Impedance calculation - SWAG method

[WGTP]

The scientific method may say cap X will roll off the bass, but when it doesn't work quite right???  Scientific Wild A** Guess

I set my single coil guitar in front of my amp positioned so it hums really nice and use that as "noise" for testing.  I change caps until I can hear the bass diminish and then estimate it is rolling things off around 160Hz for my sealed back cabinet.  Same with the highs only I use 3.2kHz. 

Currently on the breadboard is a SRPP with a BS170 in the lower position and a J201 in the top.  The input impedance should be pretty high with 1M resistors, but a 100n cap is where the bass starts rolling off.  This puts the impedance around 10K???

1k - 1uf=160Hz
10k - 100n=160Hz
100k - 100n=16Hz
1M - 100n=1.6Hz

[artifus]

I set my single coil guitar in front of my amp positioned so it hums really nice and use that as "noise" for testing.  I change caps until I can hear the bass diminish and then estimate it is rolling things off around 160Hz for my sealed back cabinet.  Same with the highs only I use 3.2kHz.

you're doing this by ear? what are the dimensions of the room you are in, where in it are you in relation to the amp and how is this room treated? do you adjust the colour balance of your photo's in red light wearing shades?

[R.G.]

What is the gate-source capacitance of the BS170, and is there any circuitry there to mitigate it?

[PRR]

> 1M resistors, but a 100n cap is where the bass starts rolling off

Something is not right.

Use a big cap, plus a series resistor (inside the box, this will hum). Play normal. Does 10K series cut the sound in half?

[Paul Marossy]

Ha ha, I haven't heard the term "SWAG" in a long time. I remember what that stood for! 

[R.G.]

Ha ha, I haven't heard the term "SWAG" in a long time. I remember what that stood for! 

Yes; most people know what SNAFU stands for.

But how many remember BOHICA?

[Paul Marossy]

I had no idea what "BOHICA" meant, but google just enlightened me! 

[WGTP]

BOHICA sounds like a term applicable to many situations in life.  From my Navy friend HMFIC and FUBAR which describes some of my projects.   

Guess I should say with, the standard frequency calculation we use F=1/(2x3.14xRC), my limited knowledge of electronics, or failure to take all the factors into consideration in determining complex impedance, series/parallel capacitance, etc.

The main point, is finding which values are affecting my low/high frequencies at that point in the circuit to allow for more effective tweaking and some very rough simming.  Adjusting the values in the simulation to reflect what I hear happening is sort of tricky.  Once I figure out that a cap is too large or small to "hear" any differences, I can remove it from consideration.

My question and the answers about by-pass caps resulted in the realization that I could use smaller ones than typically seen and get a Treble boost out of the deal.

Another interesting "location" is the cap in the middle of the Mu/SRPP and 4049's disrupted me for several years.

Thanks to all for you responses.  Remember I said this was the SWAG method. 
   

[artifus]

'swag' was new to me. please forgive my previous over tired, under thought response. an entertaining and informative thread none the less.

[seedlings]

After you guess it close, double the R or C to cut F in half.  Halve one or the other to double F.

CHAD

[WGTP]

Yeah, that's the plan.  By neutralizing some values in the James or BMP sim in the Duncan Tonestack, I can get all kinds of pretty graphs that "seem" to sound like what is happening.  Very simplistic.  Or LT Spice does some cool stuff.