Guitar preamp distortion/overdrive from single side of 12AX7

[Potato Cakes]

Hello, everyone!

I have a guitar amp project that I am wanting to incorporate a clean and overdrive channel using a single 12AX7 for the preamp of both channels. I for the clean, I referenced the Silvertone 1482 schematic that uses one side of a 12AX7 that produces a very nice sounding, clean channel that doesn't not break up with a wide range of guitar types. The other side of the 12AX7 I want to be pushed into overdrive at lower volume. I have attached a section of a amp kit schematic that I have built with what they called a "Turbo" mode that accomplishes this, but I don't necessarily understand how adding a cap that bypasses the tone stack creates this effect. Without adding another tube to implement a cold clipper or an additional drive stage, are there any other (better?) methods to create obviously intentional distortion?

Thanks!

Paul

 

[musipol]

It’s just letting more level get to the 2nd triode stage. You won’t get much clipping from just that.

 

[Rusan]

I have attached a section of a amp kit schematic that I have built with what they called a "Turbo" mode that accomplishes this, but I don't necessarily understand how adding a cap that bypasses the tone stack creates this effect. Without adding another tube to implement a cold clipper or an additional drive stage, are there any other (better?) methods to create obviously intentional distortion?

Thanks!

Paul

The Turbo cap is simply bypassing the tone stack's huge insertion loss, and replacing it with a much more favorable 1M load.

You could add reverse-paralleled clipping diodes for additional distortion, though that's not a particulary "tubey" sound...

EDIT: Sorry musipol, you posted just as I did.

 

[Potato Cakes]

It’s just letting more level get to the 2nd triode stage. You won’t get much clipping from just that.

The effect is actually significant, but as you pointed out it's sending more signal to the 2nd triode stage. In my case I will be feeding a phase inverter section, so this particular circuit might not work as intended.

Thanks!

Paul

 

[Potato Cakes]

The Turbo cap is simply bypassing the tone stack's huge insertion loss, and replacing it with a much more favorable 1M load.

You could add reverse-paralleled clipping diodes for additional distortion, though that's not a particulary "tubey" sound...

EDIT: Sorry musipol, you posted just as I did.

I did try diodes but wasn't quite what I wanted or at least what I think I wanted. I have also used lower values for the cathode resistor and bypass caps which does make it "growl" a bit more. I could try feeding the distortion side from the output of the clean side of the 12AX7, but right now I am curious if I can achieve the effect I am after with just a single side of the tube.

Thanks!

Paul

 

[CJ]

Switch one or both tubes into grid leak bias?

Or build a stomp box and put it inside the amp with blue tooth switching.

Or put a circuit inside the guitar,

 

[Matador]

Rather than bypassing with a 0.47uF, you can lift the ground reference of the EQ section (where the mid pot/resistor connects to ground) which will remove the effect of the tone stack.

You can squeeze out a bit more gain by raising the anode resistor values, and adjusting the bias to favor clipping one side of the waveform more, however that will also degrade your 'clean' channel.

You open up a lot more possibilities by just adding one more tube.

 

[Rusan]

You might install a buffer after the first triode stage, to reduce insertion loss. The tone stack looks a lot less big and scary to an IRF820 source follower than it does to a 12AX7 anode.

Another option might be a cascode CCS anode load for the first 12AX7 stage, using an N-channel depletion mode MOSFET and a 3 terminal regulator in a cascode configuration. It might not be stable at the 12AX7's 1mA plate current, but if so, it would have many megohms impedance at guitar frequencies. You'd get a couple dB more gain; practically the full 12AX7 mu of 100.