Capacitors and Prasit

Shop: Hey Prasit, what’s up?

Prasit: I need a 10 microfarad 35 volt axial for a dbx compressor.

Interviewer: I suspect she’ll find one. That’s a lot of capacitors.

Shop: We’d always had a lot in stock but a few years ago, a local store called Pacific Radio decided to stop selling capacitors, and we bought out their inventory. I was quite pleased at first, but we soon realized that even though we now had most almost every style and value ever needed – what about the others? David and Cholada spent a month building a capacitor database and then buying up every weird style and value they needed to make the stock complete. Besides the usual electrolytic axials and radials they bought Orange Drops, micas, disks, polypropolenes, metalized film, ceramic – you name it. Japanese gear uses a lot of non polarized caps, so we stocked up and created a whole separate section of NP’s. Of course we also have hundreds of big can type capacitors, along with variable caps, precision caps, and safety caps.

Interviewer: What’s a safety capacitor?

Shop: Safety capacitors are used to replace old paper capacitors that were used across the AC line in older tube gear. Sometimes they’re used to provide a pseudo ground on a piece of gear with an ungrounded AC cord. Safety capacitor are extra rugged. Unlike some other capacitor types, if they fail, they won’t explode.

Interviewer: There’s a lot of voodoo and myths about capacitors.

Shop: Yeah, and David and the guys know that lots of it is bunk. Capacitors are crucial to sonics though, and any piece of gear that’s more than about 10 years old ought to have its caps replaced, or at least checked.

Interviewer: What’s the best way to test a capacitor?

Shop: Good question, but there’s no easy answer. Lots of guys use inexpensive digital meters but they can be very inaccurate, and generally won’t see leakage, which is a very common failure. ESR meters are neat, but they’re not precise either. And for tube gear, high voltage performance is very important, but few modern instruments can test at high voltages.

A few years ago we picked up a TO-5 Capanalyzer, a high quality instrument made in the 60’s by Sprague, a major capacitor manufacturer that’s still around. Ironically, most of the capacitors in the T0-5were bad, and it had other problems. David restored it but no calibration info was available, so he reverse engineered the circuit and created a detailed alignment procedure. Other Sprague owners jumped on it. (The document is available here on the antiqueradios website.

Interviewer: That’s the most shameless, thinly veiled, self serving promotion I have ever heard.

Shop: Bite me.

Interviewer: Let’s step over to this other group of parts, which is clearly your resistor stock.