I'm trying to replace some old leaky capacitors for this radio I'm working on and in the data sheet they have this rating of cycles for the large capacitors near the bottom of the sheet here: data sheet

What does this mean? Should this be taken into account when I go looking for the capacitors? I've done some research and been to a few stores but it seems none of them post the "cycles" of these larger capacitors. Could this be some specification for older technology that isn't used any more? I'm pretty sure all these capacitors in this list are films with the exception of the bottom two -- they'll be electrolytic I presume.

If anyone here with experience in older components wouldn't mind letting me know a thing or two I'd be very thankful.

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    \$\begingroup\$ There was a time when not all power was 50-60Hz. If I recall America still has some electric railroads with 25cycle power, keep in mind electric railroads are already weird because they are single-phase, which doesn't attach to a 3-phase grid very well, and so they have a lot of their own generation infrastructure already. \$\endgroup\$ Nov 24, 2017 at 1:01
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    \$\begingroup\$ Hz used to be called cycles (short for cycles-per-second). There's some old US Navy electronics training videos that talk about radio transmissions having kilocycles and megacycles. \$\endgroup\$
    – user253751
    Nov 24, 2017 at 6:44
  • \$\begingroup\$ And before GHz, we had KMC...or kilomegacycle! \$\endgroup\$ Nov 26, 2017 at 4:50
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    \$\begingroup\$ @Harper Quite a late response, but I seem to recall reading that there's still an operational, if small, grid on the US-Canada border that operates at 16.67 Hz. \$\endgroup\$
    – Hearth
    May 6, 2019 at 14:28

2 Answers 2


It's your mains frequency (cycles per second). And MFD means microfarads, which you probably know already. You won't find a spec like that on the capacitor datasheet.

That's really, really old. 25Hz has not been used widely in a very long while. Your mains frequency is either 50Hz or 60Hz, so 6uF is the value. Expect a modern replacement to be much more compact.

Similarly the voltage needs to be rated at least as high as the original, but higher isn't a problem.

You need to make sure you have the correct type of capacitor- if you put a polarized capacitor on an AC circuit you could have more excitement than you probably want.

Given the age- the other parts may be paper capacitors- paper and foil wrapped together and dipped in wax. You can generally replace them with film capacitors.

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    \$\begingroup\$ Wow thanks this actually makes a lot of sense. Not sure how I missed it. And yeah the schematic is mostly non polarized symbols with the exception of those two larger caps, they're definitely polarized. Thanks! \$\endgroup\$
    – Capn Jack
    Nov 24, 2017 at 1:04
  • \$\begingroup\$ If the preceding diode is a vacuum tube, I’d advise to stick to the 6µF value as stated. \$\endgroup\$
    – greg
    Nov 24, 2017 at 8:31
  • \$\begingroup\$ @greg it surely is- do you think that will be an issue with early vacuum tube rectifiers? \$\endgroup\$ Nov 24, 2017 at 13:29
  • \$\begingroup\$ @SpehroPefhany yes it is. Vacuum tube rectifiers rely on a space charge. Putting a too large capacitor just after the rectifier would make it to deliver more than its max peak current, exhausting its space charge. It will probably not make a audible difference at first but it will quickly degrade the cathode and dramatically shorten the tube lifetime. \$\endgroup\$
    – greg
    Nov 25, 2017 at 10:52
  • \$\begingroup\$ As a side note, in the SI system, MFD would rather be "mega farad diameter" or some nonsense like that. While µF means micro farad (or the electronic industry de facto standard: uF). \$\endgroup\$
    – Lundin
    Dec 4, 2017 at 15:29

I imagine that's either referring to the frequency of the power supply (the 50-60 cycles may be the hint, since those are the standard mains frequencies of Europe and the USA, respectively) or referring to the output frequency of an oscillator that is adjusted by the capacitance. In the first case it may be recommending specific capacitor values based on the supply frequency. Another possibility is that it's telling you which values to use in order to obtain different oscillator frequencies. Without more context it's difficult to say for sure.


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