{
  "format": "circuit-codex-reference",
  "version": 1,
  "id": "aa764-vibro",
  "url": "https://circuitcodex.com/amps/aa764-vibro/",
  "attribution": "The Analog Maker / Circuit Codex",
  "license": "https://creativecommons.org/licenses/by-sa/4.0/",
  "revision": "ebddc3fd2cf887dca010bbe60487d7391ea6c052",
  "scope": "DC and drawing checks have separate coverage. See included check results and the circuit page; not a dimensioned build template or a physical-build certification.",
  "metadata": {
    "id": "aa764-vibro",
    "name_style": "Blackface Vibro Champ-style",
    "family": "blackface",
    "era": {
      "start": 1964,
      "end": 1967
    },
    "wattage": 5,
    "tubes": [
      "12AX7",
      "12AX7",
      "6V6GT",
      "5Y3GT"
    ],
    "topology": {
      "rectifier": {
        "kind": "tube",
        "type": "5Y3GT"
      },
      "bias": "cathode",
      "phase_inverter": "none",
      "tone_stack": "tb"
    },
    "lineage": {
      "derived_from": [
        "aa764"
      ],
      "influenced": []
    },
    "conventions": {
      "designators": "sequential"
    },
    "sources": [
      {
        "desc": "Fender 'VIBRO-CHAMP AA764' drawing (I-FD), schematic + layout sheets: every component value AND the printed voltage chart read directly from the published sheet. The audio path duplicates the plain Champ's AA764 drawing component-for-component — two 68 kΩ input stoppers on a 1 MΩ leak; 12AX7 stages (100 kΩ plate loads, 1.5 kΩ/25 µF cathodes); treble-bass tone stack (250 pF, 100 kΩ slope, 0.1 µF and 0.047 µF, 250 kΩ treble and bass pots, 15 kΩ to ground) ahead of a 1 MΩ volume; 47 Ω/2.7 kΩ negative-feedback divider under the second cathode; 0.02 µF coupling into a 6V6GT with a 220 kΩ grid leak and a 470 Ω · 1 W cathode (25 µF bypass); rails +355/+340/+320 on three 20 µF · 450 V cans split by 1 kΩ · 1 W and 10 kΩ · 1 W; 5Y3GT rectifier off a 315-0-315 V AC winding; transformers TR1 125P1B and TR2 125A35A, the same part numbers aa764 prints. What is added is a second, dedicated 12AX7 (both sections) for the tremolo: a phase-shift oscillator on pins 1-3 (470 kΩ plate load off the B+2 node to +170 V; ladder plate -> .02 -> a Speed node that a 3 MΩ-RA Speed control with a 100 kΩ fixed leg returns to ground -> .01 -> the pedal node, carrying the Vibrato Pedal jack and a 1 MΩ to the cathode -> .01 -> the grid, 1 MΩ to ground; 4.7 kΩ/25 µF cathode to +1.6 V), direct-coupled into a cathode follower on pins 6-8 (plate tied directly to the B+2 node at +340 V, cathode +175 V) whose cathode feeds a 25 kΩ-RA Intensity control's wiper through 68 kΩ; one end of that pot is grounded and the other goes straight to the second 12AX7 stage's cathode (+1.5 V), with no capacitor in the path. Also printed: +342 V / +340 V at the 6V6 plate/screen pins and +21 V at its cathode (pin 8); the grid carries no figure. Notice: values ±20%, read to ground with an electronic voltmeter, all resistors 1/2 W 10% and all capacitors at least 400 V unless marked. Not reproduced here; archived at EL34World",
        "url": "https://el34world.com/charts/Schematics/Files/Fender/Fender_vibro_champ_aa764.pdf"
      },
      {
        "desc": "Mojotone/Ampwares Fender field guide, 'Blackface Vibro Champ' — schematic designation AA764, production 1964-1967, cathode-biased 6V6GT/5Y3GT, single 8-inch Oxford 8EV speaker, front panel Input/Volume/Treble/Bass/Speed/Intensity. Cited here for the production window and panel-control names; the field guide's own '6 W' figure is a rounded marketing number and this entry keeps the 5 W the corpus already uses for the shared AA764 output stage (see the AA764 Champ entry and the Champ family history)",
        "url": "https://mojotone.com/blogs/fender-field-guide/fender-blackface-vibro-champ"
      }
    ],
    "verification": {
      "status": "draft"
    },
    "added": "2026-08-30T00:00:00.000Z",
    "contributors": [
      "TheAnalogMaker"
    ],
    "sections": {
      "V4": "excluded — the vibrato bottle: a running RC phase-shift oscillator (V4A) and the cathode-follower buffer into Intensity (V4B); neither has a static operating point"
    }
  },
  "parts": {
    "notes": "The audio path (R1-R15, VR1-VR3, C1-C11, V1-V3, T1-T2) duplicates the plain Champ's AA764 drawing part-for-part — same values, same roles — because the Vibro Champ drawing is the plain Champ's circuit plus a tremolo whose cathode follower injects into V1B's cathode; only the printed rail voltages differ (see Sources). One channel across two input jacks (high/low), each with its own 68 kΩ stopper sharing the 1 MΩ leak. V1B's 1.5 kΩ cathode resistor does not go straight to ground: it lands on a 47 Ω leg that carries the 2.7 kΩ negative-feedback return from the speaker jack, and its 25 µF can bypasses only the 1.5 kΩ. Both transformer part numbers are printed on the drawing (TR1 125P1B, TR2 125A35A) — the same parts the Champ's sheet prints, though this sheet's own HT winding reads 315-0-315 V against the Champ's printed 320-0-320 V for the identical 125P1B — the Champ entry's parts list records the same transformer printing different figures across drawings. This list numbers them T1 and T2, the archive's convention where a drawing prints no designators of its own. — The tremolo oscillator (V4, both sections) is a dedicated bottle, not shared with an audio-path stage (contrast the 6G3, whose oscillator is the second half of its driver tube). Its parts are listed in full below, every value read off the drawing, but its DC operating point is excluded from the simulated deck (see the circuit story): it is a running phase-shift oscillator with no static quiescent point.",
    "items": [
      {
        "ref": "R1",
        "part": "Carbon comp resistor",
        "value": "68 kΩ · ½ W",
        "role": "Input grid stopper, jack 1 (high)"
      },
      {
        "ref": "R2",
        "part": "Carbon comp resistor",
        "value": "68 kΩ · ½ W",
        "role": "Input grid stopper, jack 2 (low)"
      },
      {
        "ref": "R3",
        "part": "Carbon comp resistor",
        "value": "1 MΩ · ½ W",
        "role": "Input grid leak"
      },
      {
        "ref": "R4",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "V1A plate load"
      },
      {
        "ref": "R5",
        "part": "Carbon comp resistor",
        "value": "1.5 kΩ · ½ W",
        "role": "V1A cathode bias"
      },
      {
        "ref": "R6",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "Tone stack — slope resistor"
      },
      {
        "ref": "R7",
        "part": "Carbon comp resistor",
        "value": "15 kΩ · ½ W",
        "role": "Tone stack — bass leg to ground"
      },
      {
        "ref": "R8",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "V1B plate load"
      },
      {
        "ref": "R9",
        "part": "Carbon comp resistor",
        "value": "1.5 kΩ · ½ W",
        "role": "V1B cathode bias — the tremolo's Intensity control also lands on this cathode"
      },
      {
        "ref": "R10",
        "part": "Carbon comp resistor",
        "value": "47 Ω · ½ W",
        "role": "V1B cathode return / feedback divider leg"
      },
      {
        "ref": "R11",
        "part": "Carbon comp resistor",
        "value": "2.7 kΩ · ½ W",
        "role": "Negative feedback, speaker → V1B cathode divider"
      },
      {
        "ref": "R12",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · ½ W",
        "role": "6V6 grid leak"
      },
      {
        "ref": "R13",
        "part": "Wirewound resistor",
        "value": "470 Ω · 1 W",
        "role": "6V6 cathode bias"
      },
      {
        "ref": "R14",
        "part": "Power resistor",
        "value": "1 kΩ · 1 W",
        "role": "Rail dropper B+1→B+2"
      },
      {
        "ref": "R15",
        "part": "Power resistor",
        "value": "10 kΩ · 1 W",
        "role": "Rail dropper B+2→B+3"
      },
      {
        "ref": "VR1",
        "part": "Audio-taper potentiometer",
        "value": "1 MΩ",
        "role": "Volume (catalog 1MA-2-35 / 0143)"
      },
      {
        "ref": "VR2",
        "part": "Audio-taper potentiometer",
        "value": "250 kΩ",
        "role": "Treble (catalog 250K-A-2-35 / 0266)"
      },
      {
        "ref": "VR3",
        "part": "Audio-taper potentiometer",
        "value": "250 kΩ",
        "role": "Bass (catalog 250K-A / 0012)"
      },
      {
        "ref": "C1",
        "part": "Electrolytic capacitor",
        "value": "25 µF · 25 V",
        "role": "V1A cathode bypass"
      },
      {
        "ref": "C2",
        "part": "Mica capacitor",
        "value": "250 pF",
        "role": "Tone stack — treble cap"
      },
      {
        "ref": "C3",
        "part": "Film capacitor",
        "value": "0.1 µF · 400 V",
        "role": "Tone stack — bass cap"
      },
      {
        "ref": "C4",
        "part": "Film capacitor",
        "value": "0.047 µF · 400 V",
        "role": "Tone stack — bass leg cap"
      },
      {
        "ref": "C5",
        "part": "Electrolytic capacitor",
        "value": "25 µF · 25 V",
        "role": "V1B cathode bypass (across R9 only)"
      },
      {
        "ref": "C6",
        "part": "Coupling capacitor",
        "value": "0.02 µF · 400 V",
        "role": "V1B → 6V6 grid"
      },
      {
        "ref": "C7",
        "part": "Electrolytic capacitor",
        "value": "25 µF · 25 V",
        "role": "6V6 cathode bypass"
      },
      {
        "ref": "C8",
        "part": "Electrolytic capacitor",
        "value": "20 µF · 450 V",
        "role": "Filter, B+1 (reservoir)"
      },
      {
        "ref": "C9",
        "part": "Electrolytic capacitor",
        "value": "20 µF · 450 V",
        "role": "Filter, B+2 (screen)"
      },
      {
        "ref": "C10",
        "part": "Electrolytic capacitor",
        "value": "20 µF · 450 V",
        "role": "Filter, B+3 (preamp)"
      },
      {
        "ref": "C11",
        "part": "Film capacitor",
        "value": "0.047 µF · 600 V",
        "role": "Across the AC line (period part; omitted in modern builds)"
      },
      {
        "ref": "V1",
        "part": "Preamp tube",
        "value": "12AX7",
        "role": "Both audio-path gain stages (V1A/V1B)"
      },
      {
        "ref": "V2",
        "part": "Power tube",
        "value": "6V6GT",
        "role": "Single-ended output"
      },
      {
        "ref": "V3",
        "part": "Rectifier tube",
        "value": "5Y3GT",
        "role": "Full-wave rectifier"
      },
      {
        "ref": "T1",
        "part": "Power transformer",
        "value": "Fender 125P1B · 315-0-315 V · 6.3 V · 5 V",
        "role": "HT + heaters + rectifier filament"
      },
      {
        "ref": "T2",
        "part": "Output transformer",
        "value": "Fender 125A35A",
        "role": "Single-ended output (impedance ratio not marked on the drawing)"
      },
      {
        "ref": "—",
        "part": "Fuse",
        "value": "1 A slo-blo",
        "role": "Mains fuse"
      },
      {
        "ref": "J1",
        "part": "Open-circuit phone jack",
        "value": "—",
        "role": "Vibrato Pedal jack — its tip is on the phase-shift ladder's pedal node, so the pedal's footswitch grounds that node and stops the oscillator"
      },
      {
        "ref": "R20",
        "part": "Carbon comp resistor",
        "value": "1 MΩ · ½ W",
        "role": "Phase-shift ladder: pedal node → V4A cathode"
      },
      {
        "ref": "R21",
        "part": "Carbon comp resistor",
        "value": "1 MΩ · ½ W",
        "role": "V4A grid leak"
      },
      {
        "ref": "R22",
        "part": "Carbon comp resistor",
        "value": "470 kΩ · ½ W",
        "role": "V4A plate load, off the B+2 (+340 V) node"
      },
      {
        "ref": "R23",
        "part": "Carbon comp resistor",
        "value": "4.7 kΩ · ½ W",
        "role": "V4A cathode bias"
      },
      {
        "ref": "C18",
        "part": "Electrolytic capacitor",
        "value": "25 µF · 25 V",
        "role": "V4A cathode bypass"
      },
      {
        "ref": "C16",
        "part": "Film capacitor",
        "value": "0.02 µF · 400 V",
        "role": "Phase-shift ladder: V4A plate → the Speed node"
      },
      {
        "ref": "R24",
        "part": "Rheostat / potentiometer",
        "value": "3 MΩ, reverse-audio taper",
        "role": "Speed control (catalog 3.0M-RA / 0015), a rheostat from the Speed node"
      },
      {
        "ref": "R25",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "Speed control's fixed leg to ground, mounted on the pot"
      },
      {
        "ref": "C15",
        "part": "Film capacitor",
        "value": "0.01 µF · 400 V",
        "role": "Phase-shift ladder: the Speed node → the pedal node"
      },
      {
        "ref": "C17",
        "part": "Film capacitor",
        "value": "0.01 µF · 400 V",
        "role": "Phase-shift ladder: the pedal node → V4A grid"
      },
      {
        "ref": "R26",
        "part": "Carbon comp resistor",
        "value": "68 kΩ · ½ W",
        "role": "V4B cathode resistor, into the Intensity control's wiper"
      },
      {
        "ref": "R27",
        "part": "Rheostat / potentiometer",
        "value": "25 kΩ, reverse-audio taper",
        "role": "Intensity control (catalog 25K-RA / 37947): wiper fed from V4B's cathode through R26, one end grounded, the other straight to V1B's cathode"
      },
      {
        "ref": "V4",
        "part": "Oscillator/driver tube",
        "value": "12AX7",
        "role": "Tremolo: phase-shift oscillator (V4A, pins 1-3) and cathode follower (V4B, pins 6-8) into the Intensity control — EXCLUDED from the simulated deck, no static DC operating point"
      }
    ]
  },
  "publishedVoltages": {
    "source": "Fender 'VIBRO-CHAMP AA764' drawing (I-FD), printed chart: +355 V (reservoir / OT primary feed), +340 V (6V6 screen pin 4), +342 V (6V6 plate, unlabeled pin near +340 V), +21 V (6V6 cathode, pin 8, printed on both the schematic and the layout page), +320 V (preamp rail), +205 V / +1.6 V (12AX7 first stage plate and cathode), +200 V / +1.5 V (12AX7 second stage plate and cathode — the tremolo's Intensity control lands on that cathode), +170 V / +1.6 V and +340 V / +175 V (tremolo oscillator, excluded)",
    "nodes": {
      "BP2": {
        "chart": 340,
        "tol_pct": 8,
        "note": "6V6 screen node, after the 1 kΩ · 1 W dropper (R14) — also the tremolo bottle's supply (the oscillator's 470 kΩ load and the cathode follower's plate), off-model"
      },
      "BP3": {
        "chart": 320,
        "tol_pct": 8,
        "note": "preamp plate rail, after the 10 kΩ · 1 W dropper (R15)"
      },
      "P1A": {
        "chart": 205,
        "tol_pct": 20,
        "note": "12AX7 first-stage plate, chart pin 1"
      },
      "K1A": {
        "chart": 1.6,
        "tol_pct": 20,
        "note": "12AX7 first-stage cathode over 1.5 kΩ, chart pin 3"
      },
      "P1B": {
        "chart": null,
        "note": "12AX7 second-stage plate, chart pin 6, printed +200 V. For reference, not gated: its cathode carries the tremolo's DC injection (see K1B), which moves this plate from 213 V to 323 V across the Intensity control's travel"
      },
      "K1B": {
        "chart": null,
        "note": "12AX7 second-stage cathode, chart pin 8, printed +1.5 V — 1.5 kΩ down to the 47 Ω/2.7 kΩ feedback divider. For reference, not gated: the Intensity control's far end lands here with no capacitor, so the tremolo's cathode follower (+175 V printed) drives current into this node through 68 kΩ and whatever part of the 25 kΩ element lies between the wiper and this end. Held at +175 V, the follower takes K1B from 1.65 V (wiper at the grounded end) to 3.87 V (wiper at this end); the drawing does not state the setting"
      },
      "K2": {
        "chart": 21,
        "tol_pct": 20,
        "note": "6V6GT cathode over 470 Ω · 1 W, chart pin 8 — +21 V on both the schematic and the layout page"
      }
    }
  },
  "operatingPoint": [
    {
      "node": "BP2",
      "chart": 340,
      "sim": 349.11,
      "disputed": false,
      "dev_pct": 2.6794117647058866,
      "tol_pct": 8,
      "gated": true,
      "within": true,
      "note": "6V6 screen node, after the 1 kΩ · 1 W dropper (R14) — also the tremolo bottle's supply (the oscillator's 470 kΩ load and the cathode follower's plate), off-model",
      "dispute_note": ""
    },
    {
      "node": "BP3",
      "chart": 320,
      "sim": 326.89,
      "disputed": false,
      "dev_pct": 2.1531249999999957,
      "tol_pct": 8,
      "gated": true,
      "within": true,
      "note": "preamp plate rail, after the 10 kΩ · 1 W dropper (R15)",
      "dispute_note": ""
    },
    {
      "node": "P1A",
      "chart": 205,
      "sim": 214.986,
      "disputed": false,
      "dev_pct": 4.871219512195117,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "12AX7 first-stage plate, chart pin 1",
      "dispute_note": ""
    },
    {
      "node": "K1A",
      "chart": 1.6,
      "sim": 1.679,
      "disputed": false,
      "dev_pct": 4.937499999999997,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "12AX7 first-stage cathode over 1.5 kΩ, chart pin 3",
      "dispute_note": ""
    },
    {
      "node": "P1B",
      "chart": null,
      "sim": 216.598,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "12AX7 second-stage plate, chart pin 6, printed +200 V. For reference, not gated: its cathode carries the tremolo's DC injection (see K1B), which moves this plate from 213 V to 323 V across the Intensity control's travel",
      "dispute_note": ""
    },
    {
      "node": "K1B",
      "chart": null,
      "sim": 1.705,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "12AX7 second-stage cathode, chart pin 8, printed +1.5 V — 1.5 kΩ down to the 47 Ω/2.7 kΩ feedback divider. For reference, not gated: the Intensity control's far end lands here with no capacitor, so the tremolo's cathode follower (+175 V printed) drives current into this node through 68 kΩ and whatever part of the 25 kΩ element lies between the wiper and this end. Held at +175 V, the follower takes K1B from 1.65 V (wiper at the grounded end) to 3.87 V (wiper at this end); the drawing does not state the setting",
      "dispute_note": ""
    },
    {
      "node": "K2",
      "chart": 21,
      "sim": 22.233,
      "disputed": false,
      "dev_pct": 5.871428571428574,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "6V6GT cathode over 470 Ω · 1 W, chart pin 8 — +21 V on both the schematic and the layout page",
      "dispute_note": ""
    }
  ],
  "checks": [
    {
      "id": "dc",
      "title": "DC operating point",
      "status": "Within target",
      "summary": "5 reference nodes are within tolerance. The circuit remains a draft awaiting maintainer review.",
      "details": [
        "This compares simulated DC conditions with the cited reference. It does not test sound, physical construction or every part of the circuit."
      ],
      "tone": "checked"
    },
    {
      "id": "schematic",
      "title": "Schematic against DC model",
      "status": "Checked",
      "summary": "Connectivity of modelled components is checked on every change.",
      "details": [
        "Parts outside the DC model need separate checks and source review."
      ],
      "tone": "checked"
    },
    {
      "id": "board",
      "title": "Board against DC model",
      "status": "Checked",
      "summary": "Connectivity of modelled components is checked on every change.",
      "details": [
        "This does not establish heater wiring or physical spacing."
      ],
      "tone": "checked"
    },
    {
      "id": "drawings",
      "title": "Schematic against board",
      "status": "No open findings",
      "summary": "No disagreement is listed in the published comparison.",
      "details": [
        "A finding identifies a difference or a coverage gap. The factory source must decide which drawing, if either, needs correction.",
        "2 sheet-only parts are explicitly excluded: C11, R22. Heaters and the pilot lamp are checked separately."
      ],
      "tone": "checked"
    },
    {
      "id": "heaters",
      "title": "Heater and filament wiring",
      "status": "Checked",
      "summary": "The declared heater circuits cover the drawing and pass their separate wiring check.",
      "details": [],
      "tone": "checked"
    },
    {
      "id": "polarity",
      "title": "Capacitor polarity",
      "status": "Partial",
      "summary": "2 capacitor markings cannot be confirmed by the DC model.",
      "details": [
        "C18: C18.a: reaches only ground, through R23",
        "C5: C5.a: on KFB, which has no simulated volts"
      ],
      "tone": "attention"
    }
  ],
  "corrections": [],
  "sourceDirectory": "https://github.com/TheAnalogMaker/circuit-codex/tree/ebddc3fd2cf887dca010bbe60487d7391ea6c052/amps/aa764-vibro"
}