Units & notation

Resistance — Ω, kΩ, MΩ

Ohms scale in steps of a thousand, same as every metric prefix. Most values in this archive sit in the kΩ range; only grid-leak and grid-return resistors commonly reach into MΩ.

Shorthand drops the decimal point in favor of the unit letter itself — 4k7 for 4.7 kΩ, 1M for 1 MΩ — so a smudged or faded decimal point can't turn 4.7 kΩ into 47 kΩ. The resistor color code calculator reads and writes exactly this shorthand.

Capacitance — µF, nF, pF

Capacitance in a tube amp spans a much wider range than resistance — from a handful of picofarads in a treble-peaking mica to tens of microfarads in a filter cap, six decades in one chassis.

American tube-era documentation almost never uses nF at all — mica and ceramic parts are given directly in pF, everything larger jumps straight to µF, written out longhand as MFD. (the µ symbol was awkward on a typewriter and harder still on a drafting pen). This archive uses µF throughout instead; both mean the same microfarad. The capacitor code chart & decoder converts between all three units and the three-digit codes printed on modern parts.

Decibels (dB)

A decibel is never an absolute quantity — it's a ratio, expressed on a logarithmic scale so that multiplying signals turns into adding decibels. Two forms turn up here, and they differ by a factor of two because power goes as voltage squared:

Voltage ratio: dB = 20 · log₁₀(V₁ / V₂)
Power ratio: dB = 10 · log₁₀(P₁ / P₂)

Every dB figure elsewhere in this archive is the voltage form — the tube substitution guide's Δ dB column compares AC voltage gains, and the coupling-cap corner in the amp builder calculators is conventionally called the "−3 dB point," the frequency where output has fallen to about 71% of input — half the power, not half the voltage.

Concretely: the gain ladder's 5751 sits at −2.9 dB against a 12AX7 — about three-quarters of the voltage, reading the table above between 0 and −6 dB. Its 12AU7 sits at −11.8 dB, about a quarter — between −6 and −12 dB. Same table, real numbers.

Tolerance, and why "±20%" means two different things

Tolerance is simply the allowed spread around a stated value. This archive uses the number 20% in two unrelated places, and it is worth keeping them apart.

The first is a component's own tolerance: the resistor color-code calculator's uncoloured fourth band, or a capacitor's M letter, both mean the part itself was sold at ±20% of its stamped value — see the resistor color code calculator and the capacitor code chart for the full tolerance tables.

The second is Fender's own reading tolerance, printed directly on the factory layout drawings this archive cites — the "NOTICE" box on both the "Deluxe" 5E3 and the "Bassman" 5F6-A layouts reads: "Voltages read to ground with electronic voltmeter, values shown ± or − 20%." That is a statement about how loosely a bench voltmeter reading was expected to match the chart — the same figure this project's own simulation-versus-chart verification is judged against (see any verified circuit's "Operating point vs. published chart" section, where the deviation is checked against exactly that convention).

The two are unrelated, and on the 5F6-A drawing they are even different numbers: its own footnote reads "Note — all resistors are one-half watt, 10% tolerance, unless otherwise noted" — a tighter component tolerance than the 20% voltage reading printed a few inches above it, on the same sheet.

Reading the shorthand on the original drawings

This archive's own redrawn schematics spell every value out in full — 0.1 µF · 400 V, 250 Ω · 5 W — but the factory drawings behind them compress a part's value and its second rating onto one short line, right beside the symbol, with no separate parts-list column at all.

The pattern is a dash standing in for a slash: value, then whichever second rating actually matters for that part — working voltage for a capacitor, power dissipation for a resistor big enough to need one stated. Ordinary ½ W carbon resistors get no such suffix at all on these drawings, just a bare number with a K or MEG. — the wattage is implied by the sheet's own footnote (see the tolerance note above), not written per part. That K/MEG. suffix is the same Ω ladder covered above, just hand-lettered eighty years before this archive's calculators were.