{
  "format": "circuit-codex-reference",
  "version": 1,
  "id": "dr103",
  "url": "https://circuitcodex.com/amps/dr103/",
  "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": "dr103",
    "name_style": "Hiwatt Custom 100-style",
    "family": "british",
    "era": {
      "start": 1969,
      "end": 1981
    },
    "wattage": 100,
    "tubes": [
      "ECC83",
      "ECC83",
      "ECC83",
      "ECC81",
      "EL34",
      "EL34",
      "EL34",
      "EL34"
    ],
    "topology": {
      "rectifier": {
        "kind": "solid-state",
        "type": "silicon bridge"
      },
      "bias": "fixed",
      "phase_inverter": "long-tailed-pair",
      "tone_stack": "cathode-follower-fmv"
    },
    "lineage": {
      "derived_from": [],
      "influenced": []
    },
    "conventions": {
      "designators": "functional",
      "notation": "uk"
    },
    "sources": [
      {
        "desc": "Four-input DR103 preamp drawing, read for every preamp value in this entry. Title block reads 'HIWATT / Late 60s Four-Input Preamp / Preamp from DR103 S/N 903 / Drawing rev. 1.0 - Mark Huss'. It is a published REDRAW taken off a specific amplifier, not a factory document, and no factory drawing of the four-input DR103 was found for this entry. Values read directly from it: four jacks in two pairs (Brilliant high/low, Normal high/low) on 68 kohm stoppers into 1 Mohm leaks; V1 ECC83 with 220 kohm plate loads on both halves and ONE shared 1.5 kohm cathode resistor bypassed by 100 uF/16 V; 1 nF (Brilliant) and 10 nF (Normal) couplers into 470 kohm-A volume pots, mixing through two 470 kohm resistors; V2 ECC83 as a 220 kohm-plate / 2.2 kohm-cathode gain stage direct-coupled to a cathode follower on a 120 kohm 2 W load; cathode-follower FMV tone stack (100 pF treble cap into a 220 kohm treble pot, 100 kohm slope, 47 nF into the 470 kohm-A bass rheostat, 47 nF into a 22 kohm middle pot) into a 470 kohm-A master volume; V3 ECC83 as a 100 kohm-plate / 2.2 kohm-cathode driver whose plate reaches the next grid through a 22 nF cap PARALLELED BY 1.8 Mohm against a 1 Mohm leak — a DC-coupled divider — feeding a second cathode follower on 220 kohm; V4 ECC81 long-tailed pair with unequal 82 kohm / 91 kohm plate loads, a 22 kohm tail over 2.2 kohm, a 1 Mohm between the two grids and 100 nF from the second grid to the tail junction; 47 nF output couplers; presence 100 kohm lin with 1 nF in, 100 R to ground and 47 nF out; feedback 10 kohm from the 16 ohm tap with 270 R and 10 nF at the junction. The sheet prints NO voltages. Not reproduced here; archived at the MHuss Hiwatt pages.",
        "url": "https://hiwatt.org/Schematics/DR103sn903pre.pdf"
      },
      {
        "desc": "Published DR103 wiring diagram titled 'HIWATT DR103 LAYOUT, EARLY 1970'S', credited on the sheet to Mark Huss (with updates from Jukka K.) and Brian Haberman, 2007 — the source for the output stage, the supply chain and the control complement of the four-input amplifier, and a second reading of the preamp. Read from it: four input jacks (Normal and Brilliant pairs); seven controls — Normal Volume, Brilliant Volume, Bass, Treble, Middle, Presence, Master Volume; V1/V2/V3 ECC83 with V4 ECC81; four EL34s each on a 22 kohm grid stopper and a 100 R screen stopper, two 100 kohm grid leaks off the bias line; supply cans of 220 uF/350 V and a 2x50 uF/450 V, each with a 220 kohm balancing resistor, behind a UF5408 bridge; droppers 100 R 5 W, 1 k 2 W and 470 R 10 W; bias network 1 k / UF5408 / 100 uF 63 V / 47 kohm / 100 kohm; feedback leg 10 k, 470 R and 10 nF. Where it and the S/N 903 drawing differ they differ in control values (500 kohm log volumes and bass, 250 kohm lin treble and master, 100 kohm lin middle against the S/N 903 amp's 470 kohm-A and 220 kohm / 22 kohm) — in-era variation, recorded rather than merged; the S/N 903 values are the ones this entry documents. Not reproduced here.",
        "url": "https://hiwatt.org/Layouts/DR103hiwattlayout.pdf"
      },
      {
        "desc": "Hiwatt DR103 owner's manual, distributed by Biacrown Ltd ('THIS MANUAL COVERS MODEL NUMBER DR103', 'Manufactured in England by the Audio Brothers'), carrying three dated factory sheets whose title blocks read 'HIWATT DR103 POWER SUPPLY CIRCUIT DIAGRAM  Drawn 22/11/94  Issue 3.', 'HIWATT DR103 PREAMPLIFIER CIRCUIT DIAGRAM  Drawn 19/05/95  Issue 4.  S/No. AB0309 Onwards' and 'HIWATT DR103 OUTPUT STAGE CIRCUIT DIAGRAM  Drawn 25/04/94  Issue 1.'. THESE SHEETS DOCUMENT A LATER, TWO-INPUT REVISION and are NOT the circuit modelled here — the Issue-4 preamp shows one jack per channel, 470 kohm volumes with a 220 kohm lin master and a 100 kohm lin middle, and labels every preamp valve ECC83. They are used for three things only: the printed bias figure 'BIAS (-38V)' on the Issue-3 supply sheet, whose 1 k / 1N5408 / 100 uF / 47 kohm network is the one the early wiring diagram also shows; the manual's DR103 SPECIFICATION page ('Valve types: Output valves 4 x matched EL34 (6CA7); Preamp valves V1,V2,V3, 3 x ECC83 (12AX7)(7025); Preamp valve V4 1 x ECC81 (12AT7)(6201)', 'Output power: 100W RMS (min) into 8 Ohms', line voltages 100/117.5/225/250 V a.c., 330 W maximum consumption, 4/8/16 ohm speaker taps); and the two working voltages the Issue-4 sheet annotates, +160 V at the V1 plate on a 220 kohm load over the same shared 1.5 kohm / 100 uF cathode this circuit uses, and +140 V at the second stage. Note that the SPECIFICATION page and the Issue-4 drawing contradict each other on V4: the specification calls it an ECC81, the drawing letters 'VALVES V1,V2,V3,V4 = ECC83'. The specification, the early wiring diagram and the S/N 903 drawing all agree on ECC81, so ECC81 is what this entry records. Not reproduced here; archived at El34World.",
        "url": "https://el34world.com/charts/Schematics/files/Hiwatt/Hiwatt_100w_dr103.pdf"
      },
      {
        "desc": "Hiwatt factory-lineage hand-drawn sheets archived with the DR504, used as the cross-reference for how far the preamp travelled across the line and for the only published Hiwatt HT figure read for this work. Page 2 is headed 'HIWATT PREAMP AND DRIVE' and lists the models it covers — 'DR504 DR405 DR103 DR201 ALL COMBO'S' — drawing the same four-valve preamp and driver this entry documents; page 3 is headed 'POWER SUPPLY / DR504 SA212 SA112 SA115', signed 'HIWATT JULY 79', and prints 460 V at the HT node after the standby switch, the 100 ohm 5 W / 1 k 1 W dropper chain to the second and third HT rails, and 'BIAS -36v'. The DR504 output-stage sheet of the same set names the transformers 'OUTPUT TRANSFORMER PARTRIDGE TG 6556 / TH 7551' and the mains transformer 'PARTRIDGE TYPE TH 7550'. These are 50-watt figures and are NOT claimed as DR103 values.",
        "url": "https://el34world.com/charts/Schematics/files/Hiwatt/Hiwatt_50w_dr504.pdf"
      },
      {
        "desc": "Wikipedia, 'Hiwatt' — the maker history behind this circuit: Dave Reeves trained in the Royal Air Force and worked at Mullard, was made redundant in 1966 and founded Hylight in September 1966; he built Sound City-branded amplifiers for Ivor Arbiter's shop, and 'Under the Hiwatt brand, Reeves' customized Sound City L100 amplifiers were renamed the DR103 Custom 100'; the DR201 and DR405 followed at bassist Glenn Cornick's urging; Partridge transformers gave the amplifiers their 'hi-fi' character; from 1971 the certified government wirer Harry Joyce standardised the schematics and capped his output at forty amplifiers a month; Reeves died in 1981 and former employees continued as Biacrown Ltd until the company folded in 1984",
        "url": "https://en.wikipedia.org/wiki/Hiwatt"
      },
      {
        "desc": "Vintage Guitar, 'Ronnie Wood's '69 Hiwatt 100 DR103' — a dated published example of the four-input DR103 in 1969, the evidence for the start of the bracket recorded in era.start",
        "url": "https://www.vintageguitar.com/14871/ronnie-woods-69-hiwatt-100-dr103/"
      },
      {
        "desc": "Mullard/Philips EL34 data sheet, January 1969 (A.F. Output Pentode) — anchor for the CC0 EL34 tube model and its octal basing: Va=250 V, Vg2=250 V, Vg1=-13.5 V -> Ia=100 mA, Ig2=14.9 mA, gm=12.5 mA/V, mu(g1-g2)=11",
        "url": "https://frank.pocnet.net/sheets/010/e/EL34.pdf"
      },
      {
        "desc": "RCA 12AT7 data sheet (March 1, 1954) — tabulated Class-A1 characteristics behind the clean-room CC0 12AT7 model used for the ECC81 phase inverter: Va=250 V, 200 ohm cathode bias -> Ia=10 mA, gm=5500 umho, mu=60; second point Va=100 V / 270 ohm -> 3.7 mA, 4000 umho. Basing 9A",
        "url": "https://frank.pocnet.net/sheets/049/1/12AT7.pdf"
      },
      {
        "desc": "RCA Receiving Tube Manual RC-19 (1959), 12AX7 average characteristics — the two-point anchor behind the CC0 12AX7 model used for the three ECC83 stages: Va=250 V / -2 V -> 1.2 mA, 1600 umho and Va=100 V / -1 V -> 0.5 mA, mu=100",
        "url": "https://frank.pocnet.net/sheets/093/1/12AX7.pdf"
      }
    ],
    "verification": {
      "status": "draft"
    },
    "added": "2026-08-08T00:00:00.000Z",
    "contributors": [
      "TheAnalogMaker"
    ]
  },
  "parts": {
    "notes": "Coupling and cathode-bypass capacitor VALUES are open at DC; those whose both leads land on named DC nodes are carried in the operating-point netlist so their board placement can be machine-checked, the rest are listed here for completeness only. Two honest caveats live in this list. (1) The S/N 903 drawing and the early-1970s wiring diagram disagree on four control values — the wiring diagram gives 500 kOhm log volumes and bass, 250 kOhm lin treble and master and a 100 kOhm lin middle where the S/N 903 amp carries 470 kOhm-A volumes, bass and master with a 220 kOhm treble and a 22 kOhm middle. The S/N 903 values are listed, because that is the drawing this entry's netlist follows; the variation is real and is recorded in the circuit story rather than averaged away. (2) The transformer part numbers TH7549/2 and TH7548/5 are lettered on the factory sheets of 1994-95, which document a later revision of this amplifier; the four-input amplifiers of the Reeves era carried Partridge transformers, whose DR103 part numbers were not read on any drawing consulted here. The mains transformer, fuses, switches, neon, impedance selector and jack sockets are annotations on the drawings and are listed here without designators, as in the sibling entries.",
    "items": [
      {
        "ref": "R1s",
        "part": "Carbon comp resistor",
        "value": "68 kΩ · ½ W",
        "role": "Brilliant high-gain input grid stopper"
      },
      {
        "ref": "R2s",
        "part": "Carbon comp resistor",
        "value": "68 kΩ · ½ W",
        "role": "Brilliant low-gain input grid stopper"
      },
      {
        "ref": "R3s",
        "part": "Carbon comp resistor",
        "value": "68 kΩ · ½ W",
        "role": "Normal high-gain input grid stopper"
      },
      {
        "ref": "R4s",
        "part": "Carbon comp resistor",
        "value": "68 kΩ · ½ W",
        "role": "Normal low-gain input grid stopper"
      },
      {
        "ref": "RG1",
        "part": "Carbon comp resistor",
        "value": "1 MΩ · ½ W",
        "role": "Brilliant input grid leak"
      },
      {
        "ref": "RG2",
        "part": "Carbon comp resistor",
        "value": "1 MΩ · ½ W",
        "role": "Normal input grid leak"
      },
      {
        "ref": "—",
        "part": "Jack socket",
        "value": "1/4 in",
        "role": "Brilliant high-gain input"
      },
      {
        "ref": "—",
        "part": "Jack socket",
        "value": "1/4 in",
        "role": "Brilliant low-gain input"
      },
      {
        "ref": "—",
        "part": "Jack socket",
        "value": "1/4 in",
        "role": "Normal high-gain input"
      },
      {
        "ref": "—",
        "part": "Jack socket",
        "value": "1/4 in",
        "role": "Normal low-gain input"
      },
      {
        "ref": "RL1",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · ½ W",
        "role": "V1A plate load (Brilliant)"
      },
      {
        "ref": "RL2",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · ½ W",
        "role": "V1B plate load (Normal)"
      },
      {
        "ref": "RK1",
        "part": "Carbon comp resistor",
        "value": "1.5 kΩ · ½ W",
        "role": "V1 cathode bias — ONE resistor shared by both channels"
      },
      {
        "ref": "C1",
        "part": "Electrolytic capacitor",
        "value": "100 µF · 16 V",
        "role": "V1 shared cathode bypass"
      },
      {
        "ref": "C2",
        "part": "Coupling capacitor",
        "value": "0.001 µF · 400 V",
        "role": "V1A → Brilliant volume — the small coupler is the whole Brilliant voicing"
      },
      {
        "ref": "C3",
        "part": "Coupling capacitor",
        "value": "0.01 µF · 400 V",
        "role": "V1B → Normal volume"
      },
      {
        "ref": "VR1",
        "part": "Audio-taper potentiometer",
        "value": "470 kΩ",
        "role": "Brilliant volume"
      },
      {
        "ref": "VR2",
        "part": "Audio-taper potentiometer",
        "value": "470 kΩ",
        "role": "Normal volume"
      },
      {
        "ref": "RM1",
        "part": "Carbon comp resistor",
        "value": "470 kΩ · ½ W",
        "role": "Brilliant mixing resistor into the V2B grid"
      },
      {
        "ref": "RM2",
        "part": "Carbon comp resistor",
        "value": "470 kΩ · ½ W",
        "role": "Normal mixing resistor into the V2B grid"
      },
      {
        "ref": "RL3",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · ½ W",
        "role": "V2B plate load (second stage)"
      },
      {
        "ref": "RK2",
        "part": "Carbon comp resistor",
        "value": "2.2 kΩ · ½ W",
        "role": "V2B cathode bias, unbypassed"
      },
      {
        "ref": "RKCF",
        "part": "Power resistor",
        "value": "120 kΩ · 2 W",
        "role": "V2A cathode-follower load — a 2 W part because this triode works across most of the HT"
      },
      {
        "ref": "C4",
        "part": "Mica capacitor",
        "value": "100 pF",
        "role": "Tone stack — treble"
      },
      {
        "ref": "RSL",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "Tone-stack slope resistor"
      },
      {
        "ref": "VR3",
        "part": "Potentiometer",
        "value": "220 kΩ",
        "role": "Treble"
      },
      {
        "ref": "C5",
        "part": "Film capacitor",
        "value": "0.047 µF · 400 V",
        "role": "Tone stack — bass"
      },
      {
        "ref": "VR4",
        "part": "Audio-taper potentiometer",
        "value": "470 kΩ",
        "role": "Bass — wired as a rheostat, wiper tied to its top lug"
      },
      {
        "ref": "C6",
        "part": "Film capacitor",
        "value": "0.047 µF · 400 V",
        "role": "Tone stack — middle, into the middle pot's wiper"
      },
      {
        "ref": "VR5",
        "part": "Potentiometer",
        "value": "22 kΩ",
        "role": "Middle"
      },
      {
        "ref": "RTM",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · ½ W",
        "role": "Treble-wiper output into the master volume"
      },
      {
        "ref": "VR6",
        "part": "Audio-taper potentiometer",
        "value": "470 kΩ",
        "role": "Master volume — a control the contemporary Marshall lead heads do not have"
      },
      {
        "ref": "RL4",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "V3B plate load (driver)"
      },
      {
        "ref": "RK3",
        "part": "Carbon comp resistor",
        "value": "2.2 kΩ · ½ W",
        "role": "V3B cathode bias"
      },
      {
        "ref": "C7",
        "part": "Film capacitor",
        "value": "0.047 µF · 400 V",
        "role": "V3B cathode bypass"
      },
      {
        "ref": "C8",
        "part": "Coupling capacitor",
        "value": "0.022 µF · 400 V",
        "role": "V3B plate → V3A grid, across the 1.8 MΩ DC-coupling resistor"
      },
      {
        "ref": "R18",
        "part": "Carbon comp resistor",
        "value": "1.8 MΩ · ½ W",
        "role": "V3B plate → V3A grid DC path — the upper leg of the divider that lifts the phase inverter's grid reference"
      },
      {
        "ref": "RG3",
        "part": "Carbon comp resistor",
        "value": "1 MΩ · ½ W",
        "role": "V3A grid leak — the lower leg of that divider"
      },
      {
        "ref": "RKCF2",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · ½ W",
        "role": "V3A cathode-follower load, DC-coupled straight into the phase-inverter grid"
      },
      {
        "ref": "RLA",
        "part": "Carbon comp resistor",
        "value": "82 kΩ · ½ W",
        "role": "PI plate load, driven side (V4A)"
      },
      {
        "ref": "RLB",
        "part": "Carbon comp resistor",
        "value": "91 kΩ · ½ W",
        "role": "PI plate load, other side (V4B) — deliberately unequal"
      },
      {
        "ref": "RGB",
        "part": "Carbon comp resistor",
        "value": "1 MΩ · ½ W",
        "role": "V4B grid referred to the V4A grid"
      },
      {
        "ref": "RTAIL",
        "part": "Carbon comp resistor",
        "value": "22 kΩ · ½ W",
        "role": "PI tail (shared cathode → feedback junction)"
      },
      {
        "ref": "RT2",
        "part": "Carbon comp resistor",
        "value": "2.2 kΩ · ½ W",
        "role": "PI tail (feedback junction → ground)"
      },
      {
        "ref": "C9",
        "part": "Film capacitor",
        "value": "0.1 µF · 160 V",
        "role": "V4B grid to the tail junction, for signal"
      },
      {
        "ref": "C11",
        "part": "Film capacitor",
        "value": "0.001 µF · 400 V",
        "role": "V3B plate → presence pot"
      },
      {
        "ref": "VR7",
        "part": "Linear potentiometer",
        "value": "100 kΩ",
        "role": "Presence"
      },
      {
        "ref": "RPG",
        "part": "Carbon comp resistor",
        "value": "100 Ω · ½ W",
        "role": "Presence wiper to ground"
      },
      {
        "ref": "C12",
        "part": "Film capacitor",
        "value": "0.047 µF · 400 V",
        "role": "Presence network → feedback junction"
      },
      {
        "ref": "RNFB",
        "part": "Carbon comp resistor",
        "value": "10 kΩ · ½ W",
        "role": "Negative feedback, 16 Ω secondary tap → PI tail junction"
      },
      {
        "ref": "RPD",
        "part": "Carbon comp resistor",
        "value": "270 Ω · ½ W",
        "role": "Feedback-junction shunt leg, in series with C10"
      },
      {
        "ref": "C10",
        "part": "Film capacitor",
        "value": "0.01 µF · 400 V",
        "role": "Feedback-junction shunt leg to ground"
      },
      {
        "ref": "C13",
        "part": "Coupling capacitor",
        "value": "0.047 µF · 400 V",
        "role": "PI (82 kΩ plate) → V5/V6 grid line"
      },
      {
        "ref": "C14",
        "part": "Coupling capacitor",
        "value": "0.047 µF · 400 V",
        "role": "PI (91 kΩ plate) → V7/V8 grid line"
      },
      {
        "ref": "RGL1",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "Grid leak for the V5/V6 pair, from the bias line"
      },
      {
        "ref": "RGL2",
        "part": "Carbon comp resistor",
        "value": "100 kΩ · ½ W",
        "role": "Grid leak for the V7/V8 pair, from the bias line"
      },
      {
        "ref": "RS5",
        "part": "Carbon comp resistor",
        "value": "22 kΩ · ½ W",
        "role": "V5 grid stopper — four times the value the Marshall heads use"
      },
      {
        "ref": "RS6",
        "part": "Carbon comp resistor",
        "value": "22 kΩ · ½ W",
        "role": "V6 grid stopper"
      },
      {
        "ref": "RS7",
        "part": "Carbon comp resistor",
        "value": "22 kΩ · ½ W",
        "role": "V7 grid stopper"
      },
      {
        "ref": "RS8",
        "part": "Carbon comp resistor",
        "value": "22 kΩ · ½ W",
        "role": "V8 grid stopper"
      },
      {
        "ref": "RSC5",
        "part": "Carbon comp resistor",
        "value": "100 Ω · ½ W",
        "role": "V5 screen-grid stopper"
      },
      {
        "ref": "RSC6",
        "part": "Carbon comp resistor",
        "value": "100 Ω · ½ W",
        "role": "V6 screen-grid stopper"
      },
      {
        "ref": "RSC7",
        "part": "Carbon comp resistor",
        "value": "100 Ω · ½ W",
        "role": "V7 screen-grid stopper"
      },
      {
        "ref": "RSC8",
        "part": "Carbon comp resistor",
        "value": "100 Ω · ½ W",
        "role": "V8 screen-grid stopper"
      },
      {
        "ref": "D5",
        "part": "Silicon rectifier diode",
        "value": "UF5408",
        "role": "Bias-supply rectifier, off its own transformer tap"
      },
      {
        "ref": "RB1",
        "part": "Power resistor",
        "value": "1 kΩ · 2 W",
        "role": "Bias-supply series resistor ahead of the diode"
      },
      {
        "ref": "C16",
        "part": "Electrolytic capacitor",
        "value": "100 µF · 63 V",
        "role": "Bias-supply filter"
      },
      {
        "ref": "RB2",
        "part": "Carbon comp resistor",
        "value": "47 kΩ · ½ W",
        "role": "Bias-supply bleed / divider — this bias is NOT adjustable on the drawings read here"
      },
      {
        "ref": "D1",
        "part": "Silicon rectifier diode",
        "value": "UF5408",
        "role": "HT bridge rectifier"
      },
      {
        "ref": "D2",
        "part": "Silicon rectifier diode",
        "value": "UF5408",
        "role": "HT bridge rectifier"
      },
      {
        "ref": "D3",
        "part": "Silicon rectifier diode",
        "value": "UF5408",
        "role": "HT bridge rectifier"
      },
      {
        "ref": "D4",
        "part": "Silicon rectifier diode",
        "value": "UF5408",
        "role": "HT bridge rectifier"
      },
      {
        "ref": "C17",
        "part": "Electrolytic capacitor",
        "value": "220 µF · 350 V",
        "role": "Reservoir, upper half of the series pair"
      },
      {
        "ref": "C18",
        "part": "Electrolytic capacitor",
        "value": "220 µF · 350 V",
        "role": "Reservoir, lower half of the series pair"
      },
      {
        "ref": "RB3",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · ½ W",
        "role": "Reservoir balancing resistor, upper"
      },
      {
        "ref": "RB4",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · ½ W",
        "role": "Reservoir balancing resistor, lower"
      },
      {
        "ref": "RD1",
        "part": "Power resistor",
        "value": "100 Ω · 5 W",
        "role": "Dropper HT1 (plates) → HT2"
      },
      {
        "ref": "RD2",
        "part": "Power resistor",
        "value": "470 Ω · 10 W",
        "role": "Feed from HT2 to the EL34 screen bus"
      },
      {
        "ref": "RD3",
        "part": "Power resistor",
        "value": "1 kΩ · 2 W",
        "role": "Dropper HT2 → HT3 (preamp rail); the factory Issue-3 sheet letters 1 W here"
      },
      {
        "ref": "C19",
        "part": "Electrolytic capacitor",
        "value": "50 µF · 450 V (×2)",
        "role": "HT2/HT3 filter can, a series pair in one housing"
      },
      {
        "ref": "RB5",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · ½ W",
        "role": "Balancing resistor for the 50 µF can, upper"
      },
      {
        "ref": "RB6",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · ½ W",
        "role": "Balancing resistor for the 50 µF can, lower"
      },
      {
        "ref": "C20",
        "part": "Electrolytic capacitor",
        "value": "220 µF · 350 V",
        "role": "HT3 filter"
      },
      {
        "ref": "RB7",
        "part": "Carbon comp resistor",
        "value": "220 kΩ · ½ W",
        "role": "Balancing resistor for the HT3 filter"
      },
      {
        "ref": "RD4",
        "part": "Power resistor",
        "value": "47 kΩ · 2 W",
        "role": "Dropper HT3 → second-stage rail"
      },
      {
        "ref": "C21",
        "part": "Electrolytic capacitor",
        "value": "32 µF · 450 V",
        "role": "Second-stage rail filter"
      },
      {
        "ref": "RD5",
        "part": "Power resistor",
        "value": "22 kΩ · 2 W",
        "role": "Dropper second-stage rail → input and driver rail"
      },
      {
        "ref": "C22",
        "part": "Electrolytic capacitor",
        "value": "16 µF · 350 V",
        "role": "Input and driver rail filter"
      },
      {
        "ref": "V1",
        "part": "Preamp tube",
        "value": "ECC83 (12AX7)",
        "role": "Input valve — one half per channel (V1A Brilliant / V1B Normal)"
      },
      {
        "ref": "V2",
        "part": "Preamp tube",
        "value": "ECC83 (12AX7)",
        "role": "Second gain stage + cathode follower into the tone stack (V2B/V2A)"
      },
      {
        "ref": "V3",
        "part": "Preamp tube",
        "value": "ECC83 (12AX7)",
        "role": "Driver + second cathode follower into the phase inverter (V3B/V3A)"
      },
      {
        "ref": "V4",
        "part": "Preamp tube",
        "value": "ECC81 (12AT7)",
        "role": "Long-tailed-pair phase inverter (V4A/V4B)"
      },
      {
        "ref": "V5",
        "part": "Power tube",
        "value": "EL34",
        "role": "Parallel push-pull output — phase A"
      },
      {
        "ref": "V6",
        "part": "Power tube",
        "value": "EL34",
        "role": "Parallel push-pull output — phase A"
      },
      {
        "ref": "V7",
        "part": "Power tube",
        "value": "EL34",
        "role": "Parallel push-pull output — phase B"
      },
      {
        "ref": "V8",
        "part": "Power tube",
        "value": "EL34",
        "role": "Parallel push-pull output — phase B"
      },
      {
        "ref": "T1",
        "part": "Output transformer",
        "value": "Hiwatt TH7549/2 · 4/8/16 Ω taps",
        "role": "Output transformer, feedback from the 16 Ω tap; the Reeves-era four-input heads carried Partridge iron whose DR103 part number was not read on any drawing consulted"
      },
      {
        "ref": "—",
        "part": "Power transformer",
        "value": "Hiwatt TH7548/5 · 100/117.5/225/250 V primary taps",
        "role": "Mains/HT transformer with the bias winding (annotation)"
      },
      {
        "ref": "—",
        "part": "Toggle switch",
        "value": "DPST",
        "role": "Mains (annotation)"
      },
      {
        "ref": "—",
        "part": "Toggle switch",
        "value": "SPST",
        "role": "HT standby, between the reservoir and HT1 (annotation)"
      },
      {
        "ref": "—",
        "part": "Rotary switch",
        "value": "3-position",
        "role": "Speaker impedance selector, 4/8/16 Ω (annotation)"
      },
      {
        "ref": "—",
        "part": "Jack socket",
        "value": "1/4 in",
        "role": "Speaker output (annotation)"
      },
      {
        "ref": "—",
        "part": "Neon indicator",
        "value": "mains neon",
        "role": "Panel indicator across the primary (annotation)"
      },
      {
        "ref": "—",
        "part": "Fuse",
        "value": "3.15 A slow-blow",
        "role": "Mains fuse, 6.3 A on the 100 V and 117.5 V taps (annotation)"
      },
      {
        "ref": "—",
        "part": "Fuse",
        "value": "3.15 A slow-blow",
        "role": "H.T. fuse (annotation)"
      }
    ]
  },
  "publishedVoltages": {
    "source": "No published valve-voltage chart exists for this circuit. Simulated values only; the printed figures named above come from the later factory Issue-3/Issue-4 DR103 sheets and from the DR504 supply sheet of July 1979, and are recorded as context, not as chart values.",
    "nodes": {
      "BP1": {
        "chart": null,
        "note": "HT1 — EL34 plates and output-transformer centre tap. DRIVEN at 480 V as the netlist's one stated assumption; the OT primary DCR is omitted, so every plate sits here"
      },
      "BP2": {
        "chart": null,
        "note": "HT2, after the 100 Ohm 5 W dropper — feeds the EL34 screen bus and the preamp chain"
      },
      "BSCR": {
        "chart": null,
        "note": "EL34 screen bus, after the 470 Ohm 10 W feed from HT2. The drop across it is the model's total screen current; each valve then has its own 100 Ohm stopper"
      },
      "BP3": {
        "chart": null,
        "note": "HT3, after the 1 kOhm 1 W dropper — the preamp rail proper: the V2A cathode follower's plate and both phase-inverter plate loads hang here"
      },
      "BP4": {
        "chart": null,
        "note": "second-stage rail, after the 47 kOhm 2 W dropper — carries only the V2B 220 kOhm plate load, which is why the drop across that resistor is so large"
      },
      "BP5": {
        "chart": null,
        "note": "input and driver rail, after the 22 kOhm 2 W dropper — the V1 plate loads, the V3B plate load and the V3A cathode follower's plate all sit here"
      },
      "P1A": {
        "chart": null,
        "note": "V1A plate — Brilliant channel, 220 kOhm load. The later factory Issue-4 sheet annotates +160 V at the equivalent plate on the same 220 kOhm / 1.5 kOhm stage; not gated, because the rail chain feeding it differs"
      },
      "P1B": {
        "chart": null,
        "note": "V1B plate — Normal channel, the same 220 kOhm load. The two channels are voiced apart by their couplers (1 nF against 10 nF), not at the cathode, so the two plates idle identically"
      },
      "K1": {
        "chart": null,
        "note": "the single 1.5 kOhm cathode resistor both V1 halves share, bypassed by 100 uF — one of the clearest differences from the Marshall lead amps, which split the two channels' cathodes"
      },
      "P2B": {
        "chart": null,
        "note": "V2B plate (220 kOhm load) — also the V2A cathode follower's grid, DC-coupled"
      },
      "K2B": {
        "chart": null,
        "note": "V2B cathode over 2.2 kOhm, unbypassed"
      },
      "KCF": {
        "chart": null,
        "note": "V2A cathode-follower output over its 120 kOhm 2 W load, driving the tone stack. Its plate sits on HT3, so this triode works across a large plate-to-cathode voltage — the reason the drawing specifies a 2 W part here"
      },
      "P3B": {
        "chart": null,
        "note": "V3B plate (100 kOhm load) — the driver stage after the master volume"
      },
      "K3B": {
        "chart": null,
        "note": "V3B cathode over 2.2 kOhm, bypassed by 47 nF"
      },
      "G3A": {
        "chart": null,
        "note": "V3A grid. This node is NOT at zero: the V3B plate reaches it through a 1.8 MOhm resistor against a 1 MOhm leak, with a 22 nF cap across the 1.8 MOhm for signal. The divider is what gives the phase inverter a lifted grid reference"
      },
      "KCF2": {
        "chart": null,
        "note": "V3A cathode follower over 220 kOhm — and, because there is no coupling cap between them, this node IS the V4A phase-inverter grid"
      },
      "PPIA": {
        "chart": null,
        "note": "phase-inverter plate, 82 kOhm side (V4A, the driven half)"
      },
      "PPIB": {
        "chart": null,
        "note": "phase-inverter plate, 91 kOhm side (V4B). The unequal loads compensate the long-tailed pair's inherent imbalance"
      },
      "KPI": {
        "chart": null,
        "note": "phase-inverter shared cathode, sitting a couple of volts above its grids — which are themselves lifted to the V3A cathode voltage"
      },
      "JPI": {
        "chart": null,
        "note": "tail junction between the 22 kOhm tail and the 2.2 kOhm to ground. The 10 kOhm negative-feedback resistor from the output transformer's 16 Ohm tap also lands here, and since that tap is at DC ground it is a second path to ground: it moves this node, unlike the arrangement in the Marshall heads where feedback returns to a grounded point"
      },
      "GPIB": {
        "chart": null,
        "note": "V4B grid, referred to V4A's grid through 1 MOhm and tied to the tail junction for signal by the 100 nF cap"
      },
      "NGA": {
        "chart": null,
        "note": "grid line of the V5/V6 pair, off the 82 kOhm plate through a 47 nF coupler, held at the bias line by a 100 kOhm leak"
      },
      "NGB": {
        "chart": null,
        "note": "grid line of the V7/V8 pair, off the 91 kOhm plate — the other push-pull phase"
      },
      "G5": {
        "chart": null,
        "note": "V5 control grid, after its 22 kOhm stopper. Carries no DC current, so it reads the bias line"
      },
      "SG5": {
        "chart": null,
        "note": "V5 screen after its 100 Ohm stopper. The figure below is the one to distrust: see the note on the EL34 model at the head of this file"
      }
    }
  },
  "operatingPoint": [
    {
      "node": "BP1",
      "chart": null,
      "sim": 480,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "HT1 — EL34 plates and output-transformer centre tap. DRIVEN at 480 V as the netlist's one stated assumption; the OT primary DCR is omitted, so every plate sits here",
      "dispute_note": ""
    },
    {
      "node": "BP2",
      "chart": null,
      "sim": 477.22,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "HT2, after the 100 Ohm 5 W dropper — feeds the EL34 screen bus and the preamp chain",
      "dispute_note": ""
    },
    {
      "node": "BSCR",
      "chart": null,
      "sim": 467.753,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "EL34 screen bus, after the 470 Ohm 10 W feed from HT2. The drop across it is the model's total screen current; each valve then has its own 100 Ohm stopper",
      "dispute_note": ""
    },
    {
      "node": "BP3",
      "chart": null,
      "sim": 469.568,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "HT3, after the 1 kOhm 1 W dropper — the preamp rail proper: the V2A cathode follower's plate and both phase-inverter plate loads hang here",
      "dispute_note": ""
    },
    {
      "node": "BP4",
      "chart": null,
      "sim": 330.244,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "second-stage rail, after the 47 kOhm 2 W dropper — carries only the V2B 220 kOhm plate load, which is why the drop across that resistor is so large",
      "dispute_note": ""
    },
    {
      "node": "BP5",
      "chart": null,
      "sim": 280.668,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "input and driver rail, after the 22 kOhm 2 W dropper — the V1 plate loads, the V3B plate load and the V3A cathode follower's plate all sit here",
      "dispute_note": ""
    },
    {
      "node": "P1A",
      "chart": null,
      "sim": 162.601,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "V1A plate — Brilliant channel, 220 kOhm load. The later factory Issue-4 sheet annotates +160 V at the equivalent plate on the same 220 kOhm / 1.5 kOhm stage; not gated, because the rail chain feeding it differs",
      "dispute_note": ""
    },
    {
      "node": "P1B",
      "chart": null,
      "sim": 162.601,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "V1B plate — Normal channel, the same 220 kOhm load. The two channels are voiced apart by their couplers (1 nF against 10 nF), not at the cathode, so the two plates idle identically",
      "dispute_note": ""
    },
    {
      "node": "K1",
      "chart": null,
      "sim": 1.61,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "the single 1.5 kOhm cathode resistor both V1 halves share, bypassed by 100 uF — one of the clearest differences from the Marshall lead amps, which split the two channels' cathodes",
      "dispute_note": ""
    },
    {
      "node": "P2B",
      "chart": null,
      "sim": 173.853,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "V2B plate (220 kOhm load) — also the V2A cathode follower's grid, DC-coupled",
      "dispute_note": ""
    },
    {
      "node": "K2B",
      "chart": null,
      "sim": 1.564,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "V2B cathode over 2.2 kOhm, unbypassed",
      "dispute_note": ""
    },
    {
      "node": "KCF",
      "chart": null,
      "sim": 176.138,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "V2A cathode-follower output over its 120 kOhm 2 W load, driving the tone stack. Its plate sits on HT3, so this triode works across a large plate-to-cathode voltage — the reason the drawing specifies a 2 W part here",
      "dispute_note": ""
    },
    {
      "node": "P3B",
      "chart": null,
      "sim": 195.437,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "V3B plate (100 kOhm load) — the driver stage after the master volume",
      "dispute_note": ""
    },
    {
      "node": "K3B",
      "chart": null,
      "sim": 1.722,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "V3B cathode over 2.2 kOhm, bypassed by 47 nF",
      "dispute_note": ""
    },
    {
      "node": "G3A",
      "chart": null,
      "sim": 69.799,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "V3A grid. This node is NOT at zero: the V3B plate reaches it through a 1.8 MOhm resistor against a 1 MOhm leak, with a 22 nF cap across the 1.8 MOhm for signal. The divider is what gives the phase inverter a lifted grid reference",
      "dispute_note": ""
    },
    {
      "node": "KCF2",
      "chart": null,
      "sim": 72.119,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "V3A cathode follower over 220 kOhm — and, because there is no coupling cap between them, this node IS the V4A phase-inverter grid",
      "dispute_note": ""
    },
    {
      "node": "PPIA",
      "chart": null,
      "sim": 332.311,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "phase-inverter plate, 82 kOhm side (V4A, the driven half)",
      "dispute_note": ""
    },
    {
      "node": "PPIB",
      "chart": null,
      "sim": 328.832,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "phase-inverter plate, 91 kOhm side (V4B). The unequal loads compensate the long-tailed pair's inherent imbalance",
      "dispute_note": ""
    },
    {
      "node": "KPI",
      "chart": null,
      "sim": 76.656,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "phase-inverter shared cathode, sitting a couple of volts above its grids — which are themselves lifted to the V3A cathode voltage",
      "dispute_note": ""
    },
    {
      "node": "JPI",
      "chart": null,
      "sim": 5.807,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "tail junction between the 22 kOhm tail and the 2.2 kOhm to ground. The 10 kOhm negative-feedback resistor from the output transformer's 16 Ohm tap also lands here, and since that tap is at DC ground it is a second path to ground: it moves this node, unlike the arrangement in the Marshall heads where feedback returns to a grounded point",
      "dispute_note": ""
    },
    {
      "node": "GPIB",
      "chart": null,
      "sim": 72.119,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "V4B grid, referred to V4A's grid through 1 MOhm and tied to the tail junction for signal by the 100 nF cap",
      "dispute_note": ""
    },
    {
      "node": "NGA",
      "chart": null,
      "sim": -38,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "grid line of the V5/V6 pair, off the 82 kOhm plate through a 47 nF coupler, held at the bias line by a 100 kOhm leak",
      "dispute_note": ""
    },
    {
      "node": "NGB",
      "chart": null,
      "sim": -38,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "grid line of the V7/V8 pair, off the 91 kOhm plate — the other push-pull phase",
      "dispute_note": ""
    },
    {
      "node": "G5",
      "chart": null,
      "sim": -38,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "V5 control grid, after its 22 kOhm stopper. Carries no DC current, so it reads the bias line",
      "dispute_note": ""
    },
    {
      "node": "SG5",
      "chart": null,
      "sim": 467.249,
      "disputed": false,
      "dev_pct": null,
      "tol_pct": 5,
      "gated": false,
      "within": null,
      "note": "V5 screen after its 100 Ohm stopper. The figure below is the one to distrust: see the note on the EL34 model at the head of this file",
      "dispute_note": ""
    }
  ],
  "checks": [
    {
      "id": "dc",
      "title": "DC operating point",
      "status": "Simulation only",
      "summary": "No undisputed reference voltages are available for a comparison.",
      "details": [
        "This compares simulated DC conditions with the cited reference. It does not test sound, physical construction or every part of the circuit."
      ],
      "tone": "neutral"
    },
    {
      "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.",
        "6 sheet-only parts are explicitly excluded: R1s, R2s, R3s, R4s, RG1, RG2. 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.",
        "No Hiwatt board-layout sheet was located for this DR103; the board is derived from the preamp drawing, so no factory drawing can establish its heater legs.",
        "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": "Checked",
      "summary": "8 capacitor markings agree with the model's voltage ordering.",
      "details": [
        "This check uses DC voltage ordering; source review establishes markings the model cannot decide."
      ],
      "tone": "checked"
    }
  ],
  "corrections": [],
  "sourceDirectory": "https://github.com/TheAnalogMaker/circuit-codex/tree/ebddc3fd2cf887dca010bbe60487d7391ea6c052/amps/dr103"
}