Throbak overdrive pre gain switch?

[dexxyy]

Hi guys, donated my colorsound overdriver to my nephew and decided to build another, did a search for colorsound and the first result was the throbak so I decided I might give this a go instead (haven`t started it yet). I read through the thread and noticed a query regarding the switch near the end and this got me looking at it. Now I just can`t seem to get my head round it.  johns replied to the query that the switch connected the pot so that you can connect/disconnect it from the circuit with the switch, but as I see it it doesn`t, and the more I look at it the weirder it looks. Firstly I thought, why wire the input wire to the board only for it to link straight out to PGSW2, why not take the input wire straight to PGSW2 and drop the top of the pulldown resistor one row to pregain2 which is the real circuit input, and also why have the wire from PGSW1/pregain3 going back to the board where it only meets a cut. Then looking at what the switch actually does it seems to me that all it`s doing is flipping between opposed resistance values of the pot and as I`m seeing it (and I`m probably well wide of the mark) if the pregain pot is set at halfway then the switch makes no difference? I`m finding it hard to see what the pregain pot does that the guitar volume knob wouldn`t do. Am I wrong in some/all of this?

[rocket88]

that was my comment, yea yea i know my names not john and i can't seem to fix it, and that's what i thought ow the switch worked. i could be, and probably am wrong about how the switch works, cause for some reason i remembered that when i hit the pregain switch there's a boost in output and signal hitting the circuit, but the pot didn't really do anything when the switch was off.

i just plugged mine in again and tried it out, and the pot affects the signal both when the switch is engaged and disengaged, so i'm not sure what it does now actually. i can tell you that when the pregain pot is full CW, it affects the signal both when the switch is engaged and disengaged, but if it's at any point below full CW, when you engaged the switch it acts like a boost. and it really slams the circuit giving not just a boost in output, but gives it a great od sound.

i haven't used mine in a long time, so i was going off of what i remembered. i probably should have looked at the schematic, but i was at work. i guess that's what i get for going off of memory from over a year ago .

[cylens]

actually, the pregain circuit is the opposite of a boost: when engaged, volume is dropped, just like the volume pot of the guitar, here's a link to the schematic: http://oi62.tinypic.com/2h5r72o.jpg

[Frank_NH]

I remember wanting to build this circuit, and looking at the "boost" as well.  It really isn't a boost at all, and I'd be inclined just to leave it out entirely.  If you want to integrate a simple clean boost, just make a daughterboard with a clean boost of your choice and add it before or after the overdrive.

[rocket88]

cylens wrote

actually, the pregain circuit is the opposite of a boost: when engaged, volume is dropped, just like the volume pot of the guitar, here's a link to the schematic: http://oi62.tinypic.com/2h5r72o.jpg

BLAH!!!! i tried mine out again, and mine definitely boosts the output when the switch is engaged, especially when the pot is about 1/2 to full CW. when the switch is not engaged the pot still controls the output, but acts more like a guitars volume. so either i did something wrong, it's something about the way the switch is wired in the layout, or that's how it is. honestly i love how mine turned out, and it sounds pretty much spot on to the original, it's to be a little different because the Ge transistor used for clipping is different from what they used. you have to remember that the original doesn't have the pregain as an external pot, but rather an internal set trimmer, so there's a factory setting.

also, i found this from a pdf of the manual:

"THE PRE GAIN: The pre gain is the lower left footswitch on the unit. When the pre gain light is off there is increased low note clarity. This is useful with higher gain settings and higher gain pickups like humbuckers. With the pre gain indicator light on gain is increased and low notes have a fatter tone. This setting is the original Overdriver circuit and may be preferred if you are using single coil pickups."

also from a review on gilmorish:
"Engaging the pre gain stage adds more saturation, which is great for a bit more bite on milder settings or for taking the pedal further into fuzz mayhem."

which makes me thing that it's some how boosting the input when it's engaged. not to mention it is called the Overdrive-Boost. something don't make sense....

[cylens]

ok, I just wired the LED wrong... but that(s what it does (to me, the LED is ON when the pot is in the circuit).

It's a matter of semantics, but to me there is no boost, it's just that there's no lowering of the signal when the pre-gain light is on.

so, when the led is off, the pot is in the circuit and lowers the volume and when it's on, the pot is bypassed (which gives the "audio boost") but you'll notice that the pot does nothing (as it is bypassed)... except when fully CCW, which grounds the signal... duh!

so if you only use the "so called boost", just get rid of the pot!