LFO: tremolo, vibrato and phaser oscillators

How a phase-shift oscillator works

A triode turns its signal upside down, half a turn of phase. The ladder of caps and resistors from its plate back to its grid pushes the phase ahead a bit per section. At one frequency the ladder adds the other half turn, the signal comes back to the grid in step, and the loop keeps itself going. That frequency is the speed. The Speed pot is one of the ladder’s resistors.

The ladder also loses a lot of signal, a factor of 29 for the textbook one, so the tube needs more gain than that to start. Then the swing grows until the tube runs out of room, pulling grid current on one side and cutting off on the other. That sets the size of the swing and how far from a sine it gets. Clipping and grid current shift the phase too, so the real speed comes out slightly below the loop gain’s figure.

How an op-amp LFO works

The op-amp version switches back and forth. Part of its output comes back to the + input, so the output snaps all the way high or low and stays. A resistor charges a cap from the output, and the cap feeds the − input. When the cap reaches the switch point, the output flips, the switch point moves, and the cap heads the other way. The output is a square wave, and the cap ramps between two switch points. The resistor and cap set the speed, and the feedback resistors set how far apart the switch points are. It starts on its own, and the switch points set its swing.

The ramps curve because the cap slows as it nears where it’s headed. On the Phase 90 the switch points sit above the middle of the output’s swing, so the cap climbs slower than it falls and the wave leans. The figures show the switch points on paper, from the op-amp’s limits and the divider, and their speed. At the fast end the real run is slower, since the small cap on the − input makes each flip slightly late.

Watching it run. Everything moves at the oscillator’s speed. The round light by the waveform’s heading pulses with the node on the scope, like a tremolo pedal’s speed light. Each wire on the schematic glows brighter as its node swings high and fades as it swings low, scaled to its own swing (under 0.05 V stays dark). On the tube versions the glow moves round the ladder from plate to grid, each node peaking slightly later than the one before: the phase shift, adding up to the half turn. On the op-amp versions the output’s wires snap on and off while the cap’s fade up and down. ⅛ speed slows it all down, and Still stops it.

Loop gain. For the tube oscillators we break the loop at the grid, drive it with a small test signal, and measure what comes back. Where the phase comes back to zero, the loop runs if the gain there is over 1 (0 dB). The further over, the faster it starts and the harder it clips.

What each one drives. In the blackface amps the second triode pulls current through a neon lamp in a light cell, whose resistance works the signal. Here the lamp is a fixed voltage drop, so it loads the oscillator as in the amp. The light and cell aren’t modeled. In the 6G7-A Bandmaster a cathode follower feeds the vibrato circuit, which moves two tubes’ bias in opposite directions. The earlier 6G7 feeds it straight from the plate through 4.7 MΩ, barely loading the oscillator. The 6G16 Vibroverb’s “vibrato” is a tremolo: the follower’s swing goes through the Intensity pot onto the power tubes’ bias. In the Vibro Champ a follower feeds the Intensity pot, whose wiper moves a preamp tube’s cathode so that stage’s gain rises and falls (the tube is in the model so the pot sees a real cathode). In the Phase 90 the cap’s ramp goes through 3.9 MΩ to four JFET gates, with the bias trimmer feeding the same line through 1 MΩ, so about a fifth of the ramp gets there. The gates swing a few tenths of a volt, moving each JFET’s resistance and the phaser’s notches.

Fender’s voltages. The drawings don’t say whether the oscillator was running when the voltages were read. A meter shows a swinging node’s average, and here the plate’s average sits well above its resting voltage, so both are shown beside the drawing’s figure.

Warnings. Red text flags an oscillator that doesn’t start, one still growing at the end of the run, or the solver not settling.

How this tool works. We solve the oscillator with the circuit bench’s engine and models: the operating point with it stopped, then (tube versions) the small-signal loop gain from 0.3 to 60 Hz, then a run in time. The grid (or the op-amp’s + input) gets a 2 V kick for 5 ms, and the run goes until the swing holds within half a percent from one cycle to the next. The Speed sweep does the same at eleven knob settings once you stop changing things. Tube capacitances are left out, since they do nothing at a few hertz. The supply and the Phase 90’s 5.1 V reference are held steady. The tremolo, vibrato and phaser circuits on the circuit bench run on the same engine.

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