# Formula question

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#### nhuwar

Ok just have a quick one.

The formula 1/2piRC= -3db point

The question I have is do I put in the capacitance is the value in farads or microfarads.

If the value is farads is the answer to the formula in hertz ???????

Thanks for the help.

Nick

#### Sherman

The formula I have in my "useful formulas" spreadsheet-

1/(2*Pi*R*C)*1,000,000

That way I can put the value in in microfarads since the spreadsheet is designed to use "real world" values and the formula converts it for me.

#### SY

C in farads, R in ohms. Or R in megohms, C in microfarads. Both give you the answer in Hz.

Gracias

Nick

#### nhuwar

Wait a sec I missed something before if you use microfarads and megohm is your out come in megahertz?

Nick

#### ErikdeBest

Hi

As SY said, the answer is still in Hz.

The F unit is divided by 1000000 to obtain microF. This 1000000 is multiplied with the R, to obtain the megaR.
100k is 0.1M (ohm).
100nF is 0.1microF.
and so on...

Good luck, Erik

#### nhuwar

Ok I was just making sure. I was getting some werd results thars all. I think my problem was I was thinking high pass instead of low.

Nick

#### richwalters

Reminds me of the early days when a 1mfd marked cap was actually 1uF in disquise ! some dreadful mistakes were made. A true 1mfd cap is quite a different animal.

SY remember this ?

r:-

#### SY

Yes, indeed. Especially on cheesy paper caps (which are now all the fashion rage).

Sadly I'm also old enough to remember mmf. And cps. I remember that QST, in one of their April issues (maybe 1967, 1968?), published a nomograph for cps to Hz conversions.

#### kevinkr

Paid Member
SY said:
Yes, indeed. Especially on cheesy paper caps (which are now all the fashion rage).

Sadly I'm also old enough to remember mmf. And cps. I remember that QST, in one of their April issues (maybe 1967, 1968?), published a nomograph for cps to Hz conversions.

I was about 9 when Batman first hit TV (1966?) and I too still remember cps, mmfd, and the physical components themselves - the electrically leaky wax covered paper things that often passed for capacitors, and those terrible carbon composition resistors with a tolerance of 20%, which always seemed to be way off the mark..

#### richwalters

This weeks pick of the junk box is this 120K 2W 10% resistor. Looks innocently cleanand not scarred. Barely used, the meter illustrates the value (155.8K) drifted with time.
And to cap it, I regulary come across old tube stuff with pretty well the same drift. And of course still work. It annoys me when tubies insist on 1% tolerance......what rubbish.

richj.-

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#### zigzagflux

We used to throw our carbon comps into the oven, restoring specifications on a good percentage of them.

Humidity seemed to be the greatest culprit.

#### nhuwar

You mean the caps covered with wax? When you would work on them and the room was warm and you touched them and it was like you sneezed on your hand.

Nick

#### Sherman

nhuwar said:
You mean the caps covered with wax? When you would work on them and the room was warm and you touched them and it was like you sneezed on your hand.

Nick

I sort of like the idea of them though, that something made from paper, aluminum foil and wax actually functions. Don't think I'd actually use them in an amp though.

In my high school basic electricity class one of the required project had us make our own caps and resistors and build a circuit that made a 1 1/2 volt light bulb flash at intervals. It was interesting and informative. Of course one of my friends and I decided to see how big a capacitor we could make with waxed paper and aluminum foil. The answer was "big enough to hurt"!

#### nhuwar

I have made caps but never with wax paper. I have used hdpe phenolic and borosilica. But then again I was running 15kvdc through them and paper anything wouldn't fly.

Nick

#### Miles Prower

kevinkr said:
I was about 9 when Batman first hit TV (1966?) and I too still remember cps, mmfd, and the physical components themselves

I have a few samples from back in those days. One even proved useful: a funkey capacitor with a 0.01uF / 5.0KV rating with two metal end caps, a glass body filled with what looks like a light oil. Used it for a damper across the HV secondary. Still works great. I also have a couple of 10uF / 450V electrolytics in a silver, cardboard box.

- the electrically leaky wax covered paper things that often passed for capacitors

They're Baaaaaack!

Cautions when using in a pentode circuit: The higher the grid resistance used, the greater the reduction of the insulation resistance will occur.

Do not use in power supplies.

And obviously as leaky as ever. That's just plain nutz these days.

, and those terrible carbon composition resistors with a tolerance of 20%, which always seemed to be way off the mark..

Well, of course they were! After all, they already culled out all the ones that were within +/-5.0% (ultraprecision in those days) the +/-10% ones, leaving the +/- 20%-ers to be "way off". I suppose every once in a while something a lot closer to nominal would slip through to be marked as a +/- 20% dealie, but probably none too often. Still seems very odd to see something like that, and even have something use such tolerances and still work. But then my original experience was with solid state, where +/- 1.0% laser trimmed metal films are routine. Solid state just is not nearly as "forgiving" as hollow state.

I regulary come across old tube stuff with pretty well the same drift. And of course still work. It annoys me when tubies insist on 1% tolerance......what rubbish.

I do insist on using 1.0% resistors as LTP plate loads. With an active tail load, I get CMRRs that are too high to measure, due to the routine noisiness of my QTH. Everything just disappears into the noise. 5.0% metal film everywhere else, except for C-comps in RF sensitive areas: grid and plate stoppers.

#### nhuwar

There are people out there that well buy anything.

Nick

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