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This page details the vibrato circuit behind my AC4 rebuild.
I wanted to add vibrato back to the circuit, but didn't have the real estate inside the amp for two dedicated triodes and plus all the caps and resistors. I had recently been breadboarding an old fashioned oscillator design called the Relaxation Oscillator. This design used a neon bulb to produce a sawtooth oscillation signal. To make the sawtooth way more of a square wave, I sent the sawtooth to to an LED and used an LDR.
These oscillators were used by Magnatone in the sixties, and that's how I because familiar with them. To understand how they work, see my Neon bulb deep-dive.
The vibrato effect effect occurs when the resistance to ground is varied at the point of the 12AX7 grid. This changes the voltage divider created by P1. If P1 is thought of as two resistors, R1 and R2 (R1 is the resistance from the center lug to ground, R2 is the resistance from the center lug up to the signal source). The LDR is parallel with R1, so as the LDR sees "light" and "dark" it varies the voltage divider circuit. It is the same as if you were turning the volume knob up and down.
In my AC4 build, I omitted R204. I also used an LDR that is 5κΩ to 20κΩ when it sees light, and 2ΜΩ when it sees dark.
Lets look at an example if the volume is set to "5":
So the divider varies from 1% to 50% attenuation as the frequency that the light hits the LDR. If you increase R204 from 0Ω up to say, 250κΩ, you'll increase the lower floor 1% from 1% to 25%. This makes R204 a depth control, and it can be a pot.
Vibrato is pitch variation, tremolo is amplitude (or volume variation), so this tremolo. If the duration of the flashing light is is 50% "light" and 50% "dark" regardless of the time frequency of the rate of flash, then the effect is a nice even tremelo. As frequency increases, if the percentage varies to say 10% "light", 90% "dark", the effect is more likely to be echo.
The circuit above is tremolo, and as it slows it becomes more of an echo effect.
The great thing about this type of oscillator is that as the The Speed pot and R201 is serial control the frequency, and it can be very slow if you want. In fact, the footswitch can be connected to a pot to vary the speed.
THE LDR can be placed directly on the NE2H neon bulb. The reason I use the oscillator to fire an LED is because at faster frequencies there isn't enough "dark", the "light" fades too slowly. For a fast tremolo, I let the sawtooth frequency spikes fire an LED. The two 470κΩ resistors form a divider to lower the voltage and make the LED flash in more of an even ON/OFF squarewave frequency.