DIY tube overdrive pedal

[jim p]

I did up this quick schematic for a tube overdrive using the 6GM8 tube. Do to the low grid resistor required with this tube there is a jfet buffer a J211 as the input which is also used as a tone control. The schematic program does not have symbols for pots so R3-R4 represents a 10 k pot. Also R19-R20 and R21-22 is a dual 100k pot used as the gain control. While R23-24 is a 1 meg audio pot for the output level control. The two triodes have current source loads in the plate circuits using 2N5457 jfets with 750 ohms in series with there source. What is not shown is the power supply section which is a rectified 12 volt AC supply that is also used for the heater with an approx 18 ohms 5 watt resistor in series. The final output buffer FET could be optional if you are connecting to a tube amplifier, required with a solid state amplifier. C10 and C11 are brightness (treble boost) that has been added and one or the other could be removed or values could be increased or decreased to your preference. You may want to add a resistor in series with the tone pot (R3-R4) to limit the maximum gain with the tone at maximum.

Took a break to see about the cost for the tube and not sure but looks like they are going up in price. The tube you can use is a 6GM8/ECC86/2N27P getting to be 20 bucks each.

Here is the basic schematic
http://i1020.photobucket.com/albums/...8overdrive.jpg


Here is a page on space charge tubes
http://www.junkbox.com/electronics/l...agetubes.shtml

This whole thing made me look around and what I think would be away to go would be to modify one of these. http://pro-audio.musiciansfriend.com...amp?sku=180581
I need to check out the schematic but for 30 bucks you have the box the tube the power supply hard too beat, just need to see what it would need to be used for a guitar.