VFE Alpha Dog notes

[Aentons]

Yet another VFE post share:

https://www.thegearpage.net/board/index.php?threads/vfe-pedals-alpha-dog-v2-all-the-mods-explained.1353771

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VFE Pedals Alpha Dog v2 - all the mods explained
Discussion in 'Effects, Pedals, Strings & Things' started by VFEpedals, Nov 26, 2013.

Nov 26, 2013#1
VFEpedals
VFEpedals
Member
Messages:112
Joined:May 24, 2011
Location:Puyallup, WA
I've been seeing lots of questions pop up on the Alpha Dog recently on TGP, so I thought it would be helpful to explain all the mods.

This will get a bit technical, but hopefully it still makes sense. First, as most of you know, the Alpha Dog is based on the Proco Rat circuit. Below is a schematic of the Rat distortion, and I will use it for reference.

[​IMG]

Here is a specific list of ALL the mods I've done to create the v2 Alpha Dog.

1) The input cap is increased (0.22uf), the input resistor is first in the chain, and the biasing resistor is replaced with a voltage divider network of two 2.2M resistors. The 0.001uf capacitor is still the same. This accomplishes two basic goals for me - makes the circuit simpler (fewer parts & values), and reduces the amount of switching pop. These changes really have no effect on the tone (I'll spare you the math equations for why this is true).

2) The gain stage uses a custom tapered 1M pot with 10p filter cap. The dual legs that connect the negative input to ground are 470 ohm + 0.22uf and variable 470 to 10.5k + 1uf. These values were chosen so that I could use film caps instead of lower quality electrolytics, while preserving the same gain & tone structure. The variable part also uses a custom made pot, and is labeled "FAT" on the outside. The two resistor/cap legs in the Rat were set to boost high-mid and treble frequencies up to 20dB more than bass & low-mid frequencies. The FAT control basically allows you to level out the gain EQ by adjusting the gain level of the leg with the flatter EQ. This also means the Alpha Dog gets about 5-6dB more gain than the Rat circuit can.

3) I use a C100K + 1K + 0.0047uf network for the FILTER control. This gives more tonal range, and a more natural EQ balance over the sweep of the control. Some older v1 Alpha Dogs use a B100K pot, which has the same range but will seem to be darker because more of the range of the control is in the dark region. As an example, if you set the v2 Alpha Dog FILTER control to 12:00, the treble cut off is around 2.3-2.8kHz. In some of the early v1 Alpha Dogs, 12:00 would cut off starting at 720Hz (a very dark setting).

4) The buffer input cap is changed to 0.22uf (no effect on tone), and the output cap is still 1uf (but I use a film cap instead of electrolytic). I also added a series 1K resistor after the volume pot to reduce switching noise (particularly when you max out the volume knob).

5) Everything from 1-4 is pretty much doing stuff that's already been done in one form or another. The HARD & SOFT controls are what are really unique to the Alpha Dog. You are all used to switches to toggle between clipping components. The new controls are setup like switches, in that you can isolate one clipping type by turning to that side. But they add an extra dimension by controlling the amount of series resistance - which affects the amount of compression/distortion and output volume. I won't go into the math here, but the basic idea is this - when the control is fully clockwise, it's like a toggle switch in the right position (and vice versa). As you move towards 12:00, series resistance is added to reduce compression - which increases volume and makes the pedal more dynamic. In order to pull this off in a way that's smooth & easy to use, the taper of the controls need to both be log & reverse log - what is commonly refered to as the "W" taper. The Alpha Dog has custom W1M (soft) and W20K (hard) 9mm pots with a nice detent at the 12:00 position.

6) The last bit to mention is the parts used to produce distortion. First, the LM308N op amp will add a very nice distortion of its own, especially at high gain settings. The M mode is a single BS170 mosfet, setup to use its protection diode and Vgs(on) point to compress the gain of the op amp. This is an asymmetrical setup, so it emphasizes even harmonics, and it uses an additional 100pf cap to smooth out any distortion artifacts. The G mode uses a single 1N34A germanium diode. Using just one diode with a super low threshold (0.3V) yields a very asymmetrical clipping (one-sided) that really sweetens things up. If you really crank the gain and turn all the way to the G mode, you'll get a pseudo-octave because the circuit is basically cutting off one side of the signal. However, the op amp will struggle to keep up in this extreme of a setting (glitchy & gritty). The V mode stands for "Vintage", and I really just put it there because it's the classic Rat setup of two silicon diodes. I personally find the original overly compressed, but being able to tune the compression really makes this mode usable to me. The T mode stands for "Twin", which DOES NOT mean a Fender Twin. It means that there are two identical stacked pairs of diodes, along with a small filter cap. The result is a smoother, more dynamic tone than the V mode.

Alright, that was a lot, but I hope it was helpful - even if you are not a technical person!
Last edited: Apr 26, 2014




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Apr 26, 2014#9
VFEpedals
VFEpedals
Member
Messages:112
Joined:May 24, 2011
Location:Puyallup, WA
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candletears7 said: ↑
Thanks for posting. Very informative.
So the Fat control really just flattens the pushed mids and highs of the Rat so all frequencies across the spectrum are heard?
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Sorry for the slow response. I don't spend much time on TGP, so if you need a quick response, just hit me up via email.

Technical response: The original rat was setup with two legs to add gain - one adds gain to the full spectrum, and and the other adds additional gain to high mid & treble frequencies. The FAT control affects the gain of the full spectrum leg, so when you turn it up it levels out the EQ while it boosts the entire signal.

Practical response: What you'll hear when you turn the FAT control clockwise in v2 is a thicker bottom end and more overall gain/saturation. Turn it counterclockwise for a tighter, punchier bottom end and lower overall gain/saturation. There is plenty of gain range no matter the setting, so it's primary importance is how it shapes the EQ of the gain section.

I hope that helps!
Last edited: Apr 26, 2014




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As for the difference with the FAT control...There are two resistor + capacitor legs off the op amp that control the EQ & gain range.

In v1, the FAT control varied the resistor in the leg of the smaller cap & resistor. This made a wider EQ impact, but also an unintuitive impact on gain. Turning it up increased relative bass response by turning down the gain of the higher frequencies.

In v2, the FAT control varies the resistor in the leg of the larger cap & resistor. This makes it sort of a "full range boost". Turning it up boost all frequencies, which in turn creates a flatter EQ response overall - creating a fuller tone, and fatter bottom end. So the gain goes up AND the bass goes up....much more intuitive!

[cooder]

Thanks for finding those gems... a lot to learn and gaze at schematics now...! Cheers

[gordo]

As detailed as the Bean docs always are, its really cool to get the designer's input on the design process.  I'd really hoped that the Patreon presence would have been this sort of insight but it never panned out.

[madbean]

As detailed as the Bean docs always are, its really cool to get the designer's input on the design process.  I'd really hoped that the Patreon presence would have been this sort of insight but it never panned out.

Yeah, these are great.