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Capacitors
Capacitors (or "caps") have a variety of uses:These are all functions of how a capacitor works - it lets a charge build up in a direction to a certain point, then will not let any more current flow. Caps have an impedance (similar to resistance) that varies with frequency. Caps will have a lower impedance at higher frequencies, meaning they pass higher frequencies more easily. The smaller the cap, the less it will pass lower frequencies.
- DC power storage
- filtering out AC
- DC blocking (keeping DC out of areas we don't want it)
- tone control - passing more or less high frequencies either to the next circuit, or to ground (to get rid of them)
Capacitors are measured in farads, but a farad is a huge value compared to what is needed in a typcial music amplifier or similar circuit. So caps are usually measured in microfarads (millionths of a farad) or picofarads (trillionths of a farad). Microfarads are usually denoted by the Greek letter mu, or often just a "u". Picofarads are usually represented by the letter "p". For instance, 100 microfarads would be "100u" and 1700 picofarads could be either "1700p" or .0017u). 22,000 picofarads would usually be ".022u".
At one time capacitors also used color codes, but this practice is extremely rare now. There were several variations for different types of caps, and for certain JAN caps. It's very unlikely you will run across such caps in instrument amps. If you do, you'll need to get a chart. (I'm not going to bother with them here since they're so scarce.)
The basic types of caps (from a visual standpoint) include:
It's actually more complex than this, but this covers the vast majority of tube amp caps.
- Disk capacitors - These are what they sound like - little disks. The most common are ceramic disks, but silver mica are also fairly common. Ceramic disks tend to drift and are prone to being microphonic; silver mica or Orange Drops are usually a better choice. They come in a variety of sizes from ~1/8" to 1" in diameter, and are fairly thin relative to that diameter. They come in a variety of colors, with brown, green, red, burnt orange and black being fairly common. The values are printed on one or both sides. The leads (wires) come out along the edge, fairly near one another. These are usually very small value caps. While you may see them as large as 10,000pF (.01 uF), most are much smaller.
Some "disk" capacitors will be oval or even rectangular, in shape.
- Orange DropsTM - These are bright orange. They are sort of a rounded, rectangular blob. The values are printed on one side. The leads come off one side, at either end. The first time you see one, you will probably recognize it. Other manufactureres make similar capacitors with different colored bodies; chocolate drops (brown) were fairly cheap and will probably need replacing, while blue and purple copies of the Orange Drops are available today and seem to work just fine.
- Tubular capacitors - These are tube shaped. They come in a huge varieity of shapes and sizes, from fairly small (1/2" long x 1/4" dia. is about as small as I'd expect to find in a tube amp) up to several inches long and more than an inch in diameter. They may have cases of paper, wax-dipped paper, plastic, metal, or something else. The leads might come out both ends or one end. The value[s] can be printed along the length or around the cap. Some of these will have + or - designations - these are electrolytic caps and must be installed with the ends oriented correctly.
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