# Need help to understand a schematic



## Jean GODBOUT (Oct 31, 2009)

Hi folks
I want to build a tubes driven overdrive from a schematic I found on the web, first tube is a 12AT7 and the second is a 12AU7.

Schematic : http://www.tube-town.net/diy/lov/lov02-pepper-shredder-mkiii.pdf

Resistor R7 have no value and C4 and C8 capacitors have no values, any suggestion ?

Second thing, this pedal run on 24Vdc and I don't understand how the filaments are alimented, the tubes filaments need 6 Volts (or 6.3 Volts) typically, but can be wire differently using 12 Volts using different pins connection, I already catch this. But here, on the schematic, it seems that the filaments will receive 24 Volts witch is way to much, what I'm I missing ?

Thank for any help on this, Jean


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## Fader (Mar 10, 2009)

The filaments are in series so each filament would see half the supply voltage but exactly the same current.
I can't help you with the rest.


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## dtsaudio (Apr 15, 2009)

I would think R7 is quite small in value. You don't need to drop any more voltage than necessary 10ohms max. You could probably leave it out.
The capacitors can pretty much be anything you want as well. Probably 1000ufd at 35volts would be adaquate. They really depend on how good your power supply is.


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## Jean GODBOUT (Oct 31, 2009)

Thank you guys, with these informations I can go ahead, and build the thing  (In fact, I already start).


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## Fader (Mar 10, 2009)

I would agree. It's looks like it's just for filtering.


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## keeperofthegood (Apr 30, 2008)

Jean, there are "sweet spots" for tubes in where they will work their magic. 

If you look down through history of the "typical voltages used" these voltages will give you an idea where to find those sweet spots. 22.5 volts was common, not because of the batteries used (though those helped) but because of that 18 to 22 volts range that would have given you that "sweet spot" for space charge in tubes.

What I would do, get my hands on a 100 ohm potentiometer, and dial it till I measured 22.5V across C8

C4 and C8 are filters if the power supply had been a wall wart, if the power supply is well filtered already these can be dropped.


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