5F2-A Tweed Princeton‑style · 1957–1961 · 5 W

draft
Schematic — redrawn in KiCad · scroll to zoom, drag to pan
Board layout — redrawn reference diagram · source noted on the drawing Print sheet ↗
5F2-A 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 circuit's simulated netlist. 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 not machine-checked; the CI report enumerates every one of their terminals so the boundary is explicit rather than silent.

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.

Node Chart Simulated Deviation Tolerance Note
BP2 325 V 308.8 V 5.0% ±10% 6V6 screen node, after the 10 kΩ dropper
BP3 250 V 267.2 V 6.9% ±10% preamp node, after the 22 kΩ dropper
P1A 170 V 174.2 V 2.5% ±20% V1A 12AX7 plate, reference pin value
P1B 170 V 171.4 V 0.8% ±20% V1B 12AX7 plate (22k feedback on its cathode)
K2 19 V 19.3 V 1.5% ±20% 6V6GT cathode across 470 Ω 1 W
K1A — 1.4 V — — informational — V1A cathode, ~1.5 V self-bias
K1B — 1.3 V — — informational — V1B cathode; 22k speaker feedback parallels the 1.5k at DC

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 — 360 V on the plates with a 470 Ω cathode resistor — and marks where the load line crosses. This circuit is published as a draft, so that operating point is not verified against a published chart: the table and notes above say what each figure rests on.

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. The 5F2-A has one channel across two input jacks (high/low), each with its own 68 kΩ stopper sharing the 1 MΩ leak. V1B's cathode is deliberately UNbypassed — the 22 kΩ feedback resistor from the speaker works against it. Power transformer is listed at the end without a designator; its HT winding voltage is not printed on the drawing.

RefPartValue / ratingRole
R1 Carbon comp resistor 68 kΩ · ½ W Input grid stopper, jack 1 (high)
R2 Carbon comp resistor 68 kΩ · ½ W Input grid stopper, jack 2 (low)
R3 Carbon comp resistor 1 MΩ · ½ W Input grid leak
R4 Carbon comp resistor 100 kΩ · ½ W V1A plate load
R5 Carbon comp resistor 1.5 kΩ · ½ W V1A cathode bias
R6 Carbon comp resistor 100 kΩ · ½ W V1B plate load
R7 Carbon comp resistor 1.5 kΩ · ½ W V1B cathode bias (unbypassed)
R8 Carbon comp resistor 220 kΩ · ½ W 6V6 grid leak
R9 Wirewound resistor 470 Ω · 1 W 6V6 cathode bias
R10 Power resistor 10 kΩ · 1 W Rail dropper B+1→B+2
R11 Carbon comp resistor 22 kΩ · ½ W Rail dropper B+2→B+3
R12 Carbon comp resistor 22 kΩ · ½ W Negative feedback, speaker→V1B cathode
VR1 Linear-taper potentiometer 1 MΩ Volume
VR2 Linear-taper potentiometer 1 MΩ Tone
C1 Electrolytic capacitor 25 µF · 25 V V1A cathode bypass
C2 Coupling capacitor 0.02 µF · 400 V V1A → tone/volume network
C3 Mica capacitor 0.005 µF Tone — treble shunt to ground
C4 Mica capacitor 500 pF Tone — network to V1B grid
C5 Coupling capacitor 0.02 µF · 400 V V1B → 6V6 grid
C6 Electrolytic capacitor 25 µF · 25 V 6V6 cathode bypass
C7 Electrolytic capacitor 16 µF · 450 V Filter, B+1
C8 Electrolytic capacitor 16 µF · 450 V Filter, B+1 (paralleled)
C9 Electrolytic capacitor 8 µF · 450 V Filter, B+2
C10 Electrolytic capacitor 8 µF · 450 V Filter, B+3
C11 Film capacitor 0.05 µF · 600 V Across the AC line
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 (part 265; impedance not marked on the drawing; typical SE 6V6 load)
— Power transformer HT c.t. · 6.3 V · 5 V Part 66079; HT winding voltage not printed on the drawing

Circuit story

The tweed Princeton is the 5F1 Champ grown up: the same one-12AX7, single-ended 6V6 signal path, but with a tone control added and negative feedback wrapped around the driver stage. Five watts of the same sweet breakup, now with a treble knob and a firmer, more focused voice than the Champ's. Produced 1957–1961; its direct ancestor is the 5F2 not yet documented in this archive, and it is the tone-control-bearing sibling of the tweed Champ already in this archive.

One naming caution: later amps that carry the Princeton name are different circuits entirely — the brown 6G2 (1961 on) and the blackface Princeton and Princeton Reverb moved to a push-pull pair of 6V6s at roughly 12–15 W. This page documents the single-ended tweed 5F2-A only.

Circuit walkthrough (short form)

Two input jacks (high/low, each a 68 kΩ stopper sharing a 1 MΩ leak) → V1A (12AX7, 100 kΩ plate load, 1.5 kΩ bypassed cathode) → 0.02 µF coupling → tone/volume network → V1B (12AX7, 100 kΩ plate, 1.5 kΩ unbypassed cathode) → 0.02 µF coupling → V2 6V6GT (cathode-biased, 470 Ω 1 W, 220 kΩ grid leak) → single-ended output transformer → speaker.

The tone/volume network is a single interactive treble-cut: two 1 MΩ linear pots with a 0.005 µF shunt cap and a 500 pF series cap. It is entirely capacitor-coupled, so at DC V1B's grid simply returns to ground through the volume pot.

Negative feedback: 22 kΩ from the 8 Ω speaker output into V1B's cathode. Because that cathode is left unbypassed, the feedback has something to push against.

Power: the 66079 transformer → 5Y3GT full-wave → first node 32 µF (two 16 µF caps in parallel) at B+1 (6V6 plate) → 10 kΩ 1 W dropper → 8 µF B+2 (6V6 screen) → 22 kΩ dropper → 8 µF B+3 (both preamp plates).

The Champ with a backbone

Two small changes separate this circuit from the 5F1 and give the Princeton its character. First, V1B's cathode bypass capacitor is gone: the 5F1 bypasses that 1.5 kΩ for maximum gain, while the 5F2-A leaves it open so a global feedback loop can work against it. Second, that loop — 22 kΩ from the speaker back to the bare cathode — trades a little raw gain for a tighter, less compressed response and a touch more headroom. Add the treble-cut tone control and the result is an amp that cleans up and articulates where the Champ simply flatters.

At DC none of this changes the operating points: bypass caps and the whole tone network are invisible to a DC meter, so the 5F2-A idles exactly like a Champ. The one visible fingerprint of the feedback loop is that V1B's cathode sits a hair lower than V1A's — the 22 kΩ quietly parallels its 1.5 kΩ to ground.

Verification

The published K-EG drawing supplies every component value but prints no voltage chart — only Fender's standard ±20 % measurement notice. Because the 5F2-A's DC signal path is identical to the tweed 6V6 single-ended Champ (the tone control is capacitor-coupled and idles at nothing), simulation is checked against Rob Robinette's published DC measurements for that circuit: B+ 360 / 325 / 250 V, a ~19 V drop across the 6V6's 470 Ω cathode resistor, and 12AX7 plates near 170 V. Driving B+1 at 360 V, every compared node lands within about 8 %: the 6V6 cathode within about 2 % (19.4 V vs ~19 V, 2.1 %), the screen and preamp rails within 5–8 %, and both 12AX7 plates within 7 %.

The match is well inside Fender's ±20 % convention, but the reference is a sibling model rather than the 5F2-A itself — a different power transformer could shift the absolute B+ — so this circuit is published as a draft pending a voltage chart measured on a 5F2-A.

Sources

Related circuits

Same family line
6G2, AA964 · The Princeton line ↗
Same tone stack
5D3, 5E3 · Single tone control — every amp ↗