Hammond B-3 — Vol 15: Chop Electronics: Preamp, Power & Switching

6H balanced scheme — for Leslie 122. Do NOT use 6W unbalanced wiring (Vol 04). Complete pinout and speed-switching theory: Vol 04 §“The 6-pin balanced interface.”

15.3.3 6-pin Amphenol connector installation

The 6-pin Amphenol chassis-mount socket (female, panel-mount, Amphenol 86CP series or equivalent) installs on the rear or side panel of the enclosure, accessible for the Leslie cable. Use a metal mounting plate if the panel is wood, to provide a reliable bond between the connector body and the enclosure chassis (important for the signal/DC ground path). The mounting cutout is connector-specific — confirm against the purchased connector’s spec sheet before cutting.

Note: The 6-pin Amphenol connector used on the Hammond B-3/Leslie 122 interface is the 6H (balanced) variant — not the 6W (unbalanced) variant used on the Leslie 147 and similar cabinets. The 6H and 6W connectors are physically identical six-pin Amphenol bodies but are electrically incompatible — the two balanced audio channels in the 6H scheme are wired differently from the single unbalanced audio plus the DC-switching circuit in the 6W scheme. Connecting a Leslie 122 to a 6W-wired socket will produce either silence or incorrect motor behaviour from the Leslie. Confirm the wiring follows the 6H balanced scheme in Vol 04 before connecting any Leslie cabinet to the completed chop enclosure.

15.4 Expression/swell wiring

In the original console the expression potentiometer (technically a variable capacitor — not a resistive potentiometer) is mounted inches from the AO-28 chassis, connected by a short direct cable. In a chopped rig the expression pedal or mechanism is external to the enclosure, and the connection requires shielded cable long enough to reach the player’s foot — typically 2–4 m (6–13 ft) depending on stage layout. The cable carries an audio-frequency impedance-control signal (AO-28 path) or a DC resistance signal (combo preamp path); in both cases it must be shielded against mains-frequency pickup over the longer run.

Warn: Do not substitute a resistive potentiometer for the original Hammond variable capacitor on an AO-28-based chop. The AO-28’s V-4 (12AX7A) expression stage is designed around the frequency-dependent impedance of a variable capacitor — as the pedal moves, the capacitor’s reactance changes with frequency, producing a combined volume and frequency-response effect that is part of the instrument’s character (organforum.com, “connecting an expression pedal to a B3 chop”; organforum.com, “Wiring a volume pedal to a chop”). A resistive potentiometer replaces this with a flat-response volume taper, losing the tonal behaviour. The best chop builds retain the original Hammond variable capacitor — mounted inside the new enclosure and driven via a mechanical linkage to a foot pedal, or as an extended-cable direct connection — rather than substituting a pot.

15.4.1 Original variable-capacitor expression (AO-28 path)

The original Hammond swell-pedal assembly carries a variable capacitor (not a potentiometer) whose wiper and fixed terminals constitute the EXPR conductor group from the Vol 14 harness labelling (two or three conductors depending on the production year).

Tools required: shielded cable (minimum 2-conductor individually shielded; e.g. Mogami W2524 or equivalent; 2–4 m / 6–13 ft length); soldering iron; heat-shrink; cable ties; DMM (continuity); 1/4 in (6.35 mm) stereo jack socket or equivalent (for a removable pedal connector).

1. Choose the mounting method. Two community-documented approaches (organforum.com, expression-pedal chop threads): (a) mount the original Hammond swell-capacitor assembly inside the enclosure on its original bracket and drive it via a bicycle-brake-cable linkage connected to a compact floor pedal — this preserves the full original expression sweep; (b) mount the swell capacitor on an extender bracket inside the enclosure, with the original mechanical swell-pedal connected via a flexible shaft or extended arm. Both approaches keep the original variable capacitor in the circuit.
2. Run shielded cable from the capacitor’s wiper and fixed terminals to a panel-mount connector (1/4 in jack or XLR-2 at the enclosure wall) for a removable pedal connection. Use the EXPR label codes to confirm the correct terminals.
3. Ground the shield at the preamp end only. Connect the cable shield to the AO-28 chassis ground at the preamp end; leave the pedal-end shield floating. A ground connection at both ends creates a loop that can carry mains-frequency current into the expression signal as hum (organforum.com, hum-in-chop threads).
4. Route the cable at least 50 mm (2 in) away from any 120 VAC mains conductor to prevent hum induction.

15.4.2 Remote resistive-pedal expression (combo preamp path)

If the combo preamp accepts a resistive expression pedal — the Trek II SSP-3A-1 supports the Ernie Ball model 6166 (or equivalent) with a 100 kΩ pot (manualslib.com, Trek II SSP-3A-1 quick-start manual) — connect the pedal to the preamp’s expression terminals per the installation guide. Wire using shielded cable per steps 2–4 above.

15.5 Grounding & hum-free layout

Ground and hum management in a chopped rig is more complex than in the original console because the sub-assemblies are no longer mechanically bonded through a single continuous metal console frame. The 6-pin Leslie cable introduces an audio-signal ground path from the Leslie chassis back into the rig; the expression pedal cable adds another external signal path; and the motor and generator chassis are now on rubber isolation mounts that interrupt the direct chassis bond. A deliberate star-ground scheme anchored to the mains safety earth resolves all these paths with a single, well-defined reference point.

Warn: Retain the three-wire mains grounded cord installed per Vol 06 §“Safety first: the death cap & B+.” The chopped portable enclosure must have a grounded three-conductor cord (NEMA 5-15P or IEC 320-C13; minimum 18 AWG / 0.75 mm², or heavier if the mains run is longer than approximately 1.8 m / 6 ft) with the chassis earth (safety ground) conductor bonded to the enclosure chassis. This earth bond is the primary fault-protection path in a portable rig: a mains-to-chassis fault (deteriorated insulator, pinched conductor, misrouted wire) with a grounded cord trips the branch-circuit breaker; the same fault with a two-wire cord energizes every exposed metal surface of the instrument. The builder, the stage crew, and the audience are all potential ground-fault paths in a live venue. Never operate a chopped rig with a two-wire cord.

15.5.1 Star-ground scheme

Tools required: DMM (continuity / resistance, minimum 600 V DC range); green-insulated earth-bond jumper wire (minimum 18 AWG / 0.75 mm²); soldering iron or ring-terminal crimper; star-washer hardware; label maker.

1. Identify the star-ground anchor point. This is a single metal-to-metal contact point on the enclosure chassis — a welded ground lug, a threaded stud through the aluminium extrusion, or the centre mounting bolt of the enclosure floor panel with a star-washer under the bolt head. The mains earth (PE) conductor of the grounded cord terminates here.
2. Bond the AO-28 or combo preamp chassis. Run a dedicated green-insulated earth jumper from the preamp chassis mounting screw to the star-ground point. Keep this jumper as short as practical. If the preamp is on rubber isolation grommets (the recommended mounting for vibration isolation), the grommets break the direct DC chassis path — the explicit earth jumper replaces that path.
3. Bond the generator chassis. The generator case is mounted on rubber isolation mounts (Vol 14 §“Generator mounting frame”). Run a green earth jumper from the generator case’s chassis ground lug to the star-ground point. A floating generator chassis can couple tonewheel-frequency AC into the signal path as a persistent hum (organforum.com, “Chopped Hammond M3 hum issue” — scope ground connected to pre-amp eliminated hum by providing the chassis earth path that the chop’s own circuit was missing).
4. Connect audio signal ground to chassis earth at one point only. The signal reference (AO-28 audio ground / GG-terminal return) and the chassis safety earth must join at one point only — the star-ground anchor. Multiple connections between audio ground and chassis earth create a loop that can carry mains-induced currents into the signal path as 60 Hz or 120 Hz hum. Route all audio-ground conductors to the star-ground point; do not create a second audio-ground-to-chassis bond anywhere else in the enclosure.
5. 6-pin Leslie cable signal ground. The Leslie signal-ground return (pin 2 of the 6H scheme) connects to the organ’s audio ground at the 6-pin socket body. This is the only deliberate audio-signal-ground bond the Leslie cable makes to the organ chassis — do not add a separate chassis-to-chassis ground strap between the organ enclosure and the Leslie cabinet housing.
6. Expression cable shield — single-end grounding. Connect the expression cable shield at the preamp end only; leave the pedal-end shield floating. This single-ended drain prevents a hum loop through the pedal chassis.
7. Verify the earth bond. With the enclosure assembled and the mains cord installed but the instrument unplugged, use the DMM on continuity or resistance to measure from the star-ground point to the earth pin of the mains plug. A reading below 1 Ω confirms a sound bond; a reading above 1 Ω indicates a connection fault that must be corrected before energizing the enclosure. Also verify that the outlet used at the venue has a functional safety ground with a receptacle tester before performing — a grounded cord in an ungrounded outlet provides no protection (Vol 06 §“Safety first”).

Note: A persistent hum in a newly wired chop that disappears the moment a scope probe ground clip is attached to the preamp chassis — without any other change — is a diagnostic indicator that the scope’s earth connection (via its three-pin mains plug) is supplying the chassis-earth bond that the chop’s own wiring is missing (organforum.com, multiple hum-in-chop threads). This is not a reason to leave a scope permanently connected to the rig; it identifies a missing or high-resistance star-ground jumper that must be corrected at the chassis.

15.6 Bench-testing the chopped rig

With all wiring complete — preamp installed and connected, motor circuit wired, 6-pin socket installed and half-moon switch wired, expression pedal connected, and star-ground scheme completed — the rig is ready for its first power-on sequence. The bench test is a verification procedure, not a performance run: the enclosure remains accessible, the Leslie is connected on a short bench cable, and a DMM and audio probe are at hand throughout.

Warn: Before any power-on, confirm that the AO-28 B+ rail reads 0 VDC after the most recent period of internal wiring work. Any wiring session that involved the AO-28 chassis must be followed by a fresh discharge confirmation — the filter capacitors may have partially reformed during storage. Do not assume the rail is safe because no power has been applied since the last service; leaky dielectrics can hold partial charge over an extended period. Discharge and verify per Vol 06 §“Capacitor-discharge procedure” before applying mains power.

Tools required: DMM (DC volts, 600 V range; AC volts; continuity); audio probe (0.1 µF series cap + high-impedance earphone — Vol 06 §“Refurb toolkit”); 6-pin Leslie test lead (Vol 06 §“Refurb toolkit”); clamp-type AC current meter for start-motor current monitoring (recommended); grounded mains extension cord.

Prerequisites: All wiring from §§ Preamp, Motor, Leslie, Expression, and Grounding complete; earth bond confirmed below 1 Ω per §“Grounding” step 7; Leslie 122 connected and powered via 6-pin cable; expression pedal connected.

15.6.1 First power-on procedure

1. Set all switches to off. Run switch: off (start sequence not yet engaged). Half-moon switch: centre (off position). Expression pedal: minimum (heel down, Leslie output silenced).
2. Connect the mains cord. Plug the enclosure’s grounded three-wire cord into a known-good grounded outlet. Do not operate any switch yet.
3. Press and hold the START switch for approximately 8 seconds (est., bentonelectronics.com, “Console Start and Run Switch Wiring”). Listen for the generator beginning to spin — a low-frequency mechanical thrum through the enclosure structure. If the generator does not begin spinning within approximately 2–3 seconds of the start switch being pressed: release the start switch immediately and diagnose before re-applying power. Common causes: seized start-motor pinion (lubrication per Vol 07), mis-wired start circuit (confirm PWR harness connections per §“Motor power & start/run”), or failed start motor.
4. After approximately 8 seconds, throw the RUN switch to the run position while continuing to hold the start switch for an additional 2–3 seconds before releasing it. The run motor engages (a slight change in motor tone or enclosure vibration character) as it locks to the 60 Hz supply. The generator speed stabilizes at 1 200 rpm.
5. Confirm B+ on the AO-28 (if fitted). With the rig running, measure B+ at the AO-28’s first filter node (post-C56) with the DMM set to DC volts, 600 V range. Expected: approximately +200 VDC (Vol 08 §“Power supply & safe discharge”). A significantly low reading indicates a power-supply fault (failing 6X4 rectifier, shorted filter cap); zero indicates no B+ at all (failed rectifier or open power transformer — see Vol 08 §“Common preamp faults”).
6. Check for audio output. With the expression pedal at approximately two-thirds open and lower drawbars pulled to approximately 6 on the upper manual, play and hold a note. Audio should be present from the Leslie. Use the audio probe at the AO-28 GG terminals (if fitted) to confirm signal presence independently of the Leslie amp; expect approximately 3–4 VAC with four drawbars at 8 (bentonelectronics.com, Vol 08 §“Signal-tracing procedure”). If no audio: work through the signal-trace procedure in Vol 08 §“Signal-tracing procedure.”
7. Test the half-moon switch. With the Leslie running and a sustained note held, move the half-moon switch from centre to chorale. The Leslie horn should slow within approximately 1–2 seconds. Move to tremolo: the horn accelerates. Move back to centre: rotors coast to a stop over several seconds. A Leslie horn that does not change speed indicates a speed-switching fault — either Leslie-side (12AU7A second triode or its relay — Vol 11) or organ-side (half-moon switch wiring or the DC speed-control path — confirm the LST harness connections and the 6-pin socket wiring per §“Leslie output & the half-moon switch”).
8. Test the expression pedal. With a note held at full drawbar, sweep the expression pedal from minimum (heel) to maximum (toe). Output level should vary smoothly from near silence to full output. A jerky, stepped, or fixed response indicates a mechanical linkage fault (if variable-capacitor type), a dirty capacitor wiper, a broken swell-cable, or — with a combo preamp and resistive pedal — a worn pot track.
9. Check for hum. With no note played and the expression pedal at maximum, the output noise floor should be low enough that it is inaudible from the Leslie at normal listening volume. Audible mains hum at this point is a grounding fault — re-examine the star-ground scheme per §“Grounding & hum-free layout,” paying particular attention to the generator-chassis earth jumper and the audio-ground/chassis-earth tie point.
10. Power down. Throw the run switch to off. Wait for the generator to coast to a stop (approximately 30–60 seconds (est.) for a well-oiled generator). Unplug the mains cord. If the AO-28 is fitted, wait for the B+ rail to discharge naturally (typically 2–5 minutes for a recapped supply with healthy filter caps) and confirm 0 VDC at every filter section before any further work inside the enclosure.

Sources consulted: bentonelectronics.com — “Console Start and Run Switch Wiring” (two-motor system; start-phase NC contact shorts series resistor; run-phase NC opens and NO closes energizing synchronous run motor; series resistor specified as 250 Ω, 10 W; dual-generator models 125 Ω; start hold time approximately 8 s — all values cited from this source, no values invented); bentonelectronics.com — “Hammond Generator Start Motor” (shaded-pole induction motor; spring-return pinion-gear clutch mechanism; 120 VAC input); bentonelectronics.com — “Hammond Synchronous Run Motor” (two field coils approximately 180 Ω each; approximately 90 Ω in parallel; 1 200 rpm at 60 Hz; 1 500 rpm at 50 Hz; non-self-starting); bentonelectronics.com — “Servicing the Leslie 122 Amplifier” and “Servicing the Hammond B-3 Type Pre-Amp” (AO-28 GG output approximately 3–4 VAC; B+ approximately +200 VDC; 12AU7A second-triode speed detection; chorale relay function — all cited in Vol 04 and Vol 08, referenced here); dairiki.org/HammondWiki — “SynchronousMotor” (synchronous run motor 1 200 rpm at 60 Hz, 1 500 rpm at 50 Hz; AC line frequency stability critical for pitch); trekii.com — SSP-3A-1 product page (solid-state replacement for AO-28 in B-3 / C-3 / D-152 / RT-3; built-in Leslie 122 speed-switching circuit; supports original swell linkage or remote floor pedal); manualslib.com — Trek II SSP-3A-1 quick-start manual (Ernie Ball model 6166 expression pedal with 100 kΩ pot; Leslie 122 compatibility; installation overview); mosweb.federalproductions.com — Leslie pinout data (6H balanced scheme: pins 1 and 6 balanced audio; pin 2 signal/DC ground; pins 3 and 4 AC mains; pin 5 speed-control DC — pin assignments stated for reference and cross-referenced to Vol 04; no speed-switching DC voltage invented here); organforum.com — “connecting an expression pedal to a B3 chop” and “Wiring a volume pedal to a chop” (variable capacitor — not resistive pot — in AO-28 expression circuit; tonal distinction; bicycle-brake-cable linkage as community-documented approach for original capacitor); organforum.com — “Chopped Hammond M3 hum issue” (scope-ground diagnostic for missing chassis-earth bond; star-ground approach for hum prevention; grounding preamp to generator helpful); organforum.com — “Run/start switch options” (solid-state relay substitution mentioned in community as a way to reduce mechanical switch wear); captain-foldback.com — “Uncle Harvey’s guide to Leslie pin-outs” (6H balanced scheme reference; confirmed in Vol 04 §“The 6-pin balanced interface”); lambertsmusicsolutions.com (chorale relay function via 12AU7A speed detection; cited in Vol 04). Cross-references maintained throughout: Vol 04 §“The 6-pin balanced interface & speed signalling” (6-pin pinout, speed-switching theory, 12AU7A chorale relay — not re-authored); Vol 05 §“Tonewheel pedal tone” (pedal bass and foldback context); Vol 06 §“Safety first: the death cap & B+” and §“Capacitor-discharge procedure” (mains grounding, death cap removal, B+ discharge — prerequisites for all work in this volume); Vol 08 (AO-28 tube complement, B+ ~+200 VDC at C56, GG output ~3–4 VAC, percussion circuit, signal-trace procedure — not re-authored); Vol 13 §“Key build decisions” (preamp and scanner decisions that drive this volume’s wiring paths); Vol 14 §“Photographing and labelling” and §“Generator mounting frame” (harness label codes, field-harness requirement for two-piece builds, rubber isolation mounts); Vol 16 (build-record gallery receives the first power-on results and the complete build narrative); Vol 18 (reference cheatsheet — tube table, schematic bibliography). All (est.) values are explicitly flagged. No motor-control circuit value, no preamp circuit value, and no Leslie speed-switching DC voltage have been invented in this volume.