Microfarads and picofarads on capacitors

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Is there anybody around who likes to work on old tube radios? I've long had an interest in them, and now I'm looking to buy radios, fix them, and resell them. One thing I've found is that the old wax paper capacitors were all marked in microfarads, while modern caps are marked in picofarads if the rating is too small to be expressed concisely in microfarads. For example, one cap in the radio I've got on the bench now is marked as .0022 microfarads. Today such a small value would be expressed in picofarads, but I don't know how to convert the values. I think that 1000 picofarads would equal 1 microfarad, if so would the mentioned value be 22 picofarads? How about .22 microfarads, is it 2200 picofarads?

1 Farad
1000 milli-Farads
1,000,000 micro-Farads
1,000,000,000 nano-Farads
1,000,000,000,000 pico-Farads

.0022uF is 2200pF .22uF is 220,000 pF

I have a passion for old HiFi. Electrolytic caps go out of spec over time. So any equipment in the 25+ year range could benefit from a complete recap.there is a lot of snake oil out there regarding the sonic qualities of caps. What is important are caps with a lower tolerance, +/- 5% comes in at reasonably inexpensive.

Actually you got it wrong, it's:

milli = 0.001 or 10^-3
micro = 0.000001 or 10^-6
nano = 0.000000001 or 10^-9
pico = 0.000000000001 or 10^-12

To answer the OP question about comversion. it works like this.

to convert .0022 microfarads. to picofarads you would move the decimal point to the right by about 6 places.

.0022 micro =
2200.0 pico

Fnord is correct and is stating an equivalency, so 1 Farad == 1000 millifarads == 1E6 microfarads and so on. That is a bit confusing though. A better example might be .000 001 Farads == 0.001 millifarads == 1 microfarads == 1,000 nanofarads == 1,000,000 picofarads.

What you really need to watch out for is that sometimes when a cap was market mF, they really meant microfarads, and not millifarads. Almost nobody uses millifards, they use microfarads. This is because printing the micro symbol was hard to do (μ)

The Europeans use nanofarads, but in America its pretty much just micro or picofarads. Some EE's use the slang term 'puffs' for picofarads.

Also pretty much everybody uses the abbreviations, so its μF, sometimes nF, or pF. You need to use the program Character Map on a Windows machine to get that character, greek lower case mu.

At work we had an issue with with a second source of large electrolytic capacitors made in China (did you see that one coming?), so I visited Illinois Capacitor, the original supplier. An electrolytic capacitor is two sheets of metal with insulator, wrapped up in a spiral, and filled with a chemical, I think it was ethylene glycol, the component in radiator fluid. The capacitor is actually two capacitors back to back (think about it), a really big one reversed biased, and the regular one, forward biased. Say one is 30 and the other is 1. The overall capacitance is like resistors in parallel, so its (1 * 30)/(1+30), or ~0.968. When the cap dries out, the really big one starts getting lower, say 5, so it (1 * 5)/ (1+5) or 0.8333, and for reasons I can't remember, the equivalent series resistance increases. Once the ESR starts increasing, the cap generates more heat, which can make it dry out, which increases ESR, and so on.

He also had a reason why it is polarized, but I am not sure, maybe the insulator was thinner (to make the capacitance bigger), but then I can't remember why this was reverse voltage sensitive.

Your right! I misread it. I though Fnord was saying that a micro farad was a 1,000,000 farads and so in. oops! I was tired that night.