12V one power supply tube distortion pedal?

[timthetortoise]

Is this possible? I want to build a tube-based distortion pedal but don\'t yet feel comfortable working around 120VAC supplies or, by extension, transformers. However, I do feel very comfortable with 9-15VDC and am wondering if there\'s a schematic out there for a 12AX7-based distortion pedal or if someone can point me in the right direction for designing one. I\'ve been reading up on tubes a whole lot and am starting to understand a little bit more why they do what they do. I got the heaters working with 9 and 12V last night, so that was a positive step for me. I guess what I\'m not so sure about is how much a signal needs to be amplified before it hits the tube, and what the relation between grid, cathode, and plate 1 and 2 are. From what I\'ve been reading, basically xxx 1 is where the signal enters, and xxx 2 is where it exits. So put in the signal on grid 1 and it exits on grid 2, but this obviously didn\'t work. I tried having it exit on plate 2 also and that didn\'t work either. Hell, I\'m not even sure where to feed the voltage on the plate. I\'d appreciate any guidance on this, as it\'s a very foggy subject for me at this point. Thanks!

Edit: I just found this, so hopefully I can do this tonight.

 

[mshilarious]

Is this possible? I want to build a tube-based distortion pedal but don\'t yet feel comfortable working around 120VAC supplies or, by extension, transformers. However, I do feel very comfortable with 9-15VDC and am wondering if there\'s a schematic out there for a 12AX7-based distortion pedal or if someone can point me in the right direction for designing one.

Yeah, you can do a pedal with 12AX7 off a 12VDC supply, but it will be severely starved-plate. Instead you might want to try 6GM8, which has a slightly different pin configuration, but is happy at much lower plate voltages.

how much a signal needs to be amplified before it hits the tube

It doesn't. The schematic you link is a hybrid design with an opamp front end, with the first opamp stage as a buffer, and the second opamp stage provides gain into the tube. That will enable you to drive the tube harder, but there are two tube stages as well. Look at the schematic for a Fender Champ by comparison. The only reason I can think of to use an opamp front end is because the gain off the first tube stage will be very limited due to its starved plate configuration, making it harder to drive the second stage into saturation.

and what the relation between grid, cathode, and plate 1 and 2 are. From what I\'ve been reading, basically xxx 1 is where the signal enters, and xxx 2 is where it exits. So put in the signal on grid 1 and it exits on grid 2, but this obviously didn\'t work. I tried having it exit on plate 2 also and that didn\'t work either. Hell, I\'m not even sure where to feed the voltage on the plate. I\'d appreciate any guidance on this, as it\'s a very foggy subject for me at this point. Thanks!

The 12A_7 family are twin triode tubes. That means they have two separate amplifier stages, each with its own cathode, grid, and plate. They can be used independently. Therefore the signal can be applied on either grid (or both, in a differential design), and can "exit" on the plate or the cathode. Usually in guitar amp, the signal is taken off the plate and that first stage is sent to the second stage grid, and the output is off the second stage plate. The cathodes are biased slightly above ground by their resistor, and bypassed with a cap so the cathode is an AC ground.

 

[timthetortoise]

Thank you for that in-depth info. I still want to use a 12AX7-based circuit, mostly because I have sockets and tubes readily available. What is the minimum voltage you\'d recommend for supplying the plates?

 

[mshilarious]

One other thing about that schemo, it doesn't use a 12VDC supply, it uses 12VAC and rectifies that to a +-12VDC supply. So its plate voltage is 24VDC.

If you are using a 12VDC adaptor, you will need to bias the opamp inputs . . . and make a few other changes too . . .

 

[dementedchord]

What is the minimum voltage you\'d recommend for supplying the plates?

around 200V...