500mmf =?

[cream123]

what is a 500 mmf cap? that's really confusing. is it micro still?

 

[Blue_Fingers_Jay]

Re: 500mmf =?

Did some reading on this and from what i got it's micromicrofarads.

Use of Feedback to Compensate for Vacuum-Tube Input-Capacitance Variations with Grid Bias
Freeman, R.L.
Proceedings of the IRE
Volume 26, Issue 11, Nov. 1938 Page(s): 1360 - 1366
Digital Object Identifier
Summary: The input capacitance of amplifier tubes decreases with increasing grid bias, thereby detuning the input circuits of intermediate-frequency stages in a superheterodyne receiver. The phenomenon is caused by a shift in space charge and also by feedback through the grid-plate capacitance. The change in input capacitance is of the order of 1.5 micromicrofarads for each of the two causes. A simple form of feed-back coupling compensates for this change of capacitance. The grid-cathode capacitance and an unbypassed resistor in the cathode lead provide the feed-back coupling.

From: http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel5/10933/35547/01686542.pdf?isnumber=35547&prod=JNL&arnumber=1686542&arSt=+1360&ared=+1366&arAuthor=Freeman%2C+R.L.

Now for the juicy part:

# In radio service literature such as Sams Photofacts, paper and electrolytic capacitors are usually expressed in terms "microfarads". Short forms for microfarad include mfd, MFD, MF, UF and uF. The mica capacitors in your tube radio will have lower capacitance values than the paper and electrolytic capacitors. Micas are expressed in terms of micromicrofarads (picofarads). Short forms for micromicrofarads include mmfd, MMFD, MMF, PF and pF. A pF is one-millionth of a uF. For example, a mica capacitor valued at 500 mmfd (pF) would be 0.0005 mfd (uF). When reading schematics and buying capacitors, you sometimes have to be able to convert uF to pF or pF to uF. For your convenience we have a Capacitor uF-nF-pF Conversion Chart that you can refer to. You may want to tape this conversion chart to your workbench.

Taken from (lots of great info here): http://www.justradios.com/captips.html

 

[cream123]

Re: 500mmf =?

ahhhh


ok thanks. mmf is pico...wth didn't they say that??


thanks

 

[jmv]

Re: 500mmf =?

yeah thats really odd, but good info to know. I don't know why they would stray from the standard prefix nomenclature. I guess it does sort of make sense, micro means x10 to the -6, pico means x10 to the -12.... so double micro makes it -12.

go figure.

 

[GoldenVulture]

Re: 500mmf =?

MMFD;Its a very old way of referring to MFD/PF.What,s the cap in?Does it appear old?
As Capacitors used to also be called Condensors[they still are in CARS].

 

[cream123]

Re: 500mmf =?

MMFD;Its a very old way of referring to MFD/PF.What,s the cap in?Does it appear old?
As Capacitors used to also be called Condensors[they still are in CARS].

it's in an old ampeg reverberocket schematic

 

[GoldenVulture]

Re: 500mmf =?

Surprising! Your more likely to find caps with that type of description in vintage radios between the 30,s and 50,s.:wall:

 

[ES350]

Re: 500mmf =?

mid-50's tweed schematics use the 'mmf' (aka 'mike-mike') on the layouts; see the treble bypass cap on the 5E8A layout...