Guitar preamp distortion/overdrive from single side of 12AX7

[Alva Goldbook]

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

As a general rule of thumb, to get a decent lead sound using 12ax7s, you'd need 3 gain stages plus a phase inverter if it's push pull. Yes, there's ways, to really get 2 gain stages to get some hair going, but 3 is the practical threshold for a good lead sound. For a good clean sound, 2.

I would note, however, that adding more gain stages to a clean sound for cleanly biased 12ax7s sounds lovely. Many blackface era Fenders had a 2 gain stage plus PI thing going, but if they had added a 3rd, they would have sounded much better.

You can add a single triode to get a clean and lead sound. I mean the JCM 800 2203 did that. But if the clean and lead sounds only have a single gain stage, I'm afraid it's just not going to get there.

 

[Potato Cakes]

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
View attachment 138675

Attached is the preamp circuit I am using for the clean section which I have already build and tested in another amp. It works and sounds fantastic. The plate voltage will wind up being adjusted.

Thanks!

Paul

 

[Wavelength]

Alva,
He doesn't have a full tone stack sucking gain and most of those marshalls have bypassed and attenuators built in. The big problem here is the driving force of the first stage. At 0.4ma drive current the slew rate current to drive the tone and volume control with much response. Especially with a gain of about ~53.

PC,
Well I would change a bunch of stuff but if you have noise the first thing I would do is create a more cascaded drops especially for the first and second stage.
If you want to create a simple distortion... put a bypass on the second 2.2K of 2.2uF if you want full range or 0.47uF if you want to cut off bass.