Machine-checked wiring. Every modelled part the operating-point netlist places on this board — the tube sockets, the plate, cathode and grid-leak resistors, the power-rail droppers, and the coupling and bypass capacitors whose two leads are both named DC nodes — has been verified in CI, terminal for terminal, to be electrically equivalent to the simulated netlist this circuit is verified against: the same net structure, no missing or extra connections. Parts outside that DC model — the volume / tone / mixer control-network internals, the negative-bias front end, and the heater chain, pilot lamp and power-transformer / rectifier AC side — are drawn from the cited sources but are not machine-checked; the CI report enumerates every one of their terminals so the boundary is explicit rather than silent. This diagram documents connectivity and part arrangement — it is not a dimensioned 1:1 build template. Board and part positions are drawn at diagram scale, and lead dress, grounding paths, and physical spacing all matter in a high-voltage chassis. If you build from it, verify every connection against the schematic above and the operating-point table below as you go.
Operating point vs. published chart
Chart values read from the published drawing — or, where the drawing carries no voltage chart, from the cited published measurements (never reproduced). Tolerances are this project's verification targets — tighter on power rails than the ±20% measurement convention Fender printed on its charts. Simulation matches every node within target — worst deviation 9.6%.
| Node | Chart | Tolerance | Note |
|---|---|---|---|
| BP2 | 430 V | ±8% | screens node after choke (DCR estimated at 130 Ω) |
| BP3 | 385 V | ±8% | PI supply node after 4.7 kΩ |
| BP4 | 325 V | ±8% | preamp node after the 10 kΩ dropper |
| PAY1 | 150 V | ±20% | 12AY7 plate, printed pin value |
| PAY2 | 150 V | ±20% | 12AY7 plate (normal channel) |
| KAY | 2.5 V | ±20% | shared 12AY7 cathode, 820 Ω |
| P2A | 180 V | ±20% | V2A plate = CF grid (direct-coupled) |
| KCF | 180 V | ±20% | cathode follower output, printed +180 |
| PPIA | 235 V | ±20% | PI plate, 82k 5% side |
| PPIB | 230 V | ±20% | PI plate, 100k 5% side |
| KPI | 34 V | ±20% | PI cathodes above 470 Ω |
| JPI | 32.5 V | ±20% | tail junction (grid-leak return) |
| S51 | — | informational — 5881 screen after 470 Ω | |
| G51 | -48 V | ±8% | fixed bias via 220k from the -48 V supply |
Tube-pin nodes are checked against the chart's own printed ±20% convention; power-rail nodes are held to tighter internal verification targets.
Parts list
Reference designators match the schematic above. The schematic draws one jack per channel (the chassis has four). Parts drawn only as annotations (bright cap, NFB, presence, balance cap) are listed without designators.
| Ref | Part | Value / rating | Role |
|---|---|---|---|
| R1s | Carbon comp resistor | 68 kΩ · ½ W | Bright-channel grid stopper |
| R2s | Carbon comp resistor | 68 kΩ · ½ W | Normal-channel grid stopper |
| RG1 | Carbon comp resistor | 1 MΩ · ½ W | Bright input grid leak |
| RG2 | Carbon comp resistor | 1 MΩ · ½ W | Normal input grid leak |
| RL1 | Carbon comp resistor | 100 kΩ · ½ W | V1A plate load |
| RL2 | Carbon comp resistor | 100 kΩ · ½ W | V1B plate load |
| RK1 | Carbon comp resistor | 820 Ω · ½ W | Shared 12AY7 cathode bias |
| C3 | Electrolytic capacitor | 250 µF · 6 V | Shared 12AY7 cathode bypass |
| C1 | Coupling capacitor | 0.02 µF · 400 V | V1A → bright volume |
| C2 | Coupling capacitor | 0.02 µF · 400 V | V1B → normal volume |
| VR1 | Audio-taper potentiometer | 1 MΩ | Bright volume (100 pF bright cap across it) |
| VR2 | Audio-taper potentiometer | 1 MΩ | Normal volume |
| RM1 | Carbon comp resistor | 270 kΩ · ½ W | Bright-channel mixer |
| RM2 | Carbon comp resistor | 270 kΩ · ½ W | Normal-channel mixer |
| RL3 | Carbon comp resistor | 100 kΩ · ½ W | V2A plate load |
| RK2 | Carbon comp resistor | 820 Ω · ½ W | V2A cathode bias |
| RKCF | Carbon comp resistor | 100 kΩ · ½ W | Cathode-follower load |
| RSL | Carbon comp resistor | 56 kΩ · ½ W | Tone-stack slope resistor |
| C4 | Mica capacitor | 250 pF | Tone stack — treble |
| VR3 | Potentiometer | 250 kΩ | Treble |
| C5 | Film capacitor | 0.02 µF · 400 V | Tone stack — bass/mid |
| VR4 | Audio-taper potentiometer | 1 MΩ | Bass |
| VR5 | Linear potentiometer | 25 kΩ | Middle (rheostat-wired) |
| C6 | Coupling capacitor | 0.02 µF · 400 V | Tone stack → phase-inverter grid |
| RGA | Carbon comp resistor | 1 MΩ · ½ W | PI grid leak (V3A), returned to tail junction |
| RGB | Carbon comp resistor | 1 MΩ · ½ W | PI grid leak (V3B), returned to tail junction |
| RLA | Carbon comp resistor | 82 kΩ · ½ W · 5% | PI plate load (V3A) |
| RLB | Carbon comp resistor | 100 kΩ · ½ W · 5% | PI plate load (V3B) |
| RTAIL | Carbon comp resistor | 470 Ω · ½ W | PI tail bias |
| RT2 | Carbon comp resistor | 10 kΩ · ½ W | PI tail (junction → foot) |
| C7 | Film capacitor | 0.1 µF · 200 V | V3B grid AC-ground to the tail junction |
| C8 | Coupling capacitor | 0.1 µF · 400 V | PI → V4 grid |
| C9 | Coupling capacitor | 0.1 µF · 400 V | PI → V5 grid |
| RGL1 | Carbon comp resistor | 220 kΩ · ½ W | V4 grid leak, from the −48 V bias line |
| RGL2 | Carbon comp resistor | 220 kΩ · ½ W | V5 grid leak, from the −48 V bias line |
| RS1 | Carbon comp resistor | 470 Ω · 1 W | V4 screen resistor |
| RS2 | Carbon comp resistor | 470 Ω · 1 W | V5 screen resistor |
| RD1 | Power resistor | 4,700 Ω | Rail dropper B+2 → B+3 |
| RD2 | Power resistor | 10 kΩ | Rail dropper B+3 → B+4 |
| L1 | Filter choke | Fender 14684 | B+1 → B+2 (screens) |
| C11 | Electrolytic capacitor | 20 µF · 600 V | Filter, B+1 |
| C12 | Electrolytic capacitor | 20 µF · 600 V | Filter, B+2 |
| C13 | Electrolytic capacitor | 20 µF · 600 V | Filter, B+3 |
| C14 | Electrolytic capacitor | 8 µF · 450 V | Filter, B+4 |
| D1 | Rectifier (bias) | selenium (silicon diode in modern builds) | Bias supply rectifier |
| RB1 | Carbon comp resistor | 15 kΩ · ½ W | Bias supply series resistor |
| RB2 | Carbon comp resistor | 56 kΩ · ½ W | Bias supply bleeder |
| C15 | Electrolytic capacitor | 8 µF · 150 V (×2) | Bias supply filters |
| V1 | Preamp tube | 12AY7 | Both input stages (V1A/V1B) |
| V2 | Preamp tube | 12AX7 | Second stage + cathode follower (V2A/V2B) |
| V3 | Preamp tube | 12AX7 | Long-tailed-pair phase inverter (V3A/V3B) |
| V4 | Power tube | 5881 | Push-pull output (upper) |
| V5 | Power tube | 5881 | Push-pull output (lower) |
| V6 | Rectifier tube | GZ34 | Full-wave rectifier (V6A/V6B) |
| T3 | Output transformer | Fender 45249 · 2 Ω secondary | Push-pull output into four 8 Ω speakers |
| — | Power transformer | Fender 8087 · 325-0-325 V | HT + heaters + rectifier filament |
| — | Mica capacitor | 100 pF | Bright cap across VR1 (annotation only on schematic) |
| — | Mica capacitor | 47 pF | PI balance cap, V3A grid–plate (annotation only) |
| — | Carbon comp resistor | 27 kΩ · ½ W | Negative feedback, speaker → tail foot (annotation only) |
| — | Linear potentiometer | 5 kΩ | Presence (annotation only) |
Circuit story
The most influential guitar amplifier circuit ever drawn: four inputs, a 12AY7 front end, a direct-coupled cathode follower driving the treble-mid-bass tone stack, a long-tailed-pair phase inverter, and a fixed-bias 5881 pair — the template Marshall copied for the JTM45 and half the industry copied after that. Produced 1958–1960. Lineage edges to 5F6 (ancestor) and JTM45 (descendant) land when those circuits do.
Circuit walkthrough (short form)
Bright + normal channels (1M leaks, 68k stoppers) → V1 12AY7 (100k plates, shared 820 Ω cathode with 250 µF bypass) → 0.02 µF couplers → 1M volume pots (100 pF bright cap) → 270k mixers → V2A 12AX7 (100k plate, 820 Ω cathode) → V2B cathode follower, DC-coupled (100k cathode load) → TMB tone stack (56k slope, 250 pF treble, 0.02 µF caps; 250k/1M/25k pots) → 0.02 µF → long-tailed-pair PI: 82k (5%) and 100k (5%) plates, 470 Ω + 10k tail, both 1M grid leaks returned to the tail junction, 47 pF balance cap → 0.1 µF couplers → 5881 pair, fixed-biased at −48 V through 220k leaks, 470 Ω 1W screen resistors → 2Ω output (four 8Ω speakers), 27k NFB into the tail foot with the 5k presence control.
Power: 325-0-325 (PT 8087) → GZ34 → standby → +432 V plates (20 µF) → choke (14684) → +430 V screens → 4.7 kΩ → +385 V PI (20 µF) → 10k → +325 V preamp (8 µF). Bias supply: selenium rectifier, 15k/56k, two 8 µF/150 V → −48 V.
Verification — against the printed factory chart
The I-EG drawing prints a full voltage chart, and simulation matches all 13 compared nodes (S51 carries no chart value and is informational only): rails within 0.8 %, every tube pin within 9.6 % (the chart's own convention is ±20 %). Working from the drawing also settles two details that often circulate incorrectly:
- The phase-inverter tail is 10k (with a 470 Ω bias resistor) — not the 6.8k sometimes quoted. The chart's own +32.5 V junction figure confirms it: 32.5 V across 10k matches the ~3.2 mA the plate drops imply.
- At DC the tail's 10k returns effectively straight to ground; the 27k feedback resistor and presence pot sit at the foot at roughly 0 V.