Capacitors Guide

Capacitor Fundamentals: Types, EIA Code Converter & Testing Guide

A comprehensive look at the core capacitor types carried by Evatco, their sonic and electrical differences, common tube-amp applications, and an interactive tool to translate 3-digit printed markings into microfarads ($\mu\text{F}$), nanofarads ($\text{nF}$), and picofarads ($\text{pF}$).

1. Key Capacitor Types Carried by Evatco

Each dielectric construction offers distinct electrical characteristics, temperature tolerances, and tonal properties inside valve circuits.

Electrolytic Capacitors

Why They are Different: Uses a liquid or gel electrolyte to achieve ultra-high capacitance values in a compact body. They are polarized (must be installed with correct orientation).

Common Uses: High-voltage power supply filter reservoirs, B+ smoothing stages, and cathode bypass circuits.

Polyester Film (Mylar / Axial)

Why They are Different: Offers a warm, smooth top-end roll-off and classic vintage frequency response. Very durable with excellent self-healing properties under minor over-voltage spikes.

Common Uses: Signal coupling between gain stages and tone stacks in vintage Fender, Marshall, and Gibson circuits (e.g., Mallory 150 series).

Polypropylene Film

Why They are Different: Features an extremely low dissipation factor, low dielectric absorption, fast transient response, and pristine high-frequency clarity with tight bass punch.

Common Uses: Modern high-gain amplifiers, hi-fi valve audio preamps, and heavy-duty power supply snubbers (e.g., Orange Drop 715P/716P, Solen).

Silver Mica

Why They are Different: Built with silver electrodes screened directly onto natural mica sheets. Exceptionally stable, extremely tight tolerances ($\pm5\%$), high voltage ratings, and zero microphonics.

Common Uses: Low-value pF applications: treble tone controls, bright caps across volume pots, and phase inverter snubbers.

2. Capacitor Code Converter (EIA 3-Digit Lookup)

Select a standard 3-digit EIA capacitor code to instantly convert its rating to Picofarads ($\text{pF}$), Nanofarads ($\text{nF}$), and Microfarads ($\mu\text{F}$), along with its common dielectric match.

Select Capacitor Printed Code: Format: First 2 digits = value, 3rd digit = multiplier ($10^x\text{ pF}$)

Letter Code (Tolerance & Voltage): Common Suffixes: J ($\pm5\%$), K ($\pm10\%$), M ($\pm20\%$)

Picofarads (pF) 22,000 pF Base Unit ($10^{-12}\text{F}$)

Nanofarads (nF) 22 nF European Standard

Microfarads (µF) 0.022 µF US / Vintage Schematic Rating

3. Essential Tools for Testing Capacitors

How to accurately diagnose open, shorted, leaky, or high-ESR capacitors depending on whether they are out-of-circuit or wired in an amplifier.

In-Circuit Testing (Connected to Amp)

Primary Tools: In-Circuit ESR Meter & Multimeter (DC Voltage mode).

Procedure:

ESR Meter: Uses low-amplitude AC pulses (<100mV) to test Equivalent Series Resistance without turning on surrounding semiconductor junctions. High ESR indicates a dried-out cap.
DC Leakage Check: Power up the chassis and test the grid pin side of a coupling capacitor. Any positive DC voltage passing through to the grid pin indicates a leaking coupling cap that must be replaced.

Out-of-Circuit Testing (Isolated Bench Test)

Primary Tools: Dedicated LCR Meter or Insulation Resistance Tester (Megohmmeter).

Procedure:

LCR Meter: Measures exact capacitance value, dissipation factor ($D$), and ESR across targeted frequencies (e.g., 100Hz / 1kHz).
High-Voltage Megohmmeter: Applies actual working DC voltage (250V–500V) across film or paper-in-oil caps to detect dielectric breakdown under high-voltage load. Standard DMM capacitance modes cannot perform this test!

HIGH VOLTAGE SAFETY WARNING

Power supply filter capacitors store high-voltage Direct Current (300V to over 500V DC) that can remain stored for days after an amplifier is turned off and unplugged. Never touch internal circuits or work with a screwdriver without first safely discharging all filter capacitors and confirming 0V DC with a multimeter.

Guide to Capacitors: Voltage Ratings, ESR, Safe Discharging & Coupling Caps

Understanding capacitor specifications is crucial for maintaining audio clarity, power supply stability, and bench safety when building or servicing valve amplifiers.

Filter Capacitor Discharge Time Calculator

Determine the required Bleeder Resistor value and estimate how quickly a high-voltage filter cap will safely drop below 10V DC.

Initial B+ Supply Voltage (DC Volts):

Total Power Supply Capacitance (µF):

Discharge Bleeder Resistor (kΩ): e.g., 220kΩ bleeder resistor or 10kΩ safety discharge tool

Resistor Power Dissipation Rating:

Safe Discharge Time (<10V) -- sec Approx. 4 time constants ($4\tau$)

Peak Resistor Power Dissipation -- W Select resistor wattage rating higher than this

Initial Discharge Current -- mA Current draw through bleeder tool

1. Voltage Ratings & Equivalent Series Resistance (ESR)

Working Voltage (WVDC) vs. Peak Voltage

WVDC (Working Voltage DC) is the maximum continuous DC voltage a capacitor can sustain without dielectric breakdown. At power-on before valves heat up, solid-state rectified B+ voltage spikes up to $V_{RMS} \times 1.414$. Always select capacitors with a voltage rating at least 20% higher than your peak unloaded B+ voltage.

Understanding ESR (Equivalent Series Resistance)

ESR represents internal resistive losses within the capacitor structure. High ESR in power filter capacitors reduces ripple filtering capability, increases internal heat build-up, and causes high-frequency power supply sag. Old electrolytic caps gradually dry out, resulting in high ESR and audible power supply hum.

2. How to Safely Discharge High-Voltage Filter Capacitors

Never short filter capacitor leads directly to ground with a screwdriver! Screwdriver shorting produces explosive spark discharges, melts capacitor lugs, degrades the internal foil dielectric, and can ruin power supply rectifiers.

Standard Bleeder Resistor Circuit

Install a permanent 220kΩ to 470kΩ, 2W to 5W power resistor across the first reservoir filter capacitor (from B+ to ground). This slowly bleeds stored voltage down to 0V within 1 to 2 minutes after switching off power.

Building a Workbench Discharge Tool

Solder a 10kΩ 5W power resistor in series between an insulated probe and an alligator clip lead. Clip the lead to the metal chassis and touch the probe to Pin 1 or Pin 6 of an anode preamp valve socket for 10 seconds to safely bleed the supply without sparks.

3. Selecting Coupling Capacitors (Tone & DC Blocking)

Coupling capacitors pass AC guitar signals between gain stages while blocking massive DC operating voltages (200V–400V DC) present on the valve anodes.

Capacitor Type	Dielectric Material	Tonal & Technical Characteristics	Common Applications
Polyester (Mylar)	Film (e.g., Mallory 150, Orange Drop 225P)	Warm, smooth high-frequency rolloff; classic mid-range warmth. Very reliable.	Vintage Fender, Gibson, and Marshall re-issues for classic blues/rock tones.
Polypropylene	Film (e.g., Orange Drop 715P/716P, Solen)	Ultra-clear, fast transient response, tight bass response with minimal distortion.	High-gain modern amps, hi-fi valve audio, dynamic bass amplifiers.
Paper in Oil (PIO)	Hermetically Sealed Oil	Ultra-smooth vintage top-end, highly musical harmonic distortion when pushed.	Boutique vintage restorations, custom audiophile preamps.
Silver Mica	Mica Dielectric	Precise low-capacitance values (pf range), high stability, zero microphonics.	Tone stack treble caps, bright caps across volume controls, phase inverter snubbers.