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.

 

[Potato Cakes]

I was mostly exploring how much I could squeeze out of a single side of a 12AX7 without utilizing an additional gain circuit, but I after reading posts here I have concluded what I suspected initially which is I have to drive the input of the tube to achieve my goals and not by maxing the gain at the output. So I will explore options along that route. I'll do some drawings and hopefully cobble something together in the middle of next month.

Thanks!

Paul

 

[Potato Cakes]

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.

The design I am wanting to do already implements two 12AX7s and two 6L6's in a compact lunch box. I don't think I would have enough room to accommodate a fifth tube.

Thanks!

Paul

 

[merlin]

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.

+1

 

[CJ]

You could add a step up xfmr between stage 1 and 2.

 

[Lemontheo]

Is there a phase inverter after this 12ax7?
The answer could be there.

if not, If you combine the tone stack bypass, a 220k resistor in anode of the fist stage, you can enter The territory of classic rock.

 

[Matador]

I was mostly exploring how much I could squeeze out of a single side of a 12AX7 without utilizing an additional gain circuit, but I after reading posts here I have concluded what I suspected initially which is I have to drive the input of the tube to achieve my goals and not by maxing the gain at the output.

Can you add any gain stages, even if not a tube?

How about a LND150 depletion MOSFET gain stage? It biases very similar to a tube, is tolerant of the B+ voltages in a preamp, and even functions pretty well if you take typical anode/cathode resistor values and cut them in about half (e.g. about 50k for the drain resistor, and 680 ohms for the source resistor). It would make a very easy 'clean pre-boost' to push your second channel more into overdrive.

 

[Potato Cakes]

I'd like to not add anything if possible. Because I'm lazy. Really, really, lazy...

I am going to see what kind of results I can get taking a tap off of the clean 12AX7 and using that to feed the other side to create a distortion effect.

+1

I have done this with other builds on a potentiometer and I like it a lot. For the design currently being discussed there will be a separate volume and tone for each channel. Although as I think about it more I may just do one input where you can mix clean and distortion off of one guitar input. The challenge was just looking at the circuit laid out as is and with minimal effort create a high distortion signal on the half of the preamp 12AX7 that wasn't being used. This was an exploration into what is possible with tubes, of which I know barely the basic concepts of their design.

Thanks!

Paul

 

[CJ]

you could try a 6J11 compactron which is a dual pentode with 13,000 mhos

 

[Wavelength]

PC,
Just a couple of things, you'll get a big pop with that 0.047uF switch. Best to put a 1M resistor across the switch so the voltage is equal on or off.
Your bias on both sections is really low. If you want a clean input then you need more voltage. I made an amp for Ed Robertson with a low bias like that and two of his guitars straight in clipped and that is not a good sound.
You'll probably get a gain of 35dB (~50) in the first stage loose 20dB in the tone stack and then head into the 1V biased second stage. Not a really good idea if you want it to be clean.

 

[Potato Cakes]

PC,
Just a couple of things, you'll get a big pop with that 0.047uF switch. Best to put a 1M resistor across the switch so the voltage is equal on or off.
Your bias on both sections is really low. If you want a clean input then you need more voltage. I made an amp for Ed Robertson with a low bias like that and two of his guitars straight in clipped and that is not a good sound.
You'll probably get a gain of 35dB (~50) in the first stage loose 20dB in the tone stack and then head into the 1V biased second stage. Not a really good idea if you want it to be clean.

That switch was on kit on another build that I did which was used as a reference. I do not get a big pop when engaging/disengaging the turbo switch. The tone stack shown is also for reference. I am using only a tone knob each for clean/distortion.

The bias values are not the same in the schematic. The only thing I was drawing attention to was the tone stack bypass cap, which as already been determined to not produce the desired result in the design I am using.

The clean section is derived from a Silvertone 1482, which maintains a great clean tone at higher volumes and with different pickup outputs. That part is staying as I have built this section in another amp and it works very well. The point of this thread was to see if I could make a channel out of the unused side of the 12AX7.

I think where I have landed is that I will take a tap from the output of the clean side of the tube and feed the grid of the distorted sided via a potentiometer (Gain) and then add another to the output of the distortion tube (Volume). The only question now is whether I use just a switch for an clean/distortion that would feed the phase inverter or also have the option to blend the two together, which would require another switch, which puts us back to me being lazy.

Thanks!

Paul

 

[Wavelength]

Paul,
If you notice on the 1482 the cathode is not bypassed meaning it has local feedback which gives higher clean room bias.

Maybe what you really need to do is draw what you are using now complete. A tone circuit like this or any of the other various tone circuits will not drop the gain as much as a full tone circuit. Therefore your gain will not increase that much.

If I was you I would go to the simulator and determine some better circuits and look at the differential in gains.
https://www.guitarscience.net/tsc/info.htm
Personally I am not a fan of current 1M pots, if you have to go there go get some good ones from PEC. All these others drop the top end like crazy. Had it out with a Bourns engineer about it. I only use 470K or 500K pots now.

The 1482 has 250V from the 330K into the plate at 113V, this means the Rp is really high and the drive sucks. These are important things to think about. That LuchBox amp in my moniker has a B+ at 200V and so I opted for 12AY7/6072A tubes instead of 12AX7 just for that reason so the drive can carry the signal through the tone and volume control.

Gordon

 

[capacitorless]

Another option for an easy additional gain stage: https://grangeramp.com/product/superfet-jack-3/