Alan Parsons, UK, & Brian Eno who employed it extensively on the early Roxy Music albums & even processed Peter Gabriel's voice on The Lamb's "Grand Parade" by Genesis.
EMS was the English equivalent of Moog during the era & pursued a separate, but parallel path in terms of innovation & the desire to produce smaller, more affordable & marketable instruments. In this regard, EMS beat Moog's Minimoog to market by a year with The Putney as the first compact synthesizer. Despite being a small unit, it lived up to its name of "Voltage Controlled Studio" (VCS) by including a pin matrix style patch bay allowing almost any component to be connected to any other without cumbersome patchcords thus facilitating experimentation & yielding infinite creative possibilities. This unique patching system would become a hallmark feature of EMS synths as they developed refined versions of The Putney & later incarnations such as the Synthi-A & Synthi-AKS.
#5. Hammond C-3, B-3, L-100 series, etc. The Hammond "tone wheel" organs were the bread & butter bedrock keyboard sound of prog. Used nearly universally their distinctive growl was really the first "rock & roll sounding" keyboard instrument that blended perfectly, yet distinctly with electric guitars. If a prog keyboardist had only one instrument, it would be a tonewheel Hammond played through a Leslie rotating speaker cabinet which utilized fixed internal speakers projecting through mechanically-rotated horns to create a moving, thickening, chorus-type sound, or "doppler effect". This could be varied through typically two speeds gradually increasing or slowing down to the new speed as a result of the momentum involved in accelerating or braking to alter the speed of the rotating horns.
The tone wheels were a series of nearly 100 rotating toothed discs of equal diameter, but varying tooth counts spun at different constant speeds each next to a corresponding electromagnetic pickup(think guitar pickup). They produced one of all of the specific harmonics, or fixed sine wave pitches, necessary to cover the entire musical range. The volumes of these pitches were then varied with the drawbars(slider controls) to balance the lows & highs to achieve the desired sound through "additive synthesis".
The main drawback of the Hammonds was their incredible weight. Consequently, touring bands(see organ on left of pic) used cut-down models which separated the top half(as seen in the photo) from the base which reduced it from incredibly heavy to merely hernia-inducing proportions.
#4. ARP Pro Soloist This was one of the first synths to provide a wide variety of quickly selectable preset sounds thus eliminating the stress of frantically adjusting multiple controls between songs on dim stages to achieve a new sound ...and most were good, usable presets that could be further altered during performance. Originally ARP's low-cost entry into the home organ accessory market, it included a rear underside kickstand to angle it for easier playing atop an organ & a detachable music stand, but significantly had one of the first pressure-sensitive keyboards which allowed adding pitch bend, a filter wah effect, a "growl" effect(modulated filter), brilliance, volume, &/or vibrato changes in any combination while playing, thus opening up a new range of expression for keyboardists. While there were various users including Chick Corea, John Entwistle, & Joe Zawinul; Tony Banks put this instrument on the map. It's all over the '73-'77 Genesis albums on nearly every solo & keyboard melody where he made effective use of its aftertouch as well as switching through multiple preset sounds during the course of a single song or solo usually drenched in reverb or run through a Leslie to thicken its single oscillator sounds.
#3. Moog Taurus Commonly mispronounced as "moooog"(like a cow), the correct pronunciation is "mow-g"(rhymes with "vogue" & "rogue"). This was a 13-note, one octave, organ-style bass-pedal-operated synth(although it had about a five octave range). There were three basic preset sounds: "Taurus", "Tuba", & "Bass" as well as a fourth labeled, "Variable" which activated a user-designed sound determined by the positions of several basic slider controls under a clear plastic door on top. Additionally, the player had foot control of Volume, Filter(brightness), Glide on/off(sliding between notes), Decay(versus a sustained tone), & Octave. This instrument allowed guitarists as well as keyboardists to add additional sounds with their feet & was most commonly employed to add a powerful, venue-rattling & potentially speaker-blowing sub-bass to an arrangement for the ultimate in massive, rumbling, climactic power. Notable among countless users were Genesis, Yes, & also Rush who toured for years with three on stage for both high & low parts. The model's biggest failing was its key contacts which were exposed underneath the unit, uncovered, & free to collect all manner of dust & stage debris therefore requiring frequent cleaning & adjustment of the contacts.
#2. Moog Minimoog Model D This compact synthesizer was used by more musicians, on more albums, than any other synth; a record which likely still stands. About 16,000 were produced from 1970 to 1981. It's fat, sliding, organic sound is instantly recognizable on hundreds of recordings across all genres, not only prog, but reggae, rock, pop, disco, funk, jazz, fusion, new wave, techno, etc. It was born out of the idea of scaling-down the huge Keith Emerson-style modular synths into something that was affordable, portable, & simplified by including only the most commonly-used components of the huge & costly systems(which were getting harder to sell as nearly every super-group, studio, & university had already bought one). Bob Moog actually had little to do with its conception as the first prototype, the "Model A", was actually built by employee Bill Hemsath during his lunch hours from scavenged parts & B-stock modules from the Moog "junk pile". Only the Model D saw production &, aside from minor cosmetic & left-hand controller variations, there were three different Model D versions commonly differentiated by having either the "discrete", "old", or "new" oscillator board. Each sounded quite different, but the latter was developed to address the Minimoog's primary fault: tuning instability due to temperature-related pitch drift. However, players tend to prefer the less stable versions citing "more character".
#1. Mellotron M400S It & its related models were "the sound" of early prog rock (listen to King Crimson's "In the Court of the Crimson King", the opening of Genesis' "Watcher of the Skies" from Foxtrot, Yes' Close to the Edge, etc). They were universally used, but also reviled by the bands of the era for their notorious unreliability. However, short of adding lots of extra musicians, bands had no other way of producing realistic choir, strings, brass, flutes, etc so they tolerated the high maintenance machines ...usually. Rick Wakeman of Yes, after a long, frustrating studio session with a finicky pair(& more than a few pints) says he dragged them into an open field, dowsed them with petrol & set them ablaze. John Paul Jones of Led Zeppelin said he was always terrified on stage of what was going to come out when he played them live because local voltage fluctuations could cause them to be unpredictably out of tune.
It was quite an electro-mechanical contraption: every key had its own length of 3-track magnetic recording tape with the sound of real instruments recorded on each track. One could switch between each of the three sounds or set the selector in between two to play back two adjacent sounds simultaneously. Each key's tape was not a loop, but a straight length(only about 8 seconds long) which was dragged across its own playback head until the key was released & a spring yanked the tape back to its starting position. Early models went flatter when more keys were pressed as more load was placed on the capstan motor. If you wanted a different three sounds you had to open the top & remove the large tape frame holding all the strips of tape & insert a different frame.
P.S. Feel free to leave a comment about any other keyboards you feel had an impact on the early days of prog from the late-'60s to mid-'70s!