funky clipper

[Andy Le Blanc]

I'm always looking to use common parts and explore how tubes can be used.
This mornings "try me" was using a 12ax7, dressed as two diodes, as a clipper.
The output wave form begins to take on a triangle or sawtooth shape.
It seems to make a decent limiter.

[Phil_S]

Andy: I've got a pile of 6AL5's that look as if they would be ideal for this sort of circuit. Would you mind expanding here for the uninformed amateur, what does one do with this thing that you are referencing?

[mlp-mx6]

Not really seeing how this is different than 2 1Nxxxx diodes (or LEDs, or whatever) in the same configuration. Not really much "tubiness" in this circuit, as far as I can tell.

This is the Peavey "saturation" circuit done with tubes. O'Connor addresses this type of circuit in great detail in TUT 1.

[Andy Le Blanc]

Its two diodes, tube diodes, they shunt to ground when they conduct.
They are wired so that one conducts while the other shuts off with an AC signal.
So if you present the circuit with a sine wave, the diodes turn on when the
signal reaches enough of an amplitude (get big enough) limiting the signal,
squaring off a portion of the sine wave. The good thing about using a
converted triode is that it is in no way an ideal diode.
It can't give you a perfect clipped (square wave) signal, its soft clipping,
more of a way to "wave shape" a signal

Many guitar effect pedals that make distortion, overdrive or fuzz are a
gain stage (small amp) followed by a couple diodes, just like this, that clip the signal.
Marshall amps have used clipped for decades to get that "cranked" tone.

Remember, all tubes are just big diodes with extra bits thrown in
between the plate and the heater.

mlp-mx6 isn't wrong, the point is that this is another option, and you can use
very common tubes to do it, bench scraps..... get the sand out of the circuit.

[Andy Le Blanc]

It works and looks practical. It's like a two transistor dirt pedal, two gain
stages and two diode's as a clipper between. I've first tried a zero bias dress
stage driving a 500k gain pot. , the clipper and then a follower stage and another 500k volume pot..
The output with this is less than unity.
Tomorrow I'll try another zero bias stage to pump up the gain.
Even with limited gain the tone wasn't bad, soft but nice. The plate volts are low.
The R-C gain tables for zero bias, an old Sylvania tube manual is interesting, because....
the distortion figure is presented as the most signal you can apply before 5% distortion.
That makes it perfect for the job, that's a lot of harmonic distortion.

[Andy Le Blanc]

It is very simple and works as a boost pedal, and adds a nice soft distortion.
Two 12ax7, zero bias stages with a couple 500k or 1M pots, and a tube clipper.
Electrical tone or waveform generator, and electronic tone formation is
really what your doing with your amp and guitar, your playing synth.
The guitar supplies a fundamental tone, and the amp supplies a harmonic compliment.
This gizmo makes a triangle wave form out of a sine wave form, opposed to
a square form. I think, as has been brought up, any diode affair will work,
silicone or otherwise. But a square form would only lend to more typical
tones. Wouldn't want to sound like an amp with a cathode coupled long tailed
inverter, with or without a lot of global feed back. Tone color is where it's at.

[KT66]

Andy, I found this thread very interesting. I am not familiar with the zero-bias gain stage, can you explain the purpose of it and why you chose it for this circuit ? What is the amplification factor of a stage like that ?

Any insight you could offer would be appreciated.

[Andy Le Blanc]

When you put a load on a tube it doesn't matter where it is , the plate or cathode.
Its a diode oriented so it always conducting, when you divide the resistance
between these you build the character of the stage. But, no matter what you do,
if you want gain you have to provide the condition for it, bias. Zero bias is
another name for grid leak. If your used to providing bias with a cathode resistor it makes sense.
The problem is that it has a horrible distortion figure, however, I'm using it to an advantage here.
It makes a bit more gain, there's no division of current thru a cathode resistor.
The amplification factor for the tube is unchanged.