Germ Warfare

[Kipper4]

I've been trying to put a varaible input cap in the axis fuzz but it aint happening. Why?
help me out fellas

heres the input cap idea stolen from Macs Tres Hombres

http://www.aronnelson.com/gallery/main.php/v/mac/my+circuits/Tres+Hombres.GIF.html

How do i get this to work with the Axis Fuzz?
see here
http://www.diystompboxes.com/pedals/axisfuzz.gif

Thanks
Rich

[midwayfair]

The fuzz already happening and you just don't hear a difference?

[Kipper4]

I suspect that the 4600 pf is interacting and I can't hear it.
Something funky is happening.
I've no idea I just tacked it on expecting it to work

[anotherjim]

4600pF? 47nF is a 473 or 47000pF.

[Kipper4]

Should have been clear I was talking about the 4600pf on q1 base in the axis.

[PRR]

> the 4600pf on q1 base in the axis.

I'm not seeing that; maybe I'm blind?

However, if it offends you, pluck it out. What's the worst can happen? They take away your pedal license? You pick up Mexican Polka stations? (Hey, could be a good gig.)

In any case:

The Tres Hombres has an input impedance like 6K, and gain will be sensitive to series impedance (coupling cap).

The Axis has input impedance closer to 300K, and isn't whole-lot sensitive to reasonable values of input series impedance.

You could maybe change the 820K+680K input resistors to 82K+68K or even lower (22K+18K?).

[Kipper4]

Sorry my mistake. Wrote in haste just before leaving for work.
It's the 470pf.
I tried to remove it and got no signal at all.
I will try some of your alternate values PPR. Thanks
I should point out that I will be using a germanium pnp q1.
My next trial will be to put 2 differant value input caps on a switch and see how that pans out.
Just got in from work and the iron is hot. I'm ready to go.
Cheers

[bloxstompboxes]

Good lord. I have been following this thread since it started. He has been to work and home again. I am still at work. These 12 hour shifts stink, lol.

[Kipper4]

Nevermind Eric it will soon be happy hour.

Update
So I implimented the two parrallel input caps on a switch (100nf//20nf) and still couldnt hear any differance.
Following this i've tried your suggestion of lowering the input impedance PPr with a 82k  68k combination
Assuming that the input impedance is calculated from the 2M2 , 820k+680k interaction.
I now have a misbiased Q1 2n3906 transistor.
What if I lower the 2M2 to a 1M and lower the input impedance even more?
Will this help with rebiasing the transistor?
Transistors aint my strong point.
Admitidly I would not have thought of lowering those 820k 680k values as I thought they would  have a currant limiting effect.
Thanks

[Kipper4]

Just a thought will uping the 180k value be a better proposition for rebiasing the tranistor or will that affect the input impedance again?

[Kipper4]

What if I lower the 2M2 to a 1M and lower the input impedance even more?
Will this help with rebiasing the transistor?


Answer nope.

[ashcat_lt]

Use a much smaller cap in place of that 473.  I'm seeing something like 30K impedance on the Hombres and around 600K for the Axis, which is a factor of about 20.  So make the cap 1/20th or what the Hombre calls for and it should be pretty close, I'd imagine.  That'll be pretty close to a 222, if I'm not mistaken.

[LightSoundGeometry]

strike the hammer while, the iron is hot!

[PRR]

APOLOGIES!! To scale the input without mucking the bias, you must scale *three* resistors: the 180K the 820K and the 680K.

So change the 180K+820K+680K input resistors to 18K+82K+68K or even lower (4.7K+22K+18K?).

This circuit uses a Voltage Divider to set Q1 Base voltage. If the resistor values are too large, Q1 Base current loads the divider and screws the bias. I would assume the designer picked these resistors as large as possible without upsetting Q1, for maximum input impedance and least-loading of guitar.

We can use lower values. Bias will be almost unchanged. The drawback is that signal input impedance drops. However the other plan which you are copying from has a low input impedance. To use a reasonable cap value to change the sound, you probably have to go in this direction. Heavy loading of guitar is often part of "fuzz sound".

As Ashcat says, you could indeed go to a much smaller cap. Counting on thumbs I got a silly-small value to make much of a difference. Super small coupling caps are prone to allow external buzz into the first transistor. It will also interact with that _470_pFd cap to ground (though this cap should not be essential).

> strike the hammer while the iron is hot!

And empty your cap-box at it.

[Kipper4]

Brilliant thanks guys.
I think I'll leave the currant perfboard as a straight up axis fuzz and start up a new on the breadboard with the germ version with interchangeable input caps.
I will try the 2nf Ashcat maybe a 5nf instead too.

[mac]

Rich,

I HAD to use low bias resistors because it was intended for low hfe germs, or silicons.
As a side effect, a good one I think, the interaction between guitar circuitry, input caps and bias resistors makes a sort of pass-band input filter.

In the Axis, my bet is that to get an audible difference you need to go from 10n to 1u.
if you lower bias you might narrow that range and get a bell-like input filter.

BTW, there is a 220 resistor not bypassed at Q1 emitter, if hfe is 200 that's 44k to gnd when drive pot is near zero... I guess.

You pick up Mexican Polka stations?

uacala!!!

mac
mac