Looking for Help in Understanding some mods/changes

[somnif]

Howdy folks,

So the latest in my string of obsessive building is going to be a Vox repeat percussion, or at least a derivative there of. Specifically, the Moosapotamus "Skippy Tremolo" (PUT version)

I found a vero layout up on Tagboard, but many people talk about problems with ticking. So, over on Diystompboxes, someone posted an alternate layout that supposedly addresses the issues. But the updated circuit incorporates several changes that I cannot quite understand.

The original Skippy schematic: http://moosapotamus.net/images/Skippy-v1-scheme.jpg

Comparing the layouts (tagboard on left, DIYSB on right):


The DIYSB thread: http://www.diystompboxes.com/smfforum/index.php?topic=103700.20

First obvious change, updated version lacks the tone and speed switches (easy enough to add back in). But, the updated version uses odd cap values for both sides. In the Skippy the Tone switches between a 4.7nF (stock VRP value) and 100nF. On the update its a 15nF.

The Mode switch toggles between 470nF and 4.7uF (incorrectly shown as a 47uF and 4.7uF on the tagboard, probably because Moosapotamus used a very strange convention of listing the part as "u47", meaning 0.47uF, but its silkscreened as a box-type). The stock value on the VRP is 10uF, and the update uses a 2.2uF.

I'm guessing he just chose "middle" values for the updated parts in those positions, but maybe I'm missing something.

It is the other changes I am more curious about:
The 100 ohm resistor from +9v to the rest of the circuit. (is this just here to limit things to 90mA?)
The cap coming off the emitter of Q3, 10uF on the stock Skippy, 4.7uF on the update.
One of the resistors on the PUT changed from 2k to 2.2k (G on Q2)

I can usually muddle through circuit changes, but these are outside my expertise. Any one have any ideas why things were changed? What is the significance of the changes. Are there audible differences?

Any help or advice would be appreciated.

[reddesert]

It's pretty common to see small value resistors in line with the + voltage rail to isolate sections of a circuit from power supply fluctuations. I read a better explanation of this once, but you can think of it as forming an RC filter to low-pass filter the voltage rail. I think the 470R resistor, R10 in the Skippy schematic, is also doing this (possibly also current limiting?), separating the supply voltage of the LFO and audio sections. The current draw of this circuit in normal operation is probably very low (few mA?), so the DC voltage drops across these resistors are not large.

Changing 2K to 2.2K is probably just to use a more standard value.

The other changes could just be a matter of taste / breadboarding.

[somnif]

The 100 ohm guy still confuses me a bit, as its not isolating a part of the circuit, but rather is between +9V and everything. Apparently the EQD version of this circuit does something similar:



Granted that circuit has polarity protection and power filtering, but that 100ohm resistor is still there. I know the circuit won't come close to drawing 90mA, and all I can think of is that resistor is going to act as a space heater. But I am sure it has some legitimate purpose beyond that. Blargh.

[reddesert]

It still provides some isolation / smooths out voltage fluctuations from the power supply. Back when people mostly used batteries to power pedals, this might have been superfluous. But with first, unregulated wall warts, and second, the somewhat more regulated but daisy chained adapters that people often use now (where noise/thump might travel between effects), it's not a bad idea.

Here's some commentary from Jack Orman: http://www.muzique.com/lab/hum.htm
Some fuzzes and transistor effects are more huminacious than op-amp based effects.

[bsoncini]

 A good explanation about the 100 ohm resistor.

http://www.diystompboxes.com/smfforum/index.php?topic=49603.0

[somnif]

A good explanation about the 100 ohm resistor.

http://www.diystompboxes.com/smfforum/index.php?topic=49603.0

Ok, that helped. I'm used to just seeing a filter cap, rather than an RC filter. Good to know there is math behind it.