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Coupling Caps

From dave_harri–(at)–oteserv.dodge.com Fri Sep 8 21:12:26 CDT 1995
From: dave_harri–(at)–oteserv.dodge.com
Newsgroups: alt.guitar.amps,rec.audio.tubes
Subject: Capacitors: Value Conversions and Substitutions
Date: Fri, 08 Sep 1995 21:05:57 GMT
Xref: geraldo.cc.utexas.edu alt.guitar.amps:3481 rec.audio.tubes:1050

MICRO- TO PICO- CONVERSIONS

If you can remember what “micro” and “pico” mean,
you will not have to worry about remembering
conversion formulas because they will be obvious.

“Micro” means “one-millionth part,” which is 1/1,000,000.
“Pico” means “one-trillionth part,” which is 1/1,000,000,000,000.

One-trillionth (pico) is one-millionth of one-millionth.
This is the same as micro-micro, which is why you will
sometimes see “mmf” instead of “pf.”

To convert micros to picos, you multiply by 1,000,000 because
there are 1,000,000 picos in each micro.

Examples

.00047 mf * 1,000,000 = 470 pf
.001 mf * 1,000,000 = 1000 pf

To convert picos to micros, you divide by 1,000,000 because
each pico is one-millionth of a micro. For example:

470 pf / 1,000,000 = .00047 mf
1000 pf / 1,000,000 = .001 mf

CAPACITOR SUBSTITUTIONS AND TONE

Using a larger capacitance allows lower frequencies
to get through, thus darkening the sound. Using smaller
capacitances make it harder for lower frequencies to
get through, thus brightening the sound.

In the Marshall amp example that you mentioned
(page 595 Tube Amp Book IV), a 470 pf cap is parallel
to a 470K resisitor. The 470K resistor is in series
with a 1M resistor. At their intersection (2)
is the .001 mf cap, which couples the next stage
of the preamp (4):

470 pf .001 mf
|—-|(—–|—-|(—-(4) Grid of next stage
| |
>From 1st stage (1)–///–(2)–///–(3) Ground
or input. 470K 1M

The 470K and 1M resistors are a voltage divider.
A lower frequency signal cannot easily make it through
the 470 pf capacitor, so it flows through both
the 470K and 1M resistors. By the time the signal
gets to (3), it is at ground potential (0).
At (2) it is somewhere between its maximum voltage at (1)
and ground. The voltage that appears at (2) is what
goes through the .001 mf cap and gets amplified by
the stage at (4).

A higher frequency signal can sneak through
the 470 pf cap, bypassing the 470K resistor. Its full
voltage appears at (2), goes through the .001 cap,
and is amplified by the next stage. So the higher
frequency signal gets amplified more than the
lower frequency signal.

If you increased the value of the 470 pf cap (or removed it)
you would darken the sound of the amp. If you decreased
it, you brighten the sound of the amp.

The .001 mf coupling cap affects all signals going to
the stage at (4). If you increased the value of this cap,
say to .01, the amp would sound darker. If you decreased,
say to 470 pf, the amp would sound brighter.
Both the 470 pf and .001 mf caps need to be rated for high voltages.

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