Wednesday, January 14, 2009

Reading Capacitor Values Part 1

Reading capacitors values from schematics or even parts lists can be confusing. There are a few basic rules to keep in mind that will make it a bit less daunting. First off the basic unit of capacitance is the farad. A farad is so large that you will never encounter a capacitor measured in farads. Most capacitors in tube amps are measured in microfarads. It takes one million microfarads to equal one farad. If a unit for a capacitor in a tube amp schematic is not specified the value is almost certainly microfarads (abbreviated uF). The rest will for the most part be picofarads (pF). I'll take some examples from the Fender Bassman 5F6A. Below is the complete schematic.

Here's an excerpt showing the 5F6A tone stack. The cap show in red is labeled .02-400. The first number indicates that the capacitance is .02 microfarads, the second number indicates the voltage rating, 400 volts in this case.
The treble cap is a much smaller value, .00025 microfarads. If you're looking though a parts catalog you are not likely to find a .00025 microfarad cap. That's because a cap of this size will more often be expressed in picofarads (abbreviated pF). There are on million picofarads in one micofarads. To find the value of a cap rated in micofarads in picofarads simply multiply by 1,000,000.

In this case .00025 microfarads x 1,000,000 = 250 pF.

Looking at the splitter of the phase inverter, there is a cap bridging the output of the inverter:

This cap is shown as at 47MMF. What does this mean? I've said already that the caps in tube amps are going to be fairly exclusively rated in microfarads (uF) or picofarads (pF). What does MMF mean?

In short, it is the same as picofards (pF). To understand why takes a bit of explanation. The abbreviation for "micro" is taken from the lower case of the Greek letter "mu".

Now you can see that the upper case "mu" looks just like an "M".
But what about the lower case"mu"?

Frequently you're not using the symbol for "mu" especially if you're typing. Convention has "mu" written as "u" so microfards is written uF. But in a way it's logical to think of the lower case "mu" as "m". This is why you'll sometimes see microfarads written as mF. Technically mF means millifarads (one thousandth of a farad) but since caps are not generally rated in millifarads you can safeley assume that if you see mF it is microfarads that is indicated.

MF techically means megafarad - one million farads. This is a value so ridiculously large that a cap this large could not even begin to be manufactured. If you see MF you can also assume microfarads (uF).

So the MMF from above can be considered to be mmF or micromicrofarads. "Micro" means "one millionth" or "divided by 1,000,000). A picofarad is a microfarad divided by 1,000,000. So saying micromicrofarad is really the same as saying picofarad.

So to sum up:

The majority of tube amp caps will be indicated in microfarads (uF, mF, MF) and picofarads (pf, mmf, MMF). Occasional you will see nanofarads (nF). A nanofarad is simply one thousanth of one microfarad or one thousand picofarads.

If a unit for a capacitor in a tube amp schematic is not specified the value is almost certainly microfarads.


mike said...

Hi Andy!!! I love the blog and I am learning a lot. I'm confused about one thing though... did you mean to write in the middle that one microfarad is equal to one million picofarads, instead of the other way around? As it is right now it looks like a picofarad is the same value as a regular farad (both one million microfarads).

Also, I'm having a hard time not writing Farhad.

akavalve said...

Yep, that's what I meant. It's fixed now. I ran out of time this morning before I got to my proofreading round - thanks for catching it.

mike said...

Sure thing! Keep up the awesome and educational work!

Anonymous said...

Nice - thanks for the info. I'm looking forward to part 2. Will you be covering the capacitor values that I don't understand from guitar amp schematics along the lines of "50/50" and "2200/35"?

akavalve said...

If there's enough questions for a whole post I certainly will. Let me know what else you find confusing and I'll try to cover them all in one fell swoop.

Anonymous said...

"MF" indicates milli-farads (10e-3) not mega or micro.

If units are not specified on the schematic, it is assumed MF (milli-farads)

akavalve said...


I realize that people are finding this post though more general capacitor searches but I'm talking about guitar tube amp schematics here where unstated cap values are pretty invariably microfarads NOT millifarads.

And yes, if you see MF on a schematic, of course it's not going to mean megafarads. But by labeling conventions MF indicating millifarads is incorrect. If you mean millifarads it should, technically, be a lowercase m.

Ruben said...

Very informative post. Every time I start tinkering with electronics again I get the same old doubts about capacitors especially. I really wish they were as easy as the resistor code.