Understanding Tube Tremolo Circuits

TERMINOLOGY
Before we get started, I feel compelled to define some terms. The terms Vibrato and Tremolo have been used interchangeably throughout the years. They are not the same, and most all tube amplifiers from the 50s to the future only perform the Tremolo function, not Vibrato.

  • TREMOLO - A cyclic change in amplitude (volume). Tremolo can be accomplished by turning the volume control up and down on your instrument in a rapid fashion. The amplifier Tremolo is an automatic feature that lowers the volume at a rate of around 1 cycle per second to several cycles per second.
  • VIBRATO - A change in the frequency of a note. This can only be done with a synthesizer.

Many amplifiers label their Tremolo function as Vibrato falsely. Notice the Fender schematic image below shows a "Vibrato Pedal". These amps do not produce Vibrato. They only produce a Tremolo effect.

CIRCUITS
When I was a kid, beginning to learn about vacuum tube circuits, I was very perplexed with the Fender guitar amplifier tremolo (labeled vibrato) circuit. I had all the basic amplifier and five tube radio schematics stuck in my head. The Fender Twin reverb tremolo (labeled vibrato) circuit eluded me for years. When I finally figured it out, I realized that part of my lack of perception was due to a poorly drawn schematic. Yes, the Fender Twin Reverb Amp AB763, as well as the Deluxe Reverb. Since I had trouble with it, I decided to document my findings here in case some readers find the circuit confusing. It is actually very fundamental and straightforward. Its schematic representation is what threw me. Hereis the original Fenderschematic:

I think this is confusing, It is actually drawn more conducive to a potential circuit layout scheme, rather than a true electronic schematic diagram.

The way I learned to draw schematics for intuitive understanding is that the signal flows from left to right, and positive voltages were on top, ground in the middle (or bottom if no negative), and negative voltages on the bottom. The schematicaboveis far fromthatmethodology.

PHASE SHIFT OSCILLATOR
The tremolo oscillator is simply a phase shift oscillator. Check out the schematic of the simplified oscillator only section drawn below.

What makes an oscillator? The answer is an amplifier with positive feedback. Negative feedback (180°), will reduce the gain of an amplifier. That is how opamps work. They are near infinite gain amplifiers and negative feedback is applied to "tame" the gain to the desired value. Another example of negative feedback is the PRESENCE control of a tube amp. It feeds back the output signal into the power amp input 180° out of phase, but only the low frequencies. This reduces the low frequency gain. Positive feedback causes oscillation. This is achieved in the circuit above by using three resistor capacitor networks. A pure capacitive network is a 90° phase shift. The series capacitor and shunt resistor network in this case provides about 60° of phase shift. Since the tube amplifier output to input is 180°, following the output with three 60° RC networks (3 x 60 = 180), results in 360° of phase shift, which is positive feedback. The frequency of oscillation for a phase shift oscillator is:

The gain of the amplifier must be greater than 29. So now you see a simple tube amp stage with cathode bias and 3 stages of RC networks in the feedback path. This circuit would oscillate at 6.5Hz.

SPEED CONTROL AND ON/OFF CONTROL
Lets add the necessary parts to control the oscillator frequency and turn the oscillator off. Note that R3 (1M) is replaced with a 3M pot, and a 100K resistor. This allows an adjustment range of about 2Hz to 8Hz. On a Twin or Deluxe Reverb, the pedal must be plugged in to ground the resistors. If the switch below connects the resistors to ground, the oscillator works normally.

If opened, the 2.2M resistor to the -50V bias supply shuts off the tube and effectively turns off the oscillator. We will later discuss why both resistors are pulled down to the -50 volts.

OPTO-ISOLATOR DRIVER
The other part of the circuit is the tube that drives the opto-isolator. The opto-isolator is nothing but a neon lamp in heatshrink tubing with a light dependent resistor (LDR). If the light is on, the resistance is low. If the light is off, the resistance is high. The LDR is shunted across the tremolo (labeled vibrato) channel output, and effectively amplitude modulates the signal. The intensity control applies the LDR to the signal, or shunts it more to ground, causing less change in amplitude.

The 10M resistor (R10) is there because a neon bulb is an open circuit until about 90V.

The Vibroverb and some other amps do not use the opto-isolator, and simply use the output of the driver tube to lower the bias of the output tubes.

So, here is the Fender Twin Reverb tremolo (labeled vibrato) circuit redrawn conducive to understanding the circuit. It is an oscillator, neon bulb driver, and an LDR shunting the signal. The output of the oscillator is arbitrarily chosen at the junction between C3 and C4. The Deluxe Reverb AA763 actually picks off the oscillator input of the grid of the oscillator. Almostany place in the feedback wouldbe OK.

Finally, the decision to negatively bias both R4 and R5 to -50V is so both the oscillator and driver tubes are completely biased off. You don't want that 2-8Hz oscillation existing if the tremolo is not being used.

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