Silver Mica caps

From ic–(at)–nake Tue Apr 25 14:31:23 CDT 1995
From: ic–(at)–nake (John Mastrangelo)
Newsgroups: rec.music.makers.guitar
Subject: Re: 10pf cap in reverb mix circuit
Date: 25 Apr 1995 16:51:54 GMT
Distribution: inet

In article <3nink5$ja--(at)--ews1.cle.ab.com> jack.zucke–(at)–b.com (Jack A. Zucker) writes:
>Anyone read Thunderfunk’s article in last months Vintage Guitar Mag ? He
>claims that the 10pf cap (in parallel with the 3.3M resistor) in the
>reverb mix circuit is one of the most critical components of the amps
>treble voicing. He (once again) insists that a silver mica cap here
>sounds much better than ceramic. When I recently serviced my ’65 twin, I
>replaced the original 10pf ceramic with a silver mica and the amp did
>indeed “come alive”. I don’t know whether this is because of accuracy or
>something else.
>
>Any comments ?

Sure, I have something to add. That 10 pF cap controls the
treble balance in the reverb channel. Notice the normal channel
has 2 gain stages before the phase inverter and the reverb channel
3. Then why doesn’t the reverb channel have way more gain
than the normal channel? Because of the 3.3M/10pF pad between
the second and third gain stages. The 10pF is used to brighten
the channel _slightly_. Last week a customer brought in
a ’73 Super Reverb that had been “converted” to blackface
specs by the most well known amp tech in Denver. The problem
was that the reverb channel was insanely bright, piercing and
gainy. You’de hope the tech would notice this but he didn’t.
The problem? Instead of the 10pF cap, he used a *250pF* cap.
Ouch.

Now on to the subject of ceramic vs. silver mica. A lot
of techs how are “in-the-know” suggest silver mica for
any cap that is associated with treble response. I think
this SM vs. Cer. business belongs in the Silver wire
category. How this myth started, I’m not sure but I
have a pretty good guess. It goes like this:

Once upon a time some amp tech mistakenly picked
up an engineering text and noticed that SM cap are
often recommended for high frequency applications.
“Oooo — high frequency, these must be great for
treble control circuits in my guitar amp. I think
I’ll use these and I’ll be sure to tell all my friends!”

The mistake made by the tech is how they interpreted the
words high frequency. In audio high frequency may mean
1kHz – 20kHz but in the engineering community (where this
recommendation was likely made) the words high frequency
almost certainly apply to frequencies greater than 1 MHz.

In regards to Jack’s positive experience when he swapped a SM
for a ceramic cap, I don’t doubt his observation. However,
I would attribute it to the difference in actual capacitance
values of the two caps. Changing this value just a couple of
pico farads will make a difference to the discerning ear.
Changing it by a factor of two will make a significant difference
to just about everyone. Imagine what increasing it by a factor
of 25 did to this guy’s Super!

Why this myth propagates is easy. Joe amp tech says
“doesn’t this Silver Mica cap (or silver wire, or preformed
cap or forward playing amp , or…….) sound great?”
Sam the guitar player isn’t sure he hears any difference but
thinks “Gee, this guy has a great ear, much better than mine,
and he’s a pro, I’ll do what he says..” And then of course,
Sam deceides he can hear it, tells his friends, etc….
Then if Bob amp tech can’t hear it, then he just doesn’t
have the golden ear. Bob then learns to hear it and so on..

Next question: What do I reccomend? I usually ask a
customer if they want SM caps or standard ceramic with
the explanation that I don’t think it matters, but if they
want SM I charge an extra 25 cents. Everyone wants it.

John Mastrangelo
Osprey Amplification