{
  "format": "circuit-codex-reference",
  "version": 1,
  "id": "ab763-twin",
  "url": "https://circuitcodex.com/amps/ab763-twin/",
  "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": "ab763-twin",
    "name_style": "Blackface Twin Reverb-style",
    "family": "blackface",
    "era": {
      "start": 1963,
      "end": 1967
    },
    "wattage": 85,
    "tubes": [
      "12AX7",
      "12AX7",
      "12AT7",
      "12AX7",
      "12AX7",
      "12AT7",
      "6L6GC",
      "6L6GC",
      "6L6GC",
      "6L6GC"
    ],
    "topology": {
      "rectifier": {
        "kind": "solid-state",
        "type": "silicon full-wave"
      },
      "bias": "fixed",
      "phase_inverter": "long-tailed-pair",
      "tone_stack": "fmv"
    },
    "lineage": {
      "derived_from": [],
      "influenced": []
    },
    "conventions": {
      "designators": "functional"
    },
    "sources": [
      {
        "desc": "Fender 'TWIN REVERB-AMP AB763' drawing (C-FD), schematic + layout sheets: every component value AND the printed voltage chart read directly from the published sheets (2026-08-08) — 68k input stoppers on 1M leaks; four preamp stages with 100k plate loads off the +410 V node [D] (channel inputs on 1.5k/25uF cathodes, both channels' SECOND stages sharing ONE 820 Ohm/25 uF cathode network through the drawing's boxed node [A]); three-knob tone stacks in both channels (250 pF treble cap and 100k slope off the plate node, 0.1 to the treble-lug/bass node, 0.047 to the middle pot, 250k-A treble and bass, 10k-A middle, treble-wiper output) into 1M-A volumes with switched 120 pF bright caps; reverb send 500 pF into a 12AT7 with both sections paralleled (1M leak, 2.2k/25uF cathode) driving the 125A20B transformer off node [B]; reverb recovery (220k tank leak, 100k plate) and the reverb/dry mix driver (3.3M with 10 pF across it, 100k plate) sharing a second 820 Ohm/25 uF cathode network through boxed node [E]; both channels coupled into 220k mixing resistors that meet and feed the phase inverter through 0.001; 12AT7 long-tailed pair (82k/100k 5% plates off +450 node [C], 470 Ohm cathode resistor to a 22k tail, 1M leaks to the tail junction) with the 820 Ohm speaker feedback landing on a 100 Ohm divider leg; four 6L6GC fixed-biased at -52 V through two 220k 5% leaks, 1500 Ohm grid stoppers and 470 Ohm-1W screen stoppers; supply +460 reservoir (70+70 uF in series with 220k-1W balancing) -> 125C1A choke -> +458 [B] -> 1k-1W -> +450 [C] -> 4.7k-1W -> +410 [D] on 20 uF/525 V cans, solid-state rectification (three series diodes per leg off 340-0-340 V) and a bias supply of 470 Ohm-1W + diode + 25 uF/50 V + a 10k-L balance control over a 27k leg; transformers TR1 125P34A, TR2 125C1A, TR3 125A29A, TR4 125A20B. Notice: values +/-20%, read to ground with an electronic voltmeter; all resistors 1/2 W 10% and all capacitors at least 400 V unless specified. Not reproduced here; archived at EL34World",
        "url": "https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_ab763.pdf"
      },
      {
        "desc": "Fender 'TWIN REVERB-AMP AB763' LAYOUT drawing (C-FD, Fender Musical Instruments, Fullerton) — the board layout on this page is redrawn from this sheet: the socket row left to right (four 6L6GC, 12AT7 phase inverter, 12AX7 tremolo, 7025 reverb recovery + mix, 12AT7 reverb driver, 7025 vibrato, 7025 normal), the eyelet board's part order above it, the control row (Intensity, Speed, Reverb, then the vibrato channel's Bass/Middle/Treble/Volume with its Bright switch, the two Vibrato jacks, the normal channel's Bass/Middle/Treble/Volume with its Bright switch and the two Normal jacks), the pilot lamp and the 6.3 V heater run marked 'TO ALL 6.3 VOLT HEATERS', the transformer lead colours (green heater pair, red 340 VAC pair, red-yellow centre tap, red-blue 48 VAC bias tap, black primary; blue and brown output-transformer plate leads, green speaker lead), the chassis-mounted bias board (470 Ohm-1W, rectifier, 25 uF/50 V, 10k-L balance to -52 V) and the four 6L6GC sockets' own 1500 Ohm grid stoppers and 470 Ohm-1W screen resistors. Notice on the sheet: 'all resistors 1/2 watt 10% tolerance if not specified', 'all capacitors at least 400 volt rating if not specified', voltages +/- 20%. Not reproduced here",
        "url": "https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_ab763_layout.pdf"
      },
      {
        "desc": "Mojotone/Ampwares Fender field guide, 'Blackface Twin Reverb' — 1963-1967, 85 W, four 6L6GC, 7025 preamp, fixed bias with an adjustment pot, solid-state rectifier, 2 x 12 in at 4 Ohms; Normal and Vibrato channels each with Bright switch, Volume, Treble, Middle and Bass, plus Reverb, Speed and Intensity",
        "url": "https://mojotone.com/blogs/fender-field-guide/fender-blackface-twin-reverb"
      },
      {
        "desc": "RCA 6L6-GC data sheet (DATA 1, 8-60) — tabulated Class A1 characteristics read for the clean-room CC0 6L6GC tube model added with this amp (Va=250 V, Vg2=250 V, Vg1=-14 V -> Ia=72 mA, Ig2=5 mA, gm=6000 umho), its interelectrode capacitances, its 7AC basing, and its design-maximum ratings (30 W plate dissipation, 500 V plate, 450 V screen). The amplification factor is RC-19's grid-No.1-to-grid-No.2 figure of 8; this sheet prints none",
        "url": "https://frank.pocnet.net/sheets/049/6/6L6GC.pdf"
      }
    ],
    "verification": {
      "status": "verified",
      "date": "2026-08-08T00:00:00.000Z",
      "max_deviation_pct": 14.9
    },
    "added": "2026-08-08T00:00:00.000Z",
    "contributors": [
      "TheAnalogMaker"
    ],
    "sections": {
      "V5": "excluded — the tremolo bottle: a running phase-shift oscillator and the neon-lamp driver inside the optocoupler; neither has a static operating point, and both tap the driven node BB"
    }
  },
  "parts": {
    "notes": "The schematic draws both instrument jacks (1 and 2) of each channel with their normalling contacts; this sheet draws them as the two input labels, with the 1 MΩ leak at jack 1's tip, jack side of its 68 kΩ stopper, where C-FD puts it. Two cathode networks are SHARED between stages and appear once here, under the boxed node letter the drawing gives them: RKA/CKA ([A], the two channels' second stages) and RKE/CKE ([E], reverb recovery and mix driver). The tremolo tube's parts are listed, but neither of its halves has a static DC point, so both are excluded from the netlist (see the circuit story). The main rectifier is silicon: two three-diode strings off the 340-0-340 V winding.",
    "items": [
      {
        "ref": "R1n",
        "part": "Carbon comp resistor",
        "value": "68 kΩ · ½ W",
        "role": "Normal input grid stopper (jack 1)"
      },
      {
        "ref": "R2n",
        "part": "Carbon comp resistor",
        "value": "68 kΩ · ½ W",
        "role": "Normal input grid stopper (jack 2)"
      },
      {
        "ref": "RGN1",
        "part": "Carbon comp resistor",
        "value": "1 MΩ · ½ W",
        "role": "Normal input leak, jack 1's tip to ground (jack side of R1n); the grid's DC return through R1n"
      },
      {
        "ref": "RLN1",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "Normal input plate load"
      },
      {
        "ref": "RKN1",
        "part": "Carbon comp resistor",
        "value": "1.5 kΩ · ½ W",
        "role": "Normal input cathode bias"
      },
      {
        "ref": "CKN1",
        "part": "Electrolytic capacitor",
        "value": "25 µF · 25 V",
        "role": "Normal input cathode bypass"
      },
      {
        "ref": "CTN",
        "part": "Mica capacitor",
        "value": "250 pF",
        "role": "Normal treble cap"
      },
      {
        "ref": "VRTN",
        "part": "Potentiometer",
        "value": "250 kΩ-A",
        "role": "Normal treble"
      },
      {
        "ref": "RSN",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "Normal tone-stack slope resistor"
      },
      {
        "ref": "CBN",
        "part": "Coupling capacitor",
        "value": "0.1 µF",
        "role": "Normal tone-stack bass cap, slope node to the bass pot"
      },
      {
        "ref": "VRBN",
        "part": "Potentiometer",
        "value": "250 kΩ-A",
        "role": "Normal bass"
      },
      {
        "ref": "CBN2",
        "part": "Coupling capacitor",
        "value": "0.047 µF",
        "role": "Normal tone-stack middle-leg cap, slope node to the middle pot"
      },
      {
        "ref": "VRMN",
        "part": "Potentiometer",
        "value": "10 kΩ-A",
        "role": "Normal middle"
      },
      {
        "ref": "VRVN",
        "part": "Audio-taper potentiometer",
        "value": "1 MΩ-A",
        "role": "Normal volume"
      },
      {
        "ref": "CBRN",
        "part": "Mica capacitor",
        "value": "120 pF",
        "role": "Normal bright cap, across the volume pot (switched)"
      },
      {
        "ref": "RLN2",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "Normal 2nd-stage plate load"
      },
      {
        "ref": "CCN2",
        "part": "Coupling capacitor",
        "value": "0.047 µF",
        "role": "Normal 2nd-stage output coupling"
      },
      {
        "ref": "R1v",
        "part": "Carbon comp resistor",
        "value": "68 kΩ · ½ W",
        "role": "Vibrato input grid stopper (jack 1)"
      },
      {
        "ref": "R2v",
        "part": "Carbon comp resistor",
        "value": "68 kΩ · ½ W",
        "role": "Vibrato input grid stopper (jack 2)"
      },
      {
        "ref": "RGV1",
        "part": "Carbon comp resistor",
        "value": "1 MΩ · ½ W",
        "role": "Vibrato input leak, jack 1's tip to ground (jack side of R1v); the grid's DC return through R1v"
      },
      {
        "ref": "RLV1",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "Vibrato input plate load"
      },
      {
        "ref": "RKV1",
        "part": "Carbon comp resistor",
        "value": "1.5 kΩ · ½ W",
        "role": "Vibrato input cathode bias"
      },
      {
        "ref": "CKV1",
        "part": "Electrolytic capacitor",
        "value": "25 µF · 25 V",
        "role": "Vibrato input cathode bypass"
      },
      {
        "ref": "CTV",
        "part": "Mica capacitor",
        "value": "250 pF",
        "role": "Vibrato treble cap"
      },
      {
        "ref": "VRTV",
        "part": "Potentiometer",
        "value": "250 kΩ-A",
        "role": "Vibrato treble"
      },
      {
        "ref": "RSV",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "Vibrato tone-stack slope resistor"
      },
      {
        "ref": "CBV",
        "part": "Coupling capacitor",
        "value": "0.1 µF",
        "role": "Vibrato tone-stack bass cap, slope node to the bass pot"
      },
      {
        "ref": "VRBV",
        "part": "Potentiometer",
        "value": "250 kΩ-A",
        "role": "Vibrato bass"
      },
      {
        "ref": "CBV2",
        "part": "Coupling capacitor",
        "value": "0.047 µF",
        "role": "Vibrato tone-stack middle-leg cap, slope node to the middle pot"
      },
      {
        "ref": "VRMV",
        "part": "Potentiometer",
        "value": "10 kΩ-A",
        "role": "Vibrato middle"
      },
      {
        "ref": "VRVV",
        "part": "Audio-taper potentiometer",
        "value": "1 MΩ-A",
        "role": "Vibrato volume"
      },
      {
        "ref": "CBRV",
        "part": "Mica capacitor",
        "value": "120 pF",
        "role": "Vibrato bright cap, across the volume pot (switched)"
      },
      {
        "ref": "RLV2",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "Vibrato 2nd-stage plate load"
      },
      {
        "ref": "CCV2",
        "part": "Coupling capacitor",
        "value": "0.02 µF",
        "role": "Vibrato 2nd-stage output coupling"
      },
      {
        "ref": "RKA",
        "part": "Carbon comp resistor",
        "value": "820 Ω · ½ W",
        "role": "Cathode bias SHARED by both channels' second stages ([A])"
      },
      {
        "ref": "CKA",
        "part": "Electrolytic capacitor",
        "value": "25 µF · 25 V",
        "role": "Bypass for the shared second-stage cathode ([A])"
      },
      {
        "ref": "CRS",
        "part": "Mica capacitor",
        "value": "500 pF",
        "role": "Reverb send coupling to the driver grid"
      },
      {
        "ref": "RGRD",
        "part": "Carbon comp resistor",
        "value": "1 MΩ · ½ W",
        "role": "Reverb-driver 12AT7 grid leak"
      },
      {
        "ref": "RKRD",
        "part": "Carbon comp resistor",
        "value": "2.2 kΩ · ½ W",
        "role": "Reverb-driver shared cathode bias (both sections paralleled)"
      },
      {
        "ref": "CKRD",
        "part": "Electrolytic capacitor",
        "value": "25 µF · 25 V",
        "role": "Reverb-driver cathode bypass"
      },
      {
        "ref": "T4",
        "part": "Reverb transformer",
        "value": "Fender 125A20B",
        "role": "Reverb driver → tank input"
      },
      {
        "ref": "RGR1",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · ½ W",
        "role": "Reverb-recovery grid resistor (tank output)"
      },
      {
        "ref": "RLR1",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "Reverb-recovery plate load"
      },
      {
        "ref": "CCR1",
        "part": "Coupling capacitor",
        "value": "0.003 µF",
        "role": "Reverb-recovery output coupling"
      },
      {
        "ref": "VRREV",
        "part": "Linear potentiometer",
        "value": "100 kΩ-L",
        "role": "Reverb level"
      },
      {
        "ref": "RMR",
        "part": "Carbon comp resistor",
        "value": "470 kΩ · ½ W",
        "role": "Reverb return into the mix-driver grid"
      },
      {
        "ref": "RGD1",
        "part": "Carbon comp resistor",
        "value": "3.3 MΩ · ½ W",
        "role": "Dry-path series resistor, dry node → mix-driver grid (10 pF across it)"
      },
      {
        "ref": "CBD1",
        "part": "Mica capacitor",
        "value": "10 pF",
        "role": "Bright cap across the dry-path 3.3 MΩ"
      },
      {
        "ref": "RLD1",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "Mix-driver plate load"
      },
      {
        "ref": "CCD1",
        "part": "Coupling capacitor",
        "value": "0.1 µF",
        "role": "Mix-driver output coupling, onto the Intensity control and the 220 kΩ mixing resistor"
      },
      {
        "ref": "RKE",
        "part": "Carbon comp resistor",
        "value": "820 Ω · ½ W",
        "role": "Cathode bias SHARED by the reverb recovery and the mix driver ([E])"
      },
      {
        "ref": "CKE",
        "part": "Electrolytic capacitor",
        "value": "25 µF · 25 V",
        "role": "Bypass for the shared recovery/mix-driver cathode ([E])"
      },
      {
        "ref": "RMIXN",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · ½ W",
        "role": "Normal-channel mixing resistor"
      },
      {
        "ref": "RMIXV",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · ½ W",
        "role": "Vibrato-channel mixing resistor"
      },
      {
        "ref": "CPIA",
        "part": "Coupling capacitor",
        "value": "0.001 µF",
        "role": "Mixer node → PI hot grid"
      },
      {
        "ref": "VRSPD",
        "part": "Reverse-audio potentiometer",
        "value": "3 MΩ-RA",
        "role": "Tremolo speed"
      },
      {
        "ref": "VRINT",
        "part": "Reverse-audio potentiometer",
        "value": "50 kΩ-RA",
        "role": "Intensity — from the mix driver's output (after its 0.1 µF coupler) to ground; the photocell shunts its wiper"
      },
      {
        "ref": "RTO1",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · ½ W",
        "role": "Tremolo oscillator plate load"
      },
      {
        "ref": "RKTO1",
        "part": "Carbon comp resistor",
        "value": "2.7 kΩ · ½ W",
        "role": "Tremolo oscillator cathode bias"
      },
      {
        "ref": "CKTO1",
        "part": "Electrolytic capacitor",
        "value": "25 µF · 25 V",
        "role": "Tremolo oscillator cathode bypass"
      },
      {
        "ref": "RTOG1",
        "part": "Carbon comp resistor",
        "value": "1 MΩ · ½ W",
        "role": "Tremolo phase-shift ladder, middle node → footswitch junction"
      },
      {
        "ref": "RTOG2",
        "part": "Carbon comp resistor",
        "value": "1 MΩ · ½ W",
        "role": "Tremolo phase-shift ladder, oscillator grid → footswitch junction"
      },
      {
        "ref": "RTOG3",
        "part": "Carbon comp resistor",
        "value": "2.2 MΩ · ½ W",
        "role": "Footswitch junction → the bias supply's filter node"
      },
      {
        "ref": "RTOG4",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · ½ W",
        "role": "Mix-driver grid leak, to ground"
      },
      {
        "ref": "RTOSP",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "Tremolo speed-network series resistor"
      },
      {
        "ref": "CTO1",
        "part": "Film capacitor",
        "value": "0.01 µF",
        "role": "Tremolo phase-shift cap, Speed node → middle node (the lamp driver's grid)"
      },
      {
        "ref": "CTO2",
        "part": "Film capacitor",
        "value": "0.01 µF",
        "role": "Tremolo phase-shift cap, middle node → oscillator grid"
      },
      {
        "ref": "CTO3",
        "part": "Film capacitor",
        "value": "0.02 µF",
        "role": "Tremolo phase-shift cap, oscillator plate → Speed node"
      },
      {
        "ref": "RTO10",
        "part": "Carbon comp resistor",
        "value": "10 MΩ · ½ W",
        "role": "Lamp-driver plate bleeder to the +458 V node"
      },
      {
        "ref": "RKTO2",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "Lamp-driver cathode bias"
      },
      {
        "ref": "CKTO2",
        "part": "Electrolytic capacitor",
        "value": "25 µF · 25 V",
        "role": "Lamp-driver cathode bypass"
      },
      {
        "ref": "RLAMP",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "Neon-lamp series resistor to the +458 V node"
      },
      {
        "ref": "OPTO",
        "part": "Optocoupler",
        "value": "neon lamp + photoresistor",
        "role": "Tremolo modulator — the lamp on the lamp driver's plate; the photocell shunts the Intensity control's wiper to ground"
      },
      {
        "ref": "RLPA",
        "part": "Carbon comp resistor",
        "value": "82 kΩ · ½ W · 5%",
        "role": "PI plate load (hot side)"
      },
      {
        "ref": "RLPB",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W · 5%",
        "role": "PI plate load (cold side)"
      },
      {
        "ref": "RGPA",
        "part": "Carbon comp resistor",
        "value": "1 MΩ · ½ W",
        "role": "PI grid leak (hot), returned to the tail junction"
      },
      {
        "ref": "RGPB",
        "part": "Carbon comp resistor",
        "value": "1 MΩ · ½ W",
        "role": "PI grid leak (cold), returned to the tail junction"
      },
      {
        "ref": "RTAIL",
        "part": "Carbon comp resistor",
        "value": "470 Ω · ½ W",
        "role": "PI cathode resistor"
      },
      {
        "ref": "RT22",
        "part": "Carbon comp resistor",
        "value": "22 kΩ · ½ W",
        "role": "PI tail, junction → feedback node"
      },
      {
        "ref": "CPIB",
        "part": "Coupling capacitor",
        "value": "0.1 µF · 200 V",
        "role": "PI cold-grid reference to the feedback node"
      },
      {
        "ref": "RFB2",
        "part": "Carbon comp resistor",
        "value": "820 Ω · ½ W",
        "role": "Negative feedback, OT secondary → feedback node"
      },
      {
        "ref": "RFB1",
        "part": "Carbon comp resistor",
        "value": "100 Ω · ½ W",
        "role": "Feedback divider leg to ground"
      },
      {
        "ref": "C1",
        "part": "Coupling capacitor",
        "value": "0.1 µF · 400 V",
        "role": "PI hot plate → the V7/V8 grid pair"
      },
      {
        "ref": "C2",
        "part": "Coupling capacitor",
        "value": "0.1 µF · 400 V",
        "role": "PI cold plate → the V9/V10 grid pair"
      },
      {
        "ref": "RGL1",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · ½ W · 5%",
        "role": "Grid leak for the V7/V8 pair, from the −52 V bias line"
      },
      {
        "ref": "RGL2",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · ½ W · 5%",
        "role": "Grid leak for the V9/V10 pair, from the −52 V bias line"
      },
      {
        "ref": "RST1",
        "part": "Carbon comp resistor",
        "value": "1.5 kΩ · ½ W",
        "role": "V7 grid stopper"
      },
      {
        "ref": "RST2",
        "part": "Carbon comp resistor",
        "value": "1.5 kΩ · ½ W",
        "role": "V8 grid stopper"
      },
      {
        "ref": "RST3",
        "part": "Carbon comp resistor",
        "value": "1.5 kΩ · ½ W",
        "role": "V9 grid stopper"
      },
      {
        "ref": "RST4",
        "part": "Carbon comp resistor",
        "value": "1.5 kΩ · ½ W",
        "role": "V10 grid stopper"
      },
      {
        "ref": "RSC1",
        "part": "Wirewound resistor",
        "value": "470 Ω · 1 W",
        "role": "V7 screen resistor"
      },
      {
        "ref": "RSC2",
        "part": "Wirewound resistor",
        "value": "470 Ω · 1 W",
        "role": "V8 screen resistor"
      },
      {
        "ref": "RSC3",
        "part": "Wirewound resistor",
        "value": "470 Ω · 1 W",
        "role": "V9 screen resistor"
      },
      {
        "ref": "RSC4",
        "part": "Wirewound resistor",
        "value": "470 Ω · 1 W",
        "role": "V10 screen resistor"
      },
      {
        "ref": "T3",
        "part": "Output transformer",
        "value": "Fender 125A29A",
        "role": "Parallel push-pull primary for four 6L6GC; 4 Ω secondary into two speakers, with the feedback tap"
      },
      {
        "ref": "T1",
        "part": "Power transformer",
        "value": "Fender 125P34A · 340-0-340 V",
        "role": "HT (with bias tap) + 6.3 V heaters"
      },
      {
        "ref": "T2",
        "part": "Filter choke",
        "value": "Fender 125C1A",
        "role": "Reservoir → screen/post-choke node [B]"
      },
      {
        "ref": "DHT",
        "part": "Rectifier (HT)",
        "value": "silicon diode (×6)",
        "role": "Full-wave rectifier, three series diodes per leg"
      },
      {
        "ref": "C10",
        "part": "Electrolytic capacitor",
        "value": "70 µF · 350 V",
        "role": "Reservoir filter, upper half of the series pair"
      },
      {
        "ref": "C11",
        "part": "Electrolytic capacitor",
        "value": "70 µF · 350 V",
        "role": "Reservoir filter, lower half of the series pair"
      },
      {
        "ref": "RBAL1",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · 1 W",
        "role": "Reservoir balancing resistor (upper)"
      },
      {
        "ref": "RBAL2",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · 1 W",
        "role": "Reservoir balancing resistor (lower)"
      },
      {
        "ref": "C12",
        "part": "Electrolytic capacitor",
        "value": "20 µF · 525 V",
        "role": "Filter, node [B] (+458, screens)"
      },
      {
        "ref": "C13",
        "part": "Electrolytic capacitor",
        "value": "20 µF · 525 V",
        "role": "Filter, node [C] (+450, PI plates)"
      },
      {
        "ref": "C14",
        "part": "Electrolytic capacitor",
        "value": "20 µF · 525 V",
        "role": "Filter, node [D] (+410, preamp plates)"
      },
      {
        "ref": "RD1",
        "part": "Wirewound resistor",
        "value": "1 kΩ · 1 W",
        "role": "Rail dropper [B] → [C]"
      },
      {
        "ref": "RD2",
        "part": "Wirewound resistor",
        "value": "4.7 kΩ · 1 W",
        "role": "Rail dropper [C] → [D]"
      },
      {
        "ref": "RBIAS",
        "part": "Wirewound resistor",
        "value": "470 Ω · 1 W",
        "role": "Bias-supply series resistor off the transformer's bias tap"
      },
      {
        "ref": "DBIAS",
        "part": "Rectifier (bias)",
        "value": "silicon diode",
        "role": "Bias-supply rectifier"
      },
      {
        "ref": "CB1",
        "part": "Electrolytic capacitor",
        "value": "25 µF · 50 V",
        "role": "Bias-supply filter"
      },
      {
        "ref": "VRBAL",
        "part": "Linear potentiometer",
        "value": "10 kΩ-L",
        "role": "Bias balance control; its wiper sets the −52 V grid line"
      },
      {
        "ref": "RB2",
        "part": "Carbon comp resistor",
        "value": "27 kΩ · ½ W",
        "role": "Bias divider leg to ground"
      },
      {
        "ref": "CDEATH",
        "part": "Ceramic capacitor",
        "value": "0.047 µF · 600 V",
        "role": "Ground-switch cap (period; not in modern builds)"
      },
      {
        "ref": "—",
        "part": "Fuse",
        "value": "2½ A slo-blo",
        "role": "Mains fuse (schematic '2½ AMP SLO. BLO.'; layout '2.5 AMP SLO BLO FUSE')"
      },
      {
        "ref": "V1",
        "part": "Preamp tube",
        "value": "12AX7 (7025)",
        "role": "Normal channel input + 2nd stage"
      },
      {
        "ref": "V2",
        "part": "Preamp tube",
        "value": "12AX7 (7025)",
        "role": "Vibrato channel input + 2nd stage"
      },
      {
        "ref": "V3",
        "part": "Preamp/driver tube",
        "value": "12AT7",
        "role": "Reverb driver (both sections paralleled)"
      },
      {
        "ref": "V4",
        "part": "Preamp tube",
        "value": "12AX7 (7025)",
        "role": "Reverb recovery + reverb/dry mix driver"
      },
      {
        "ref": "V5",
        "part": "Preamp tube",
        "value": "12AX7",
        "role": "Tremolo oscillator + lamp driver (DC point excluded)"
      },
      {
        "ref": "V6",
        "part": "Preamp/driver tube",
        "value": "12AT7",
        "role": "Long-tailed-pair phase inverter"
      },
      {
        "ref": "V7",
        "part": "Power tube",
        "value": "6L6GC",
        "role": "Push-pull output (hot phase)"
      },
      {
        "ref": "V8",
        "part": "Power tube",
        "value": "6L6GC",
        "role": "Push-pull output (hot phase)"
      },
      {
        "ref": "V9",
        "part": "Power tube",
        "value": "6L6GC",
        "role": "Push-pull output (cold phase)"
      },
      {
        "ref": "V10",
        "part": "Power tube",
        "value": "6L6GC",
        "role": "Push-pull output (cold phase)"
      }
    ]
  },
  "publishedVoltages": {
    "source": "Fender AB763 Twin Reverb-Amp drawing, printed voltages: +460 (6L6GC plates, OT centre tap), +458 (screens, post-choke node B, reverb-transformer feed), +450 (phase-inverter plate supply, node C), +440 (reverb-driver plate), +410 (preamp plate supply, node D), +8.6 (reverb-driver cathode), +280/+275 (mix-driver and reverb-recovery plates) and +270/+260/+250/+240 (the four channel preamp plates) all on +2.0 V cathodes, PI +245/+235 plates with +100 V joined cathodes and a +98 V tail junction, -52 (6L6GC fixed bias); tremolo 12AX7 +380/+5.6/+2.0 excluded",
    "nodes": {
      "BC": {
        "chart": 450,
        "tol_pct": 8,
        "note": "node [C], the phase-inverter plate supply — solved from BB through the drawing's 1 kOhm-1W dropper"
      },
      "BD": {
        "chart": 410,
        "tol_pct": 8,
        "note": "node [D], the preamp plate supply feeding all six 100 kOhm plate loads — solved from BC through the 4.7 kOhm-1W dropper"
      },
      "S61": {
        "chart": 458,
        "tol_pct": 8,
        "note": "6L6GC screen after its 470 Ohm-1W stopper, off the post-choke node BB"
      },
      "G61": {
        "chart": -52,
        "tol_pct": 8,
        "note": "6L6GC grid, fixed bias via a shared 220 kOhm leak and a 1500 Ohm stopper from the -52 V supply"
      },
      "PRD": {
        "chart": 440,
        "tol_pct": 20,
        "note": "reverb-driver 12AT7 plate (both sections paralleled), off BB through the TR4 primary DCR"
      },
      "KRD": {
        "chart": 8.6,
        "tol_pct": 20,
        "note": "reverb-driver 12AT7 shared cathode over 2.2 kOhm"
      },
      "PN1": {
        "chart": 260,
        "tol_pct": 20,
        "note": "Normal-channel input plate, 100 kOhm from +410"
      },
      "KN1": {
        "chart": 2,
        "tol_pct": 20,
        "note": "Normal-channel input cathode over 1.5 kOhm"
      },
      "PV1": {
        "chart": 250,
        "tol_pct": 20,
        "note": "Vibrato-channel input plate, 100 kOhm from +410"
      },
      "KV1": {
        "chart": 2,
        "tol_pct": 20,
        "note": "Vibrato-channel input cathode over 1.5 kOhm"
      },
      "PN2": {
        "chart": 270,
        "tol_pct": 20,
        "note": "Normal-channel 2nd stage plate, 100 kOhm from +410"
      },
      "PV2": {
        "chart": 240,
        "tol_pct": 20,
        "note": "Vibrato-channel 2nd stage plate, 100 kOhm from +410"
      },
      "KA": {
        "chart": 2,
        "tol_pct": 20,
        "note": "the 820 Ohm / 25 uF cathode network the two channels' second stages SHARE (drawing box [A]); the drawing prints +2.0 V at both ends of it"
      },
      "PR1": {
        "chart": 275,
        "tol_pct": 20,
        "note": "reverb-recovery plate, 100 kOhm from +410"
      },
      "PD1": {
        "chart": 280,
        "tol_pct": 20,
        "note": "reverb/dry mix-driver plate, 100 kOhm from +410"
      },
      "KE": {
        "chart": 2,
        "tol_pct": 20,
        "note": "the 820 Ohm / 25 uF cathode network the recovery and mix-driver stages SHARE (drawing box [E]); printed +2.0 V at both ends"
      },
      "PPIA": {
        "chart": 245,
        "tol_pct": 20,
        "note": "PI plate, 82 kOhm (hot) side"
      },
      "PPIB": {
        "chart": 235,
        "tol_pct": 20,
        "note": "PI plate, 100 kOhm (cold) side"
      },
      "KPI": {
        "chart": 100,
        "tol_pct": 20,
        "note": "PI joined cathodes, above the 470 Ohm"
      },
      "JPI": {
        "chart": 98,
        "tol_pct": 20,
        "note": "PI tail junction (470 Ohm / 22 kOhm), grid-leak return"
      }
    }
  },
  "operatingPoint": [
    {
      "node": "BC",
      "chart": 450,
      "sim": 445.652,
      "disputed": false,
      "dev_pct": 0.9662222222222251,
      "tol_pct": 8,
      "gated": true,
      "within": true,
      "note": "node [C], the phase-inverter plate supply — solved from BB through the drawing's 1 kOhm-1W dropper",
      "dispute_note": ""
    },
    {
      "node": "BD",
      "chart": 410,
      "sim": 407.838,
      "disputed": false,
      "dev_pct": 0.5273170731707263,
      "tol_pct": 8,
      "gated": true,
      "within": true,
      "note": "node [D], the preamp plate supply feeding all six 100 kOhm plate loads — solved from BC through the 4.7 kOhm-1W dropper",
      "dispute_note": ""
    },
    {
      "node": "S61",
      "chart": 458,
      "sim": 457.611,
      "disputed": false,
      "dev_pct": 0.08493449781659607,
      "tol_pct": 8,
      "gated": true,
      "within": true,
      "note": "6L6GC screen after its 470 Ohm-1W stopper, off the post-choke node BB",
      "dispute_note": ""
    },
    {
      "node": "G61",
      "chart": -52,
      "sim": -52,
      "disputed": false,
      "dev_pct": 0,
      "tol_pct": 8,
      "gated": true,
      "within": true,
      "note": "6L6GC grid, fixed bias via a shared 220 kOhm leak and a 1500 Ohm stopper from the -52 V supply",
      "dispute_note": ""
    },
    {
      "node": "PRD",
      "chart": 440,
      "sim": 457.175,
      "disputed": false,
      "dev_pct": 3.9034090909090935,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "reverb-driver 12AT7 plate (both sections paralleled), off BB through the TR4 primary DCR",
      "dispute_note": ""
    },
    {
      "node": "KRD",
      "chart": 8.6,
      "sim": 9.07,
      "disputed": false,
      "dev_pct": 5.465116279069775,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "reverb-driver 12AT7 shared cathode over 2.2 kOhm",
      "dispute_note": ""
    },
    {
      "node": "PN1",
      "chart": 260,
      "sim": 269.759,
      "disputed": false,
      "dev_pct": 3.7534615384615444,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "Normal-channel input plate, 100 kOhm from +410",
      "dispute_note": ""
    },
    {
      "node": "KN1",
      "chart": 2,
      "sim": 2.071,
      "disputed": false,
      "dev_pct": 3.5500000000000087,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "Normal-channel input cathode over 1.5 kOhm",
      "dispute_note": ""
    },
    {
      "node": "PV1",
      "chart": 250,
      "sim": 269.759,
      "disputed": false,
      "dev_pct": 7.903600000000006,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "Vibrato-channel input plate, 100 kOhm from +410",
      "dispute_note": ""
    },
    {
      "node": "KV1",
      "chart": 2,
      "sim": 2.071,
      "disputed": false,
      "dev_pct": 3.5500000000000087,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "Vibrato-channel input cathode over 1.5 kOhm",
      "dispute_note": ""
    },
    {
      "node": "PN2",
      "chart": 270,
      "sim": 275.741,
      "disputed": false,
      "dev_pct": 2.126296296296291,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "Normal-channel 2nd stage plate, 100 kOhm from +410",
      "dispute_note": ""
    },
    {
      "node": "PV2",
      "chart": 240,
      "sim": 275.741,
      "disputed": false,
      "dev_pct": 14.892083333333328,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "Vibrato-channel 2nd stage plate, 100 kOhm from +410",
      "dispute_note": ""
    },
    {
      "node": "KA",
      "chart": 2,
      "sim": 2.166,
      "disputed": false,
      "dev_pct": 8.299999999999997,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "the 820 Ohm / 25 uF cathode network the two channels' second stages SHARE (drawing box [A]); the drawing prints +2.0 V at both ends of it",
      "dispute_note": ""
    },
    {
      "node": "PR1",
      "chart": 275,
      "sim": 275.741,
      "disputed": false,
      "dev_pct": 0.2694545454545402,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "reverb-recovery plate, 100 kOhm from +410",
      "dispute_note": ""
    },
    {
      "node": "PD1",
      "chart": 280,
      "sim": 275.741,
      "disputed": false,
      "dev_pct": 1.5210714285714337,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "reverb/dry mix-driver plate, 100 kOhm from +410",
      "dispute_note": ""
    },
    {
      "node": "KE",
      "chart": 2,
      "sim": 2.166,
      "disputed": false,
      "dev_pct": 8.299999999999997,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "the 820 Ohm / 25 uF cathode network the recovery and mix-driver stages SHARE (drawing box [E]); printed +2.0 V at both ends",
      "dispute_note": ""
    },
    {
      "node": "PPIA",
      "chart": 245,
      "sim": 254.727,
      "disputed": false,
      "dev_pct": 3.9702040816326547,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "PI plate, 82 kOhm (hot) side",
      "dispute_note": ""
    },
    {
      "node": "PPIB",
      "chart": 235,
      "sim": 248.231,
      "disputed": false,
      "dev_pct": 5.630212765957444,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "PI plate, 100 kOhm (cold) side",
      "dispute_note": ""
    },
    {
      "node": "KPI",
      "chart": 100,
      "sim": 97.062,
      "disputed": false,
      "dev_pct": 2.9380000000000024,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "PI joined cathodes, above the 470 Ohm",
      "dispute_note": ""
    },
    {
      "node": "JPI",
      "chart": 98,
      "sim": 95.04,
      "disputed": false,
      "dev_pct": 3.0204081632652997,
      "tol_pct": 20,
      "gated": true,
      "within": true,
      "note": "PI tail junction (470 Ohm / 22 kOhm), grid-leak return",
      "dispute_note": ""
    }
  ],
  "checks": [
    {
      "id": "dc",
      "title": "DC operating point",
      "status": "Verified",
      "summary": "20 reference nodes are within tolerance. Maintainer verification is recorded for 2026-08-08; changes since then are awaiting maintainer re-review.",
      "details": [
        "The circuit's facts (netlist, reference voltages, parts list, both drawings' connections, capacitor polarity marks and heater declarations) have changed since the maintainer's review, or no review fingerprint is on record yet. The badge stands on its date until the maintainer re-reviews.",
        "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.",
        "23 sheet-only parts are explicitly excluded: C10, C11, C12, C13, C14, CB1, CBRN, CBRV, CDEATH, DBIAS, DHTA, DHTB, R1n, R1v, R2n, R2v, RB2, RBAL1, RBAL2, RBIAS, RGN1, RGV1, T2. Heaters and the pilot lamp are checked separately."
      ],
      "tone": "checked"
    },
    {
      "id": "heaters",
      "title": "Heater and filament wiring",
      "status": "Not established",
      "summary": "The drawn supply-leg connections have not been established by this check.",
      "details": [
        "Supply-leg grouping still needs confirmation at V1, V2, V3, V4, V5, V6.",
        "V1: pins [4, 5] are one leg at 6.3 V and opposite legs at 12.6 V"
      ],
      "tone": "attention"
    },
    {
      "id": "polarity",
      "title": "Capacitor polarity",
      "status": "Partial",
      "summary": "2 capacitor markings cannot be confirmed by the DC model.",
      "details": [
        "CKTO1: CKTO1.b: reaches only ground, through RKTO1",
        "CKTO2: CKTO2.b: reaches only ground, through RKTO2"
      ],
      "tone": "attention"
    }
  ],
  "corrections": [
    {
      "id": "ab763-twin-inputs-2026-10-03",
      "date": "2026-10-03",
      "amps": [
        "ab763-twin"
      ],
      "title": "AB763 Twin Reverb input grid leaks and mains fuse",
      "description": "Each channel's 1 MΩ input leak now hangs from jack 1's tip, on the jack side of its 68 kΩ resistor, as the factory drawing shows; it had been drawn on the grid. The 2½ A slow-blow mains fuse is now on the parts list. No operating point changes.",
      "commit": "1bab4626cbc417a67e2eb9689c765d87c921c77e",
      "url": "https://github.com/TheAnalogMaker/circuit-codex/commit/1bab4626cbc417a67e2eb9689c765d87c921c77e"
    }
  ],
  "sourceDirectory": "https://github.com/TheAnalogMaker/circuit-codex/tree/ebddc3fd2cf887dca010bbe60487d7391ea6c052/amps/ab763-twin"
}