5C1 Wide‑panel Champ‑style · 1953–1955 · 4 W

✓ verified 2026-08-08 · changes since then are awaiting maintainer re-review

Verification details

What these checks mean

Each check covers a different part of this reference. The circuit badge records the DC comparison and maintainer review.

View checks, findings and exclusions
DC operating point Verified
4 reference nodes are within tolerance. Maintainer verification is recorded for 2026-08-08; changes since then are awaiting maintainer re-review.
  • 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.
Schematic against DC model Checked
Connectivity of modelled components is checked on every change.
  • Parts outside the DC model need separate checks and source review.
Board against DC model Checked
Connectivity of modelled components is checked on every change.
  • This does not establish heater wiring or physical spacing.
Schematic against board No open findings
No disagreement is listed in the published comparison.
  • A finding identifies a difference or a coverage gap. The factory source must decide which drawing, if either, needs correction.
  • 0 sheet-only parts are explicitly excluded. Heaters and the pilot lamp are checked separately.
Heater and filament wiring Checked
The declared heater circuits cover the drawing and pass their separate wiring check.
Capacitor polarity Checked
4 capacitor markings agree with the model's voltage ordering.
  • This check uses DC voltage ordering; source review establishes markings the model cannot decide.

Archive revision ebddc3f. Published corrections. These checks do not certify a physical build.

Schematic — redrawn in KiCad · scroll to zoom, drag to pan

Machine-checked drawing. This sheet is checked in CI against the netlist the simulation solves: every component the netlist models is proved to have its terminals on the same nodes in this drawing as in that netlist, so the picture and the maths cannot drift apart.

Details

The check compares connectivity, and only as far as the netlist reaches. It does not check component values, and it does not prove the netlist itself a faithful reading of the factory drawing — that rests on the operating-point comparison below and on the cited sources. The netlist is a DC model, so what it leaves out is drawn here but outside this check: the supply front end ahead of the first rail, transformer winding resistance, the volume / tone / mixer control networks and the capacitors inside them, and the heater and AC-side wiring. Separate checks cover drawing-to-board agreement, declared heater circuits and polarity where the model can decide it; see Verification details for their coverage and findings. Every abstraction this circuit relies on is declared as reviewable data in its own sch_map.yaml, and the CI report enumerates each terminal left unchecked.

Board layout — redrawn reference diagram · source noted on the drawing Print sheet ↗
5C1 eyelet board layout — an original diagram redrawn from the published layout drawing, showing the principal parts in the order that drawing places them on the board, drawn in the period layout-sheet style with each value lettered on the part.

Scroll the drawing sideways to read the whole board — the source line and the colour legend run along the bottom of the sheet — or open the full-size diagram in its own tab. Values are lettered on the parts in the period shorthand — .02-400, 250-5, 1MEG: how to read it.

Machine-checked wiring. Every modelled part the operating-point netlist places on this board has been verified in CI, terminal for terminal, to be electrically equivalent to the simulated netlist this circuit is verified against. This diagram documents connectivity and part arrangement — it is not a dimensioned 1:1 build template.

Details

The modelled parts are 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; the check proves the same net structure, with 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 outside this DC equivalence check. Separate checks compare the drawings and examine declared heater circuits and polarity; see Verification details for their coverage and findings.

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). The simulated column is this circuit's own netlist solved in ngspice by the run that gates it; every deviation beside it is those two numbers compared, not a separately stated figure. Tolerances are this project's verification targets: tube-pin nodes carry the tolerance the cited source states, or ±20% — the era's printed convention — where it states none, and power rails are held to tighter internal targets. Every gated node lands within target — worst deviation 14.8%.

Node Chart Simulated Deviation Tolerance Note
BP2 320 V 324.4 V 1.4% ±20% the 500 Ω/25 kΩ junction, which feeds the output transformer's primary — a single 500 Ω dropper off the reservoir, no choke on this pre-5E1 circuit. The 6V6 plate pin itself prints +300 V, 20 V lower across the primary, whose resistance the model omits, so the plate simulates at this node's voltage
BP3 280 V 249.8 V 10.8% ±20% shared rail after the 25 kΩ dropper off BP2, printed at the 6V6 screen (pin 4) — feeds the 6V6 screen (drawn off the far side of the 25 kΩ from its own plate, not off BP2) and the whole 6SJ7 plate/screen circuit
P1 130 V 149.3 V 14.8% ±20% 6SJ7 plate, printed pin value — gated like every other pin, and inside the chart's ±20 %. The stage around it is not reproduced: its grid and screen are reported rather than gated (see G1 and SG1), so this pass shows the plate voltage agrees, not that the 6SJ7's operating point does
SG1 — 12.1 V — — 6SJ7 screen — the drawing prints +21 V here. Reported, not gated: the stage is grid-leak (contact) biased, the corpus's tube models carry no grid-current path, and so the deck holds the grid at 0 V (G1) and the tube's screen current does not follow the real stage's; the screen simulates well below the printed figure
G1 — 0.0 V — — 6SJ7 grid — the drawing prints −0.5 V here, the contact bias the tube develops across its 5 MΩ leak. Reported, not gated: with no grid-current path in the corpus's tube models, nothing in this DC deck can pull the grid below 0 V
K2 16 V 15.4 V 4.0% ±20% 6V6GT cathode, chart pin value (500 Ω, 25 µF bypass)

Tube-pin nodes are checked at the tolerance the cited source states, or at the era's ±20% convention where it states none; power-rail nodes are held to tighter internal verification targets. Simulated figures are the netlist's DC solution, not measurements from a chassis.

The output stage behind these numbers can be drawn: the load line explorer plots this circuit's 6V6GT at the DC operating point its netlist carries — 324 V on the plates with a 500 Ω cathode resistor — and marks where the load line crosses.

Parts list

Every designator in this list appears on the schematic above, and every designator on the schematic appears here — the two are checked against each other in both directions, a valve's two halves counting as one bottle (V1A and V1B are both V1). Three parts the drawing carries as annotations rather than numbered symbols are listed here without designators, and stand outside that check. Numbering follows the schematic. The AC-line section (power transformer, fuse, line cap) is drawn as annotation only and carries no designator. Values are from the published Champ 5C1 (F-DH) drawing.

RefPartValue / ratingRole
R1 Carbon comp resistor 75 kΩ · ½ W Input 1 series resistor, jack 1 tip → summing node ahead of C1
R2 Carbon comp resistor 75 kΩ · ½ W Input 2 series resistor, jack 2 tip → summing node ahead of C1
R3 Carbon comp resistor 5 MΩ · ½ W 6SJ7 grid leak (contact bias — no cathode resistor)
R4 Carbon comp resistor 2 MΩ · ½ W 6SJ7 screen dropper
R5 Carbon comp resistor 250 kΩ · ½ W 6SJ7 plate load
R6 Wirewound resistor 500 Ω · 2 W 6V6 cathode bias
R7 Power resistor 500 Ω · 1 W Rail dropper B+1→B+2
R8 Power resistor 25 kΩ · 1 W Rail dropper B+2→B+3
VR1 Audio-taper potentiometer 1 MΩ Volume
C1 Coupling capacitor 0.02 µF · 600 V Input → 6SJ7 grid
C2 Bypass capacitor 0.05 µF · 600 V 6SJ7 screen bypass
C3 Coupling capacitor 0.02 µF · 600 V 6SJ7 plate → volume pot
C4 Electrolytic capacitor 25 µF · 25 V 6V6 cathode bypass
C5 Electrolytic capacitor 8 µF · 450 V Filter, B+1 (reservoir)
C6 Electrolytic capacitor 8 µF · 450 V Filter, B+2
C7 Electrolytic capacitor 8 µF · 450 V Filter, B+3
V1 Preamp tube 6SJ7 Single gain stage
V2 Power tube 6V6GT Single-ended output
V3 Rectifier tube 5Y3GT Full-wave rectifier (V3A/V3B)
T1 Output transformer ≈5 kΩ : 8 Ω Single-ended output (impedance not marked on the drawing; typical SE 6V6 load)
— Power transformer HT · 6.3 V · 5 V HT secondary (voltage not marked; +340 V reservoir implies ≈345-0-345) + heater and rectifier windings
— Line capacitor 0.05 µF · 600 V AC line to chassis, on the line side of the fuse
— Fuse 2 A AC-line fuse

Circuit story

The earliest circuit-numbered Champ, and the corpus's first octal-preamp circuit: a single 6SJ7 sharp-cutoff pentode giving one stage of voltage gain, one 1 MΩ volume control, a cathode-biased 6V6 single-ended output, and a 5Y3GT rectifier — no tone control and no phase inverter. Wide-panel tweed cabinet, produced circa 1953–1955. It carries the Champion 600's circuit forward under Fender's new "Champ" model name (see the Champ family page) and is the direct topological ancestor of the 12AX7-based 5E1 that replaces it: the wide-panel-to-narrow-panel revision (5D1) that sits between the two is a short-lived component revision of this same circuit, not a documented redesign of its own (no landed circuit page).

Circuit walkthrough (short form)

Two input jacks, each through its own 75 kΩ series resistor, sum into a single 0.02 µF coupling cap (jack 1's contact grounds its own tip when nothing is plugged in) → V1 (6SJ7 pentode, grid-leak/contact biased: a 5 MΩ leak returns the grid to ground and the cathode grounds directly, with no cathode resistor at all — the plate carries a 250 kΩ load, the screen a 2 MΩ dropper bypassed by 0.05 µF) → 0.02 µF coupling → 1 MΩ volume pot → V2 6V6GT (cathode-biased, 500 Ω with 25 µF bypass, grid returned to ground through the volume pot) → single-ended output transformer (≈5 kΩ : 8 Ω, typical for a single-ended 6V6; the drawing doesn't mark it) → speaker. No negative feedback loop on this circuit (the 5E1 does not add one either; NFB into the Champ line waits for the 5F1).

Power: a center-tapped HT winding feeds the 5Y3GT full-wave rectifier into a reservoir at +340 V. There is no choke here (the 5E1 that follows adds one; the 5F1 and AA764 drop it again) — a plain 500 Ω resistor drops the rail to +320 V, which supplies only the 6V6 plate (through the output-transformer primary); a 25 kΩ resistor then drops that node further to +280 V, and it is this third rail — not the plate's own +320 V node — that the drawing routes to the 6V6 screen, alongside the whole 6SJ7 plate/screen circuit. So the single output tube's screen shares a dropper-filtered rail with the preamp rather than riding its own plate's node, which is the opposite of where a reader would guess it sits at a glance. Three 8 µF / 450 V cans do the filtering — the same three-can arrangement the 5E1 keeps, just filtered through resistors instead of the choke the 5E1 introduces.

The 6SJ7, and why it has no cathode resistor

The 5C1's preamp tube is an octal metal pentode, not the 9-pin 12AX7 twin triode the line switches to at the 5E1 and keeps from then on (the 5D1 in between stays on the 6SJ7) — grid No.1 comes out on a base pin rather than a top cap, which is what let Fender mount it flat on a chassis-board like everything else (see the 6SJ7 tube page). Fender bias this stage the cheapest way available: grid-leak (contact) bias. The cathode ties straight to ground, and a single 5 MΩ resistor returns the grid to ground too — with no cathode resistor anywhere in the stage. A real 6SJ7 run this way self-biases to a small negative grid voltage from grid contact potential and rectified grid current, not from a cathode voltage drop. It is a real, period-correct circuit — the printed chart puts its grid at −0.5 V with the cathode at ground — and it disappears from the Champ line entirely once the 5E1's 12AX7 arrives with conventional cathode-biased stages.

A documented model limitation, not a circuit claim

The project's tube models are fitted from datasheet anchor points in the Koren model form and explicitly carry no grid-current path — a stated limitation of the corpus's tube models, and confirmed in the 6SJ7 model's own subcircuit, which ties the grid node only to the plate/cathode through AC-only Miller capacitances. Contact bias is exactly the mechanism that gap can't reach: with nothing but a capacitor and a to-ground resistor at the grid, this DC deck has no current path that would pull the grid negative, so it settles the grid at 0 V where the sheet prints −0.5 V. The screen follows it: the sheet prints +21 V there, and the deck settles near +12 V. Those two figures are carried in the voltage table as reported, not gated. The 6SJ7 plate is gated like every other pin, and it lands inside the chart's ±20 % — +149.3 V against the printed +130 V — but with the stage's grid and screen off, that agreement says the plate voltage matches, not that the 6SJ7's operating point does. The B+ rails and the 6V6 stage do not depend on this mechanism and are chart-gated as usual.

Verification

The layout sheet prints a voltage chart (Fender's usual "read to ground with an electronic voltmeter, ±20%"): +340 V at the rectifier, +320 V where the 500 Ω meets the 25 kΩ, +280 V on the far side of the 25 kΩ (lettered at the 6V6's screen), the 6V6's plate and cathode at +300 V and +16 V, and the 6SJ7's plate, screen and grid at +130 V, +21 V and −0.5 V. The simulation is gated on four of them and lands inside Fender's own ±20% on all four: the +320 V junction 1.4% off (+324.4 V), the shared screen/preamp rail 10.8% off (+249.8 V), the 6SJ7 plate 14.8% off (+149.3 V) and the 6V6 cathode 4.0% off (+15.4 V). The shared rail runs low because the 25 kΩ carries 3.0 mA in the simulation against the 1.6 mA the printed 320 → 280 V drop implies, most of it the 6V6's screen current. The 6SJ7's screen and grid are reported, not gated (see above). The 6V6 plate pin is not a node of its own here: the model omits the output transformer's primary resistance, which drops the 20 V between the junction and the pin.

The screen node is where this circuit punishes a careless reading. Tie the 6V6 screen to the +320 V junction instead of the +280 V node the drawing actually feeds it from and the tube draws hard enough to lift the simulated cathode to +20.4 V — 27.5% over the printed +16 V — while the shared rail, relieved of the screen current, climbs to +303 V and takes the 6SJ7's plate to +180 V against its printed +130 V, a 38.6% miss. The chart catches it on two nodes at once, which is the point of gating against it.

The board, as the factory drew it

A genuine factory layout page exists for this circuit — page 2 of the same F-DH sheet — so the board order and the point-to-point wiring here are read from it rather than derived. It runs 6SJ7 preamp stage, 6V6GT output, resistor-dropped B+ chain, 5Y3GT rectifier, left to right: the reverse of the rectifier-first reading order the four later Champs in this corpus (5E1, 5F1, AA764 and the Vibro Champ AA764) all use.

The drawn wiring is proved electrically equivalent to the simulated netlist, so this board carries a verified wiring claim, and both drawing styles render with zero collision-lint findings and no waiver. One detail is worth naming because it is easy to get wrong on a circuit this small: the coupler out of the 6SJ7's plate feeds the volume pot, not the 6V6 grid directly, so a model that bridges plate to grid short-circuits past the control the drawing routes it through. It makes no difference to any simulated voltage — SPICE treats a coupling cap as open at DC either way — and every difference to whether the equivalence proof means anything.

The heater layer: a link removed, and what the sheet shows

The board drawing here used to close its heater chain with a link across the last socket's two heater pins. Nothing on the F-DH sheet joins them, and the 6SJ7 has two heater pins and no centre tap, so those two pins are the supply's two legs and a link between them is a short across it. The link is gone.

What the sheet shows, read at its full published resolution, is now stated on this page as checked data. The schematic page draws the 6.3 V secondary with one lead grounded and the other marked to all 6.3 volt filaments — the single-ended supply the other two tweed Champs here, the 5E1 and 5F1, declare as well. On the layout page each socket returns to chassis at the socket: at the 6V6GT a straight lead crosses from heater pin 7 to the base sleeve at pin 1, which the sheet grounds, leaving pin 2 the fed side; at the 6SJ7 a short bow ties heater pin 2 to the suppressor grid at pin 3, which this circuit grounds, leaving pin 7 the fed side. The two bottles therefore ground opposite heater pins, which is exactly the kind of fact no pin label can supply and only the amplifier's own drawing carries. Both bottles are octal with two heater pins and no centre tap, so the grouping itself was never in doubt here — only which of the two each socket returns on, and that is now declared and machine-checked against the leads drawn.

Corrections to this reference

  • — 5C1 input resistors. The two 75 kΩ input resistors now sit in series between each jack and the first stage's coupling capacitor, as the factory drawing shows. Both drawings had placed them as shunts to ground with the jacks wired straight to the capacitor. No operating point changes. View correction

All published corrections · Subscribe to corrections

Sources

Found a value, a date or a claim on this page that doesn't match its source? Report a correction for 5C1 ↗ — the form opens with this page and its circuit id filled in, and asks what you checked it against. Every correction is reviewed before anything changes.

Related circuits

Same family line
5E1, 5F1 · The Champ line ↗