Hammond B-3 · Volume 5
Hammond B-3 — Vol 05: Bass — Pedalboard & the Trek II Expander
This volume documents the bass end of the Hammond B-3 rig: the stock 25-note pedalboard, how its tones are generated from the tonewheel generator’s lowest wheels through the two pedal drawbars (16′ and 8′), how the pedal division is keyed (monophonic, lowest-note priority), and what the B-3 does and does not provide at the very bottom of the range. It then documents the aftermarket Trek II bass expander — specifically the Trek II String Bass unit (model SB-2500B) — that is fitted to this rig to augment the pedals. The physics of the tonewheel generator and additive synthesis are Vol 02 §2.x; the manual drawbars, presets, percussion, and expression are Vol 03 §3.x; mic’ing and levels for recording the bass are Vol 07 §7.x; the Leslie 122 that radiates the result is Vol 04. This volume references those, not duplicates them.
Note: On the B-3 the pedal tones are not a separate oscillator bank. They are tapped from the same tonewheel generator as the manuals — specifically its lowest complex wheels — and shaped by two pedal drawbars rather than the nine-drawbar sets of each manual (Vol 02 §2.x covers the generator and key-contact matrix). Everything in §“Pedal tone generation” follows from that fact.
The 25-note pedalboard
The B-3 console carries the standard Hammond 25-note pedalboard: 25 chromatic pedals running from a low C up two octaves to middle C (i.e. two octaves plus the top note — C, C up an octave, C up a second octave) (azurehillsmusic.com, “Hammond Organ Pedalboard”; Pedal keyboard, en.wikipedia.org; Hammond organ, en.wikipedia.org). This is the layout shared across the console line (B-3, C-3, A-100) and is the board fitted to the subject rig.
Two pattern questions matter for technique and for swapping boards:
- Radiating vs. parallel. The Hammond 25-note board uses radiating pedals (the pedals fan out from a point behind the player so the spacing suits the natural sweep of the feet), the same radiating geometry as the larger AGO board (azurehillsmusic.com).
- Concave vs. flat. The defining difference from a full AGO (American Guild of Organists) pedalboard is concavity. The AGO specification calls for 32 pedals, C to g′, in a concave and radiating pattern; the standard Hammond console 25-note board is radiating but essentially flat (minimal concavity) (azurehillsmusic.com; HammondWiki “Classical Organists Dislike Most Hammonds”, dairiki.org). The flat-ish, short Hammond board is exactly what gospel and jazz players favour for fast pedal work and what classical players tend to dislike (azurehillsmusic.com).
Note: The pedal dimensions and spacing of the Hammond 25-note board are, for practical purposes, the same as the AGO board’s — only the note count (25 vs 32) and the concavity (flat vs concave) differ (azurehillsmusic.com). A player trained on AGO pedals will find the reach familiar but the curvature and top range missing.
The 25-note range is the load-bearing fact for the rest of this volume: the pedals cover two octaves, the bottom note is a low C, and there is no pedal note below that low C on a stock B-3 (contrast the RT-3 in §“What the B-3 has”).
Pedal tone generation & the pedal drawbars
The pedal division is voiced by two adjustable drawbars — one labelled 16′ and one labelled 8′ — as opposed to the nine-drawbar registration set provided for each manual (HammondWiki, dairiki.org; hammondorganco.com “Pedalboards”; azurehillsmusic.com). The 16′ drawbar contributes the deep sub-octave fundamental; the 8′ drawbar adds the octave above it for definition and to help the note “speak” on small speakers where the 16′ fundamental alone is nearly inaudible.
The tones themselves come from the lowest tonewheels of the generator. The bottom octave of the generator — the complex (wide-tooth) wheels — is reserved for and routed to the pedals; the lowest pedal note corresponds to tonewheel no. 1 at approximately 32.7 Hz (Electric Druid, “Technical aspects of the Hammond Organ”, electricdruid.net; HammondWiki tonewheel/frequency data, dairiki.org). In organ-footage terms a 16′ stop places low C near 32 Hz and an 8′ stop places it near 65 Hz (organforum.com pedal-frequency discussion; electricdruid.net).
Mapping the 25-note range onto sounding fundamentals (equal temperament, low pedal C at the 16′ pitch taken as C1 = 32.70 Hz):
| Pedal note (board) | 16′ drawbar sounding f0 | 8′ drawbar sounding f0 |
|---|---|---|
| Low C (bottom) | ≈ 32.70 Hz (C1) | ≈ 65.41 Hz (C2) |
| F (5 up) | ≈ 43.65 Hz (F1) | ≈ 87.31 Hz (F2) |
| C (octave up) | ≈ 65.41 Hz (C2) | ≈ 130.81 Hz (C3) |
| G (19 up) | ≈ 98.00 Hz (G2) | ≈ 196.00 Hz (G3) |
| C (top, middle C) | ≈ 130.81 Hz (C3) | ≈ 261.63 Hz (C4) |
Frequencies are standard equal-tempered values anchored to the cited low-C tonewheel figure (≈ 32.7 Hz; electricdruid.net, dairiki.org); intermediate notes are computed, not separately sourced, and given to orient the reader rather than as device measurements.
And the pedal drawbar roles:
| Pedal drawbar | Footage | Pitch relative to played pedal | Role |
|---|---|---|---|
| Pedal 1 | 16′ | Sub-octave (one octave below 8′) | Deep fundamental; the body of the bass note (≈ 32.7 Hz at low C) |
| Pedal 2 | 8′ | Unison reference octave | Octave-up reinforcement for pitch definition and audibility on small/limited LF systems |
Tip: Because there are only two pedal drawbars, pedal “registration” on a stock B-3 is essentially a balance between deep (16′) and present (8′). Pulling only 16′ gives a soft, felt sub-bass that can disappear on a modest speaker; adding 8′ brings the line forward. Through a Leslie 122 the bass is radiated by the rotating drum (Vol 04 §“Horn & drum rotors”), so the perceived weight also depends on the cabinet, not the drawbars alone.
Note: The 16′/8′ pedal drawbars feed the same downstream audio chain as the manuals — matching transformer → AO-28 output amplifier → Leslie (Vol 02 §2.x; Vol 04). There is no separate pedal amplifier or pedal output on a stock B-3; the pedal bass and the manual signal share one balanced line to the cabinet.
Pedal keying: monophonic bass
The pedal division is monophonic with lowest-note priority: pressing two pedals at once sounds the lower of the two, which is what makes legato pedal-bass lines and quick “walking” patterns play cleanly without doubled or muddied notes (this monophonic, lowest-note behaviour is the standard Hammond pedal characteristic and is the design the Trek II String Bass deliberately mirrors — “monophonic, which is desirable for faster playing”; trekii.com SB-2500B literature, dannychesnut.com).
The natural envelope of the stock pedal bass is fast-decaying and fairly pure: a tonewheel pedal note has little inherent sustain and a relatively simple spectrum (dominated by the 16′ fundamental plus the 8′ octave), so it behaves more like a plucked/struck bass that dies away than a sustained organ pedal. Exact decay/sustain times are not published as a device spec and are not asserted here in ms; the qualitative behaviour — quick decay, little ring — is the point, and it is precisely the limitation the Trek II’s adjustable sustain is sold to address (§“The Trek II bass expander”).
Note: “Monophonic” here describes the pedal division, not the organ. The two manuals are fully polyphonic (Vol 03 §3.x). Lowest-note priority on the pedals is a feature, not a defect: it keeps a foot-played bass line unambiguous.
What the B-3 has (and doesn’t): pedal solo vs the RT-3’s 32′
A point of frequent confusion is worth stating flatly: the stock B-3 has no 32′ pedal pitch and no dedicated pedal-solo division. Its bottom pitch is the 16′ drawbar, and its lowest sounding pedal fundamental is the low-C tonewheel near 32.7 Hz (electricdruid.net; dairiki.org). The pedal voicing is just the 16′ + 8′ drawbars described above.
The organ that does add the deep solo bass is the RT-3 (and its relatives the RT-2 and D-100). The RT-3 is essentially a B-3 with two additions (Hammond organ, en.wikipedia.org; organforum.com “Can a Hammond RT3 pedalboard… be swapped out for the standard 25?”):
- a 32-note concave-radiating AGO pedalboard (C to g′), instead of the 25-note flat board; and
- a Pedal Solo Unit — a separate solo bass system with its own volume control that produces pitches from 32′ down through 2′, i.e. an octave below the B-3’s 16′ at the bottom.
| Feature | Stock B-3 | RT-3 (for contrast) |
|---|---|---|
| Pedalboard | 25 notes, low C → middle C, radiating/flat | 32 notes AGO, C → g′, concave-radiating |
| Lowest pedal pitch | 16′ (low C ≈ 32.7 Hz) | 32′ via the Pedal Solo Unit (≈ 16 Hz region) |
| Pedal voicing | 2 drawbars (16′ + 8′) | 16′/8′ drawbars plus a separate Pedal Solo Unit (32′–2′) with its own volume |
| Dedicated solo division | None | Yes — independent solo bass system |
Warn: Do not attribute a 32′ “solo pedal” sound to a stock B-3. That feature belongs to the RT-2/RT-3/D-100 concert models, not the B-3 (en.wikipedia.org; organforum.com). On the subject rig, the only way to extend or thicken the bottom beyond the 16′/8′ tonewheel pedals is the aftermarket Trek II unit documented next — which adds a string-bass voice and sustain, not a true 32′ pipe-organ sub-octave.
The Trek II bass expander
Note (product identity & a correction to the common assumption): Trek II Products makes several Hammond add-ons, and the bass expander is often loosely described as a “pedal sustain” mod. The specific, sourceable product is the Trek II String Bass, current model SB-2500B (trekii.com; HammondWiki “Trek II String Bass with Simultaneous Retention of Hammond Pedal Tones”, dairiki.org; dannychesnut.com). It is not merely a sustain enhancer bolted onto the existing tonewheel pedal tones — it is a separate synthesized string-bass voice generator with its own adjustable sustain and pluck, which can run alongside the original Hammond pedal tones. Where this volume cannot confirm a numeric spec from Trek II’s own literature, it says so and marks the value “(est.)” rather than inventing one.
What it does. The SB-2500B generates a string-bass voice keyed from the organ’s pedals, with a longer, adjustable sustain and an adjustable initial “pluck” attack — emulating an upright/electric bass rather than the pure, fast-decaying tonewheel pedal tone. Trek II’s literature states it provides 25 notes in both an 8′ and a 16′ voice, that the tones are monophonic (“desirable for faster playing with a relatively long sustain time”), and that using both 16′ and 8′ pedal tones yields “a much richer string bass with more variation in tone” than the earlier Krueger string-bass system it supersedes (trekii.com SB-2500B; dannychesnut.com; HammondWiki, dairiki.org).
Simultaneous retention of the Hammond pedals. The distinguishing feature, and the reason the HammondWiki page is titled as it is, is that the installation provides four pedal drawbars: 16′ and 8′ original Hammond, plus 16′ and 8′ Trek II string bass. The player can select Hammond only, Trek II only, or any blend of the two (HammondWiki, dairiki.org). The stock tonewheel pedal bass is therefore preserved, with the string-bass voice layered on top as desired.
Controls. From published Trek II descriptions:
| Control | Function | Source / confidence |
|---|---|---|
| Sustain (length) | Sets how long each string-bass note rings after release — the core “adds sustain” feature; adjustable from the console | Published (trekii.com; dannychesnut.com) |
| Pluck | Sets the strength/character of the initial attack transient (the simulated string “pluck”) | Published (trekii.com; dannychesnut.com) |
| 16′ / 8′ string-bass voices | Two footages of the string-bass tone, paralleling the Hammond pedal footages; selectable via the added drawbars | Published (trekii.com; HammondWiki) |
| Tuning | Quartz-referenced tuning for accuracy/stability | Published as “quartz tuning accuracy” (trekii.com) |
| Output level / separate outputs | Separate output(s) available to feed an external amplifier (so the string bass can be routed apart from the organ’s main signal) | Published in general terms; exact level/impedance not published — (est.) |
The Pluck/Sustain control is typically mounted to the underside of the organ’s front wooden rail within reach of the player and connects to the main circuit board via a Molex connector (trekii.com; dannychesnut.com).
Installation. The unit taps the pedal contacts, not the audio bus: a contact strip is mounted to the pedal support structure, and nineteen wires run from that pedal contact strip to the Trek II board, bundled into a DB25 connector so the assembly can be unplugged for removal. The main circuit board mounts anywhere convenient inside the console, and the unit’s power transformer is mounted near the organ’s run-motor end — i.e. well away from the audio signal path — to avoid inducing hum into the organ’s signal chain (trekii.com; dannychesnut.com; HammondWiki, dairiki.org). The added string-bass drawbars sit alongside the original 16′/8′ pedal drawbars.
Warn (unconfirmed specifics — not invented): Trek II’s public literature describes the SB-2500B’s functions and controls but does not publish hard numbers for several parameters relevant to a bench writeup. The following are not confirmed from Trek II sources and are deliberately left unspecified rather than estimated into the text: the sustain time range in seconds/ms, the output level (V/dBu) and impedance of the separate output, the internal supply voltages, and the exact attack/decay shaping of the pluck. Any of these that appear elsewhere as “(est.)” are approximations for orientation only. The string-bass voicing is an analog emulation of a plucked string bass, distinct from the tonewheel pedal tone — it does not add a true 32′ sub-octave (that remains an RT-3 feature, §“What the B-3 has”).
Note (model/ambiguity): The “B” suffix (SB-2500B) denotes the current revision of the String Bass unit; earlier Trek II string-bass and pedal-sustain variants exist, and Trek II’s catalogue also includes unrelated Hammond accessories (e.g. line-out kits). The product documented here is the String Bass / SB-2500B, identified as the bass expander on the subject rig. If a different Trek II bass accessory is in fact fitted, the controls and installation specifics above may differ and should be re-verified against the unit’s own label and Trek II’s page for that model.
Integrating pedals + Trek II in play
In the installed rig the two bass voices live in parallel: the stock tonewheel pedal bass (16′ + 8′, fast-decaying, pure) and the Trek II string bass (16′ + 8′ string voice, adjustable pluck and sustain), both keyed monophonically from the same 25 pedals, both ultimately summed into the organ’s balanced line to the Leslie 122 (Vol 04).
Pedal-bass signal path with the Trek II inserted
Worked example — a sustained gospel bass note. A player wants a low F under a chord, held long, with an upright-bass feel:
- The foot presses the F pedal (5 semitones up from low C). Monophonic keying sounds that one note; the tonewheel path produces F at the 16′ pitch ≈ 43.7 Hz plus the 8′ octave ≈ 87.3 Hz (§“Pedal tone generation”).
- The stock tonewheel pedal bass speaks immediately and, on its own, would decay quickly (little natural sustain).
- The Trek II String Bass, keyed from the same pedal contact, adds its 16′/8′ string voice with the Sustain control set long, so the note rings on after the foot lifts, and the Pluck control set to taste gives the attack an upright-bass “thump.” The player has dialled the four pedal drawbars to blend Hammond 16′ + Trek II 16′/8′ (HammondWiki, dairiki.org).
- Both voices sum into the organ’s balanced line and are radiated by the Leslie 122’s rotating drum (Vol 04 §“Horn & drum rotors”), which adds the low-frequency wash. Recording this — mic choice and the very-low-frequency content near 30–45 Hz — is a Vol 07 §7.x topic.
Tip: Because the Trek II runs in parallel with the stock pedals, the most useful starting point is to set the Hammond 16′/8′ for the fundamental weight and the Trek II string voice for the sustain and articulation, then balance the two with their respective drawbars. Pulling the Trek II to zero returns the rig to a pure stock B-3 pedal bass at any time.
Sources consulted: Hammond organ and Pedal keyboard — en.wikipedia.org (25-note range, RT-3 32-note AGO board and 32′ Pedal Solo Unit); “Hammond Organ Pedalboard — Pros and Cons of 25 and 32 Note Versions” — azurehillsmusic.com (25-note C–C range, radiating vs concave, dimensions); HammondWiki — dairiki.org (“Classical Organists Dislike Most Hammonds”; “Trek II String Bass with Simultaneous Retention of Hammond Pedal Tones”; tonewheel/frequency data); “Technical aspects of the Hammond Organ” — electricdruid.net and The Organ Forum — organforum.com (lowest pedal tonewheel ≈ 32.7 Hz; pedal frequency limits; RT3 pedalboard swap thread); Hammond Organ Co. — hammondorganco.com (pedalboards, two pedal drawbars 16′/8′); Trek II Products — trekii.com (SB-2500B String Bass: adjustable sustain/pluck, quartz tuning, 25 notes in 8′ and 16′, separate outputs); dannychesnut.com (Trek II String Bass installation — pedal contact strip, 19 wires/DB25, Molex console control, transformer placement). Frequencies in the pedal table are equal-tempered values anchored to the cited ≈ 32.7 Hz low-C tonewheel; intermediate notes are computed for orientation. Trek II numeric specs not in its published literature (sustain time, output level/impedance, supply voltages) are explicitly left unspecified rather than invented; the B-3-vs-RT-3 distinction (no 32′ / no pedal-solo division on the stock B-3) is stated per en.wikipedia.org and organforum.com.