I am building an amp and I wanted the ability to add another stage for distortion. Unfortunately, I have some issues which will prevent me from using another 12ax7 stage. Looking at some high gain amp examples, I noticed that a great many of them contain one stage that is biased so cold and at such a low amperage/high voltage, they go into severely hard clipping that is a square wave with rounded edges. Because of the bias point, one side of the signal is going to clip much more than the other; this is called asymmetrical clipping and tubes do it naturally and very well.
Diodes are a low cost way to add distortion. If designed with some foresight, it can be designed to be “tube-like”, but really doesn’t sound exactly like a tube on it’s own. Placed between stages, it will get enough filtering through the tubes to not sound cold, hard, or whatever the purists like to say as they push their amps with solid state pedals.
The above simplified schematic is my take on it. R1 is the grid stop resistor, the pot is the grid leak resistor, and R2 is there to limit the pot’s lowest range. C2 should be in the pico range and help to subdue the highs. I would attenuate somewhere above 5kHz to 10kHz. R2 will have a value based on the pot value. If you use a 500k pot, a 1k to 10k value would likely work just fine.
The pot will vary the voltage threshold of the diode clipping. Basically, it will be the voltage being dropped below the wiper plus the forward voltage of the diodes. It will make it possible to have massive, hard clipping and limiting, and to dial it back to something more subdued that has coloring from the diodes, but not total squaring.
Once I finish the build, I’ll update this, talk about the merits (good and bad), and hopefully get a video example of it’s impact on the amplifier.
My notes, tips, ideas:
- I would use this after the second stage. The controls on the first stage are for the gain control and you may want to be able to dial in and out the voltage threshold for the diode clipping without affecting the overall distortion of the amp.
- You can mix and match your diode selections. I would have LEDs followed by silicon and/or germanium. Have the highest Vf first in line.
- Asymmetrical clipping is going to give it the most tube-like sound.
- LEDs are less harsh and more tube-like.
- Keep in mind the amount of attenuation any pots or other potential dividers upstream will have on the signal when choosing values and diodes.
- An alternate circuit could made by changing the pot to a resistor, and moving POT/R2/Diodes/GND to the grid of the upcoming tube.