Hammond B-3 · Volume 4
Hammond B-3 — Vol 04: The Leslie 122
This volume documents the Leslie Model 122 — the rotating-speaker cabinet that is the canonical acoustic partner to the Hammond B-3. It covers why a rotating loudspeaker produces the signature swirl (Doppler frequency modulation, amplitude modulation, and comb filtering), the cabinet’s mechanical and electrical anatomy, its ~40 W tube power amplifier and high-voltage supply, the passive 800 Hz crossover, the two independent rotors, the two-speed (tremolo/chorale) motor system with its half-moon switch, and the balanced 6-pin connection that distinguishes the 122 from its unbalanced 6-pin sibling, the 147 (both use a 6-pin connector — the difference is balanced “6H” wiring vs unbalanced “6W” wiring, not the pin count). The B-3 console, its AO-28 output amplifier, and the scanner vibrato/chorus upstream of the cabinet are Vol 02 §2.x and Vol 03 §3.x; mic’ing the rotating field is Vol 07 §7.x and cabinet service is Vol 06 §6.x — both are referenced here, not duplicated.
Note: The Leslie effect sits acoustically downstream of everything in Vols 02–03. The scanner chorus/vibrato (Vol 03 §3.4) is an electronic modulation applied to the signal before it leaves the console; the Leslie rotation is a mechanical, acoustic modulation applied to the sound in the room. The two stack independently, which is why players treat them as separate colours.
Why a rotating speaker
A stationary loudspeaker radiates a fixed sound field: the listener hears one direct path plus the room’s reflections, unchanging in time. Donald Leslie’s insight was to put the radiating element in motion so that the sound field itself sweeps continuously. The perceived effect is not a single phenomenon but the superposition of three, all driven by the same rotation:
- Doppler frequency modulation. As the mouth of a rotating horn swings toward the listener, the wavefronts are compressed and the pitch rises slightly; as it swings away, the wavefronts stretch and the pitch falls. Over one revolution the source velocity varies sinusoidally, so the radiated tone is frequency-modulated, producing a sequence of FM sidebands around every partial in the signal (Leslie speaker, en.wikipedia.org; theatreorgans.com Hammond-Leslie FAQ). This is the core of the “vibrato-like” pitch movement.
- Amplitude modulation (tremolo). A horn is directional — it beams its energy. As the mouth rotates past the listener, the on-axis lobe sweeps across them, so the level at the listening position rises and falls once per revolution. The same rotation therefore also imposes a tremolo, in phase-locked relation to the pitch sweep (en.wikipedia.org). The name “tremolo” for the fast speed comes from this amplitude component.
- Comb filtering and room interaction. The moving source continuously changes the path-length difference between the direct sound and the wall/floor reflections, so the interference notches (a comb filter) slide up and down the spectrum as the rotor turns. Layered over the FM and AM, this gives the effect its shimmering, three-dimensional “chorusing” richness rather than a clean single-rate vibrato.
The 122 doubles the richness by using two independent rotating sources crossed over at 800 Hz: a treble horn carries the highs (where Doppler and beaming are strong and audible) and a separate bass drum/rotor carries the lows. Because the horn and the drum spin at different rates and are not phase-locked to each other (see §“Horn & drum rotors”), the high-frequency swirl and the low-frequency wash drift in and out of alignment, producing a complex, evolving field that a single rotor — or any electronic emulation of one — struggles to match.
Tip: The most dramatic part of the sound is the inertia ramp at a speed change. The rotors are heavy and belt-driven, so switching tremolo→chorale (or back) does not snap between speeds — the horn and drum spin down or up over a second or more, and because they have different masses they ramp at different rates, smearing the pitch and tremolo through a sweep that is itself a signature gesture (Vol 03 §3.x covers the half-moon switch as a control; the mechanical ramp is detailed in §“Fast/slow” below).
Anatomy of the 122
The 122 is a tall floor cabinet that partitions, top to bottom, into the upper treble-horn compartment, the lower bass-rotor compartment, and the amplifier chassis at the base. Sound and signal flow through it as follows:
- Balanced input arrives on the 6-pin connector from the Hammond’s AO-28 output amplifier (§“The 6-pin balanced connection”).
- The signal is amplified by the ~40 W push-pull tube power amplifier (§“The amplifier & power supply”).
- The amplifier output feeds a passive 800 Hz crossover that splits the band (§“The 800 Hz crossover”).
- Highs (above 800 Hz) drive a compression driver that fires through a rotating two-horn treble assembly — one horn radiates, the other is a sealed dummy acting as an aerodynamic and mass counterweight.
- Lows (below 800 Hz) drive a downward-firing 15-inch woofer whose output is redirected sideways by a rotating bass drum/rotor baffle.
- Two motors per rotor (a fast and a slow motor) spin the assemblies via belts; the half-moon switch on the organ selects the speed, with the switching information carried back over the same 6-pin cable (§“Fast/slow”).
Note: “Drum” and “rotor” refer to the same lower element: the woofer itself does not rotate. It fires into a rotating drum-shaped baffle (open on one side) that scoops and slings the low-frequency energy around the cabinet, producing the bass Doppler/AM. Likewise only the horn assembly rotates up top, not the compression driver, which is stationary on the rotor axis and couples to the horn through the hollow rotating shaft.
The amplifier & power supply
The 122 carries a single-channel vacuum-tube power amplifier rated at roughly 40 W output (en.wikipedia.org; bentonelectronics.com, “Servicing the Leslie 122 Amplifier”). It is deliberately modest in headroom — a hard-driven B-3 will push it into the warm tube overdrive that is part of the classic sound. The tube complement, as documented in the 122/142 amplifier service literature, is:
| Stage | Tube | Role |
|---|---|---|
| Input / driver | one 12AU7A (dual triode) | Balanced input amplifier and phase splitter; one section also gates the speed-changeover relay (see §“Fast/slow”) |
| Power output | a matched pair of 6550A | Push-pull output stage delivering ~40 W (bentonelectronics.com) |
| Voltage regulator | one OC3 gas-discharge regulator | Holds the screen/driver supply at a regulated rail (~310 VDC) (bentonelectronics.com) |
| Rectification | solid-state bridge (four diodes) on most units; a rectifier tube on the earliest examples | Produces the main high-voltage B+ (bentonelectronics.com) |
The high-voltage supply runs the output stage at a B+ of approximately +430 VDC at the rectifier, around +415 VDC at the 6550A plates, with the OC3 holding the regulated rail near +310 VDC (bentonelectronics.com). These are lethal potentials and they persist on the filter capacitors after the cabinet is unplugged.
Warn: The 122 amplifier chassis is connected directly to the AC mains and develops B+ rails on the order of +400 V to +430 VDC (bentonelectronics.com). Reservoir/filter capacitors can hold a dangerous charge long after power-off and after the cable is pulled. The cabinet must never be opened or serviced live, and the supply must be discharged and verified before any internal work. Mains, grounding, and capacitor-discharge procedure are covered in Vol 06 §6.x — do not improvise.
Note: Output-tube substitution is a known field variation. The factory 122/142 complement is the 6550A pair (bentonelectronics.com; en.wikipedia.org notes “typically 6550 tubes”), but cabinets in the wild are sometimes found rebiased for other large-bottle pentodes. Any restored unit should be confirmed against its own chassis-stamped revision before assuming the values above; tube-by-tube bias and rail measurement is a Vol 06 §6.x task.
The 800 Hz crossover
Between the power amplifier and the two transducers sits a passive crossover with a transition frequency of 800 Hz (en.wikipedia.org; theatreorgans.com Hammond-Leslie FAQ). Everything above 800 Hz is routed to the treble compression driver feeding the rotating horn; everything below 800 Hz is routed to the 15-inch woofer feeding the rotating bass drum. The split is acoustically motivated: 800 Hz sits low enough that the directional, Doppler-rich behaviour of a horn covers the entire range where the swirl is most audible, while the omnidirectional bass is handed to the drum where its slower, broader wash is wanted.
The crossover is wired for the cabinet’s specific driver impedances; the service literature notes the 15-inch woofer is a 16 Ω unit in the 122 (bentonelectronics.com). The exact reactive component values and the crossover slope are part of the cabinet’s passive network and are not restated here as bare numbers; the 800 Hz transition is the load-bearing figure.
Note: A single fixed crossover frequency is why the horn/drum “handoff” sits where it does in the spectrum. Frequencies right at ~800 Hz are radiated by both rotating systems simultaneously, contributing to the dense, overlapping modulation in the midrange.
Horn & drum rotors
The two rotating systems are mechanically and acoustically distinct:
- Treble horn assembly. A stationary compression driver couples through a hollow shaft into a rotating two-horn casting. Only one horn is acoustically open; the second horn is a sealed dummy that exists purely to balance the assembly’s mass and aerodynamics so it spins smoothly and quietly. The horn covers the band above 800 Hz, where Doppler FM and beam-induced AM are strongest.
- Bass drum/rotor. Below 800 Hz, a downward-firing 15-inch woofer (16 Ω; bentonelectronics.com) couples into a rotating drum baffle that flings the low-frequency energy around the cabinet. The drum is larger and heavier than the horn, so it accelerates and decelerates more slowly.
Because the two rotors are driven by separate motors and belts at different nominal speeds (next section), their modulations are not synchronized — the high swirl and the low wash continuously drift relative to one another, which is a large part of why the 122 sounds “alive” rather than like a single oscillating source.
Fast/slow: motors & the half-moon switch
Each rotor is driven by two AC motors — a fast (tremolo) motor and a slow (chorale) motor — coupled to the rotor shaft through belts and idler pulleys. Selecting a speed energises the corresponding motor pair; the rotor’s own inertia then ramps it to the new speed over a second or more, producing the signature accelerating/decelerating sweep.
The speeds documented in the Leslie/Hammond literature are summarised below. Published figures vary slightly between sources and drift with belt wear, line frequency, and motor condition, so the typical values are given with the resulting modulation rate in Hz and flagged where a single authoritative figure could not be pinned:
| Rotor | Chorale (slow) | → Hz | Tremolo (fast) | → Hz |
|---|---|---|---|---|
| Treble horn | ~40–50 RPM (est.) | ~0.7–0.8 Hz | ~400 RPM | ~6.7 Hz |
| Bass drum | ~40 RPM (est.) | ~0.67 Hz | ~340 RPM | ~5.7 Hz |
Speeds from en.wikipedia.org (Leslie speaker: chorale horn ~50 / drum ~40 RPM; tremolo horn ~400 / drum ~340 RPM) and corroborating Hammond/Leslie forum and owner’s-manual references (organforum.com; hammondorganco.com owner’s manuals); the chorale figures in particular vary source-to-source and are marked (est.). The horn always runs faster than the drum, and the two are not phase-locked.
Note: The horn’s tremolo rate of ~400 RPM works out to roughly 6.7 Hz, in the same neighbourhood as the console’s scanner vibrato (~7 Hz, Vol 03 §3.4) — which is why the two effects feel related yet distinct: the scanner is a clean electronic ~7 Hz pitch wobble, while the Leslie’s ~6.7 Hz is a spatial sweep carrying AM and comb filtering as well.
The speed is chosen at the organ by the half-moon switch — a semicircular two- (or three-) position lever mounted within reach of the manuals that selects tremolo (fast), chorale (slow), and on many installations an off/brake position. The switch does not carry the audio; it controls the changeover relay inside the 122, and that control information travels back to the cabinet over the same 6-pin cable that carries the audio and AC power (next section). Mic positioning around the rotating field is teased in the SVG below and developed in Vol 07 §7.x.
The 6-pin balanced connection
The defining electrical distinction of the 122 (and its console-organ sibling the 142) is that it presents a balanced audio input on a 6-pin Hammond connector, and it carries the speed-switching information as a DC voltage superimposed on that same balanced audio pair — so no dedicated switching conductor is needed. The 147 (and 145), by contrast, present an unbalanced input on a differently-wired 6-pin connector — the “6W” (Wurlitzer) type, versus the 122’s “6H” (Hammond) type — and switch the motors with AC line voltage on a freed-up pin rather than DC on the audio pair. Both cabinets use a 6-pin Amphenol connector; the real distinction is 6H-balanced vs 6W-unbalanced wiring, not the pin count. This is the single most important compatibility fact about the cabinet.
A 122’s balanced input is the natural match for the Hammond console’s balanced AO-28 output (Vol 02 §2.x): the AO-28 drives the line at roughly 3–4 V RMS (organforum.com, “Leslie 122 input — line level or what?”), well above modern line level. The original 122’s exact input impedance and sensitivity are not stated as a single figure in the period service data; for reference, the modern reissue Leslie 122H/142H specifies a line input of 10 kΩ, sensitivity ~100 mV (−18 dBu) (Leslie Heritage 122H/142H Owner’s Manual, hammondsuzuki.com), which is a useful modern analogue rather than the vintage figure.
Note (resolves the Vol 02 flag): Vol 02 §2.x left the B-3→Leslie hand-off as “balanced line level, voltage/impedance UNCONFIRMED.” It can now be stated, with citation, that the AO-28 drives the balanced 122 input at roughly 3–4 V RMS (organforum.com). The vintage 122’s precise input impedance remains unspecified in the period literature; the modern Heritage 122H’s 10 kΩ / ~100 mV (−18 dBu) line spec (hammondsuzuki.com) is cited as a present-day reference point, not as the original value.
The pinout below follows the canonical Leslie reference (captain-foldback.com, “Uncle Harvey’s guide to Leslie pin-outs”):
| Pin | Wire colour | Function |
|---|---|---|
| 1 | Black | Balanced audio input (one leg) |
| 2 | Yellow | Signal / DC ground (reference for the superimposed switching DC) |
| 3 | Gray | AC mains in |
| 4 | Blue | AC mains in |
| 5 | Brown | B+ feed, ~300 VDC (captain-foldback.com) |
| 6 | Red | Balanced audio input (other leg) |
Warn: The 6-pin cable carries AC mains (pins 3 and 4) and, per the cited pinout, a high-voltage DC feed (~300 VDC on pin 5) alongside the audio. It is not a low-voltage signal cable. Connectors must be mated and de-mated only with the system powered down; never probe or hot-plug this connector. Mains/HV safety is Vol 06 §6.x.
Note (how the switching rides the audio): Because the audio is balanced on pins 1 and 6, the speed-changeover DC can be applied equally to both audio legs — a common-mode signal that the 122’s balanced 12AU7A input rejects (and therefore does not amplify), while a relay-control stage downstream detects it and switches the fast/slow motors. Reported switching levels are on the order of tens of volts DC (commonly cited around 60–100 VDC at the relay control) (organforum.com; groupdiy.com). This is why the 122 needs no extra switching pin, whereas the unbalanced 147 must dedicate pins to grounded fast/slow control. Confusing the two cabinets’ cabling is the classic Leslie hook-up error: a 122 and a 147 are not interchangeable without the correct console kit or an adapter (HammondWiki, Leslie Hookup Kit Part Numbers).
Leslie 122 signal & acoustic flow
Sources consulted: Leslie speaker — en.wikipedia.org; “Servicing the Leslie 122 Amplifier” — bentonelectronics.com; “Uncle Harvey’s guide to Leslie pin-outs” and Leslie amplifier schematics — captain-foldback.com; Hammond-Leslie FAQ — theatreorgans.com; The Organ Forum (organforum.com) and GroupDIY (groupdiy.com) on the 122 balanced input and DC speed switching; Leslie Heritage 122H/142H Owner’s Manual — hammondsuzuki.com; HammondWiki (dairiki.org) Leslie hookup-kit references; Hammond owner’s manuals — hammondorganco.com. Values marked “(est.)” are approximate where a single authoritative figure could not be confirmed; no electrical value has been invented. The AO-28 output level and 122-vs-147 connector facts resolve the open flag carried from Vol 02 §2.x.