5F1 Tweed Champ‑style · 1958–1964 · 5 W

✓ verified 2026-07-18 · 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
7 reference nodes are within tolerance. Maintainer verification is recorded for 2026-07-18; 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 ↗
5F1 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 14.9%.

Node Chart Simulated Deviation Tolerance Note
BP2 295 V 291.5 V 1.2% ±5% 6V6 screen node, after the 10 kΩ dropper (R10)
BP3 250 V 252.1 V 0.8% ±5% preamp node, after the 22 kΩ dropper (R11)
P1A 150 V 163.9 V 9.2% ±20% 12AX7 plate, chart pin value
P1B 150 V 161.2 V 7.5% ±20% 12AX7 plate, chart pin value
K1A 1.5 V 1.3 V 11.7% ±20% 12AX7 cathode, chart pin value
K1B 1.5 V 1.3 V 14.9% ±20% 12AX7 cathode (NFB 22k parallels at DC)
K2 18 V 18.0 V 0.2% ±20% 6V6GT cathode, chart pin 8

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 — 340 V on the plates with a 470 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). One part the drawing carries as an annotation rather than a numbered symbol is listed here without a designator, and stands outside that check. Numbering follows the schematic; gaps (R12, C2, C4) are intentional. Power transformer and chassis hardware are listed at the end without designators.

RefPartValue / ratingRole
R1 Carbon comp resistor 1 MΩ · ½ W Input leak, jack 1's tip to ground (the V1A grid returns through R3)
R2 Carbon comp resistor 68 kΩ · ½ W Input 2 grid stopper
R3 Carbon comp resistor 68 kΩ · ½ W Input 1 grid stopper
R4 Carbon comp resistor 1.5 kΩ · ½ W V1A cathode bias
R5 Carbon comp resistor 100 kΩ · ½ W V1A plate load
R6 Carbon comp resistor 1.5 kΩ · ½ W V1B cathode bias
R7 Carbon comp resistor 100 kΩ · ½ W V1B plate load
R8 Wirewound resistor 470 Ω · 5 W 6V6 cathode bias
R9 Carbon comp resistor 220 kΩ · ½ W 6V6 grid leak
R10 Power resistor 10 kΩ · 2 W Rail dropper B+1→B+2
R11 Power resistor 22 kΩ · 2 W Rail dropper B+2→B+3
R13 Carbon comp resistor 22 kΩ · ½ W Negative feedback, speaker→V1B cathode
VR1 Audio-taper potentiometer 1 MΩ Volume
C1 Coupling capacitor 0.02 µF · 600 V V1A → volume
C3 Coupling capacitor 0.02 µF · 600 V V1B → 6V6 grid
C5 Electrolytic capacitor 16 µF · 450 V Filter, B+1
C6 Electrolytic capacitor 25 µF · 25 V 6V6 cathode bypass
C7 Electrolytic capacitor 8 µF · 450 V Filter, B+2
C8 Electrolytic capacitor 8 µF · 450 V Filter, B+3
V1 Preamp tube 12AX7 Both gain stages (V1A/V1B)
V2 Power tube 6V6GT Single-ended output
V3 Rectifier tube 5Y3GT Full-wave rectifier (V3A/V3B)
T2 Output transformer ≈5 kΩ : 8 Ω Single-ended output (impedance not marked on the drawing; typical SE 6V6 load)
— Power transformer 325-0-325 V · 6.3 V · 5 V HT + heaters + rectifier filament

Circuit story

The smallest amp in the tweed canon and the clearest illustration of a complete guitar amplifier: one 12AX7 providing two gain stages, a cathode-biased 6V6GT single-ended output, a 5Y3GT rectifier, and nothing else — no tone control, no phase inverter, five watts of pure signal path. Produced 1958–1964; direct ancestor: the mid-1950s 5E1.

Circuit walkthrough (short form)

Two input jacks, each through its own 68k grid stopper (a 1M leak from jack 1's tip to ground) → V1A (12AX7, 100k plate load, 1.5k cathode — unbypassed on the drawing, as is V1B's) → 0.02 µF coupling → 1M audio volume pot → V1B (12AX7, 100k plate, 1.5k cathode) → 0.02 µF coupling → V2 6V6GT (cathode-biased, 470 Ω 5 W, 220k grid leak) → single-ended output transformer (≈5 kΩ : 8 Ω, typical for a single-ended 6V6; the drawing doesn't mark it) → speaker. Negative feedback: 22k from the speaker jack into V1B's cathode (at DC this parallels the 1.5k through the secondary's near-zero DCR).

Power: 325-0-325 PT → 5Y3GT full-wave → filter nodes 16 µF / 8 µF / 8 µF (450 V) separated by a 10k and a 22k dropper (all confirmed in print on the K-EE sheet): B+1 340 V (output plate) → B+2 295 V (screen) → B+3 250 V (preamp plates).

How simulation pinned down the rail dropper

The published chart marks B+1 340 V, B+2 295 V, B+3 250 V. Driving B+1 at 340 V, simulation discriminates the second dropping-resistor value cleanly — a nice example of what simulation-verified archiving can do:

Node Chart Sim with 10k Sim with 22k
B+2 295 V 291 V (1.3%) 291.5 V (1.2%)
B+3 250 V 273 V (9.2% off) 252 V (0.8%)

With 22k, the downstream stage voltages fall into line as well. Conclusion: the second dropper is 22k — a value some descriptions of this circuit get wrong — and the schematic's printed 22K marking confirms it.

A correction: the heater wiring this entry drew, and what the sheet shows

The board drawing on this page shipped its 6.3 V heater wiring wrong, and the correction is recorded here rather than quietly made.

The 12AX7 has a centre-tapped heater, which lets one valve run from either supply: at 12.6 V the two heater ends sit on opposite legs and the centre tap carries none, while at 6.3 V the two ends are strapped together as one leg and the centre tap is the other. Both arrangements use the same three pins, so the pin names alone cannot say which an amplifier wires — and the drawing was built by a rule that read the pin names and always chose the series form. On this circuit that produced a lead to each heater end and then a link joining them, which reads as a short across the supply, with the centre tap left unwired.

The factory drawing shows the parallel arrangement, and shows the supply it belongs to. The schematic draws the 6.3 V secondary with one lead grounded at the transformer and the other marked to all 6.3 volt filaments and pilot lite — a single-ended supply, one wire out and the chassis back. The layout wires exactly that: one green transformer lead to the chassis-ground point beside the high-tension centre tap, the other to the pilot lamp and on to the 6V6GT's heater, then to the 12AX7; the 12AX7's two heater ends strapped together and its centre tap taken to ground; the 6V6GT's other heater pin taken to ground on its own. The 5Y3GT keeps its separate 5 V filament winding, which floats at the rectified B+ and is grounded nowhere.

What the drawing now states, it states as data: this circuit's supply voltage, the connection group each socket pin belongs to and which leg returns to ground are declared in the layout and checked — against the valve's own datasheet grouping, and against the leads the drawing draws.

Verification

Simulation is checked against the drawing's full printed voltage chart: the 6V6 cathode within 0.2 % (18.0 V vs +18 V), rails within 1.2 %, and the 12AX7 pins (+150 V plate, +1.5 V cathode) within the chart's own printed ±20 % convention — Fender measured on 1958 production tubes, while these models are datasheet-typical, so the preamp simulating slightly leaner than the era measurement is expected behavior. Planned refinements: output-transformer primary resistance in the DC deck, and full curve-traced tube models.

Corrections to this reference

  • — 5F1 second input resistor. Each input jack now has its own 68 kΩ resistor into the first stage, as the factory drawing shows. The parts list, schematic and board had carried one shared resistor. 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 5F1 ↗ — 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
5C1, 5E1 · The Champ line ↗