GGG Tube Driver (Hi & Lo EQ + Bias Control)

[rantony]

Hello everybody,

I want to build an overdrive tube pedal. I took a look at the the Shaka Tube and the GGG (General Guitar Gadgets) Tube Driver. Both are interesting projects. But both also have strengths and weaknesses:

The GGG Tube Driver has two Tone controls (one for High frequencies and one for Low's). On the other side, it does not have a Bias control.

The Shaka Tube has the Bias control, but only one Tone control !!!

I'm sure we can get the best of both worlds by adding a Bias control to the GGG Tube Driver. Someone suggested to me that it could be done by putting an electrolytic capacitor and a resistor in parallel with a 10k potentiometer, between the 3 & 8 tube pins.

Here is the link to the GGG schematics:

http://www.generalguitargadgets.com/diagrams/tube_driver_sc.gif

I prepared this to help you to visualize:



What do you think about it ? Would it be OK ? What are the factors that must be taken into account to determine the correct values of the resistor and the capacitor ? Should there be any other changes ? Does it all depend on the V+ and V- voltages mesured with a multimeter ? (Should you need this, a nice guy from this forum who already built a GGG Tube Driver told me that he mesured +14.3v and -12.5v voltages.)

Also...

Should I lower the value of R8 & R11 ?

Should I invert the polarity of the electrolytic capacitor and put it to ground instead of V- ?

Finally, should the potentiometer be linear or logarithmic ?


Not an easy thing to me... A lot of questions, I know ! Please help me !   

Thanks to everyone

[Jackie Treehorn]

The 10k resistors from grid to -voltage set the bias.  I just replaced them with 15k trim pots.

They make a big difference in sound.  You can get a lot more sustain through fiddling with them.