Type the number stamped on a capacitor and read its value in microfarads, nanofarads, and picofarads — plus the letter that tells you the tolerance. A conversion chart and an illustrated tour of vintage-amp cap types follow below.
Three-digit code: the first two figures are significant, the third is the number of zeros — so 103 is 10 followed by three zeros, 10,000 pF. A one- or two-digit number is the value in picofarads directly (47 = 47 pF); an R stands in for a decimal point (4R7 = 4.7 pF). A trailing letter is the tolerance.
One farad is enormous; amplifier values live far down the ladder. Each step is a factor of a thousand.
Common amplifier values, in all three units and their coded form. The last two columns are what you will actually see printed on the part.
| µF | nF | pF | Code | Typical use |
|---|---|---|---|---|
| 0.1 | 100 | 100,000 | 104 | Cathode bypass, coupling |
| 0.047 | 47 | 47,000 | 473 | Coupling |
| 0.022 | 22 | 22,000 | 223 | Coupling, bright channel |
| 0.02 | 20 | 20,000 | 203 | Coupling (older value) |
| 0.01 | 10 | 10,000 | 103 | Coupling, bypass |
| 0.0047 | 4.7 | 4,700 | 472 | Tone, coupling |
| 0.001 | 1 | 1,000 | 102 | Bright / treble cap |
| 0.0005 | 0.5 | 500 | 501 | Treble peaker |
| 0.00025 | 0.25 | 250 | 251 | Bright cap |
| 0.0001 | 0.1 | 100 | 101 | Treble bleed, stability |
| 0.000047 | 0.047 | 47 | 470 | Grid, stability |
The letter after the numeric code is the tolerance, read from the standard IEC 60062 (formerly EIA) component tolerance-letter code — so H is ±3%, J is ±5%, K is ±10%. On small picofarad parts it is an absolute figure; on larger film capacitors it is a percentage.
| Letter | Tolerance | Notes |
|---|---|---|
| B | ±0.1 pF | Absolute — small values |
| C | ±0.25 pF | Absolute — small values |
| D | ±0.5 pF | Absolute — small values |
| F | ±1% | — |
| G | ±2% | — |
| H | ±3% | — |
| J | ±5% | Common on film caps |
| K | ±10% | Common on film caps |
| M | ±20% | Common on ceramics |
| Z | +80% / −20% | Ceramics, non-critical |
Open an old chassis and you meet a handful of recurring shapes. Knowing them at a glance helps you find the coupling, bypass, and tone caps without tracing every lead. These are line drawings, not any one manufacturer's part.
Axial leads out each end, a band marking the outer-foil connection. The paper-dielectric versions are the classic leaky suspects in an old amp — worth measuring before you trust them.
A dipped disc on two radial leads. Cheap and stable enough for small values — you will find these as bright caps and around the phase inverter.
Polarized — mind the mark. Filters the power supply and bypasses cathodes. Vintage multi-section cans pack several capacitors into one aluminum tube.
A small molded block — very stable and low-loss. It holds its value for decades, which is why it turns up in tone-shaping and treble-peaking spots.
Not everything is coded. The earliest amps often mark a capacitor with its value spelled out — .02 400 V — rather than a three-digit number. When the printing has worn away, the era-component references in the library help pin down what belongs in a given position. For that shorthand exactly as printed on a factory drawing — .05-600 and the like — see the units & notation guide.