The Tremulous Bear

The Tremulous Bear is a boutique-quality tremolo that can be constructed from readily-available components. It does all of the good things you would expect of a modern, flexible trem: traditional Fender-style chop, subtle background quavering and weird modulations. The LFO is an interesting design; you get choice of triangle or square wave and lots of room for hacking and mods. Small Bear offers a complete kit that includes a ready-to-solder PC board. This article describes a perfboard version for advanced builders and includes technical details on how the pedal works. If you bought the kit, here's a link to the instruction manual.

An early version of the Tremulous Bear originally appeared in the August, 2001, issue of Gernsback's Poptronics magazine. This redesign is similar in concept but much easier to build. Here are a few sound clips:

Trem1       Trem2       Trem3

How It Works

The Tremulous Bear gets its time-tick from a somewhat unusual setup of the popular 555 timer. The CMOS version is used here for its low current drain. For those who have never poked inside this chip, here's a look and an explanation of what goes on:

Fig. 1 shows the internal components of the chip inside the dotted lines, with the external connections and components that make it run as an oscillator. Op-amps U1 and U2 are connected as comparators; the chip's internal resistors bias U2 such that its output will change states if the voltage at the inverting input is greater than 2/3 of V+. As you can see, this will happen when external capacitor C charges to that level through resistors RA and RB. When U2's output goes low, flip-flop FF toggles and turns on field-effect transistor Q. The capacitor now discharges to ground through RB and the drain-source of the transistor. When the voltage on the cap is down to about 1/3 of V+, comparator U1 changes states. The flip-flop is cleared, the discharge path is closed, and the charging cycle starts again. We get a square wave from the output of the flip-flop, and the triangle wave at pin 2 results from the charging and discharging of the capacitor. But we need a way to make use of the triangle wave without lousing up the charging of the cap. Take a look at the Tremulous Bear circuit:

IC2 is wired as the LFO. The square wave output at pin 3 feeds voltage divider R5-R6, which sets a suitable level to feed the modulator. Capacitor C9 softens the edges of the waveform, which helps suppress audio ticks. The triangle wave at pin 2 controls the gate of Q1, a MOSFET connected as a source follower. The high input resistance of the MOSFET isolates the timing capacitor from the modulator, and it outputs an exact replica of the charging  voltage across R7. Toggle switch S2 feeds the selected control signal to modulator transistor Q2, through potentiometer R8 and blocking diode D2. D2 keeps voltage divider R9-R10-R11 from loading either control signal. Potentiometer R11 sets the quiescent bias of Q2. As the voltage of the control signal rises and falls, so does the bias on the base of Q2 and therefore the brightness of the LEDs. LED1 gives a visual indication that the effect is operating, while LED2 excites photocell LDR1. The LDR may be a separate component or part of a photocoupler assembly.

The "Var" Switch

For greatest flexibility, it's desireable for a tremolo to offer control of the duty cycle of the controlling signal, i. e., what percent of the time is it "on" and what percent "off"? Getting an uneven duty cycle from a 555 oscillator is easy; it's getting equal on and off times that takes a little doing.

Looking back to Fig. 1, the timing capacitor charges through two resistances, RA + RB, but only discharges through RB. Because of this, we can't get equal charge and discharge times. This is OK if we want an odd effect, but we also need a way to get a "normal" 50% duty cycle. Diode D1 is switched in or out to take care of this problem. With S1 open (the "Variable" position), C7 charges and discharges through two resistances. With S1 closed (the "50%" position), C7 charges mostly through R1 + R2 because the forward resistance of the diode is much lower than the combination of R3 and R4. C7 then discharges entirely through R3 and R4, because the diode blocks in its reverse direction. Since D1 creates separate charge and discharge paths for C7, it is easy to set R1 and R3 for equal on and off times.

The Modulator

The design of this stage is based on the Commonsound Tremulus Lune. IC1a is a simple amplifier that provides a constant input impedance for a guitar. The resistance of LDRl determines the input level to IC1b. IC1b provides a convenient way to match the output level of the circuit to your taste, and a low output impedance for easily driving effects loops.

Bypass and Control

Switch S3 is a DPDT alternate-action type. It provides true bypass with control of the in-use/modulation indicator LED using what is commonly called the "RAT bypass" circuit. When S3 is in bypass mode, level control R25 grounds the base of Q3, a darlington transistor, and so cuts it off. This floats the emitter of Q2 and prevents illumination of the LEDs. I have done extensive testing of this part of the circuit, and I have found the switching both positive and completely silent.

How To Build It

The number of components and the tooling requirements make the Tremulous Bear an intermediate-level project. Before trying this one, you should have built at least one pedal that required tooling of a metal case and working with perfboard or Veroboard.

One caution to hobbyists who have never built a tremolo: the hardest part of making a good trem is keeping the LFO from introducing thumps or ticks into the audio output. It is doubly difficult when using the 555, because the switching action of the comparators puts noise on the power supply rails and produces harmonics that go into the RF region. Notice that the board layout clearly separates the LFO from the modulator. This also makes it easy to physically separate the leads that go off-board from the modulator from those for the LFO. If you hand-wire or create a different board layout, I strongly suggest that you follow these guidelines. If you build the Tremulous Bear as shown here, youíll find that it is tick-free.

Here is a look inside the case:

I managed to scrunch my first prototype onto a medium pad-per-hole board, but I chose to spread this one out to a large-size for ease of construction.

Begin by following my instructions for building a 5-knob Shell, but use this drilling template. You want to create the tiny pilot hole for the stomp switch, then holes for the pots, toggle switches, LED bezel, input and output jacks and power jack.

Note: The enclosure shown here is a pre-powder-coated white. If you start with an unpainted box, don't do any painting or decorating yet. If you are working with a pre-coated box, make sure that you grind or sand away the color around the holes for the input and output jacks. When you have finished drilling the holes and grinding out any molding artifacts (step 7 in building the shell), temporarily mount the jacks and the toggle switches. The result should look like this:

The next step is to tool the blank perfboard, create cutouts for the jacks and switches, and establish the locations for the mounting studs. The layout drawing and the detail photos show the areas that need to be cut away. Mark these carefully with a Sharpie or similar marker. I left the cutout for the battery for later when doing this proto.

Use any combination you want of cutting, scoring or grinding to create the cutouts. (IMORTANT NOTE: Machining epoxy-glass circuit board creates fiberglass dust, which is noxious stuff! When doing any tooling of the board, wear disposable gloves and a face mask!)

After you have done the cuts, drill a 1/8" hole at index R17 for the LED indicator wires. Then bore another 1/8" hole at the lower right corner for a mounting stud. If you have made the cuts correctly, the board should slip right over the switches and down to the floor of the case, like this:

While holding the perfboard in place, bore through the pilot hole that you created earlier on the top of the case. This may create a new hole in the board, or the drill may go through an existing hole. Either way, you now know the starting point for creating the cutout for the stomp switch. Mark this point with a Sharpie.

Now mark the center of each of the corner mounting holes for the board. Using carborundum paper (or a Dremel tool if you have one), sand thoroughly over and around the mark for each mounting hole to create a cleaned location for a mounting stud.

Using a Unibit, or twist drills and reamer, bore the hole in the case for the stomp switch. Drill a small pilot hole in the board at the point you marked for the switch cutout, and slowly enlarge it and square it off. Mount the switch temporarily. If your cutout is just right, the board will once again slip into place over the switches:

You are ready to install the mounting studs. With 3/8" studs, I found that board just barely cleared the cans of the pots. To provide a little more room between them, use quick-setting epoxy cement to attach two flat washers under each mounting hole as  shims:

Temporarily install screws and studs in the mounting holes. Re-position the board, and make sure that the flanges of the jacks have room to move freely when you insert a plug. The result should look like this:

Glue the studs in place and reinforce with more epoxy in the usual way.

Remove the board, tool the cutout for the battery, and install either a battery clip  or a bump-on attached to the cover to keep the battery from shaking:

Now disassemble, and paint if you need to or just do your decals. The shell is finished, and you are ready to stuff the board. Here's the layout, and a parts list:


Some suggestions and notes:

I have covered techniques for wiring on pad-per-hole perfboard in the article on the Tweak-O, so I will not repeat here.

The leads for capacitors C7 and C8 and diode D2 need to be formed so that they span four holes.

Some builders will want to try different photocouplers or use a separate photocell and LED, so a three-pin length of single-in-line socket material is installed for each side of the assembly. Accommodating that change may also require trimming the value of R14, so a four-pin length is included for that.

R12 should be a minimum of 100 meg to avoid switch pop. I got this by putting five 22 meg resistors in series, though 100 meg resistors are sometimes available as surplus. I installed them radially, soldering the bottoms in place first, and only later connected them when doing the wiring.

To help ensure that a build works the first time I power it up, I use a yellow highlighter to mark a connection on the layout drawing as I make it, and then test the connection for continuity:

Seeing The Light

The switching speed and resistance range of LDR1 make a difference in the percussiveness of the effect, and the brightness of its associated LED will affect its resistance curve. Many photocouplers, both commercial and home-brewed, will  work. I have tried the Vactec VTL5C3 and the Silonex NSL-32, as well as a bunch of Clairex parts of different specs. I have also successfully used photocouplers that I cobbled from a red high-brightness LED and a photocell. If you go this route, try an LED rated 3000 millicandles at 20 ma. forward current, like the red high-brightness water-clear on my stock list. A Kingbright (Mouser) L53SRCE is similar. Both the Clairex CL7P5HL and the Silonex NSL-5542 are fine for the cell.

If the illuminated resistance of the cell is too low, you'll hear distortion on volume peaks when the depth and/or bias is heavy. In that case, raise the value of R14. I found that the VTL5C3 worked well with R14 at 3.3K. The Silonex part wanted a resistance for R14 of 5.1K. If you are looking to qualify parts from your junk box for this job, I found that the modulator wants an illuminated resistance for LDR1 in the range of 10K with an LED drive current slightly over 2 ma. The kit includes a Clairex photocoupler with a recommended resistor value for R14.

Make sure that you observe polarity when installing the LED side of your photocoupler!

Install the LED assembly, and make sure that you correctly color code its leads. Thread its leads through the hole in the board above Q1 before screwing the board down. Install the completed board in the case and wire to the outboard components. The grounding scheme I used was: Cold side of R25 to sleeve of J2, to sleeve of J1, to board ground at I-17.

Setup and Testing

Connect a battery or power supply and plug your instrument into J1. Start with the depth and bias pots about half way up, both speed pots about half way up, level about 3/4 of the way up. You should see activity from the in-use LED and the effect should be live. If it doesn't kick in after you click the stomp switch, there's an error somewhere. See the troubleshooting tips.

Using It

This is not a two-knob trem. The controls in the Tremulous Bear interact with each other, and it will take some practice to learn what settings you find pleasant for the kind of music you are playing. The bias control affects the percussiveness of the trem by filling in the space between beats with unmodulated signal, and the level pot will allow you to balance the loudness of the resulting output with the straight guitar signal.


If the LED doesnít flash, itís time to break out the multimeter. Remember that the battery voltage is switched by the input jack, so you have to have a plug in J1 when you take measurements. First, make sure that pin 8 of the 555 has about 8.5 volts on it (presuming that your battery is actually 9 volts.) If you donít see this, review all of the power connections and test the continuity of the associated board traces. When you have the power straightened out, start testing the continuity of every joint and trace in this part of the circuit. Once the oscillator is working, try the effect again.

If there are problems with the sound, first check the supply voltages on the modulator side. You need to have +8.5 volts at pin 8 of the chip and +4.25 on pin 3 and pin 5. If the voltages are there, see if you can narrow the trouble to one stage by using the guitar amp as a signal tracer. Again, you may have to check each PC board trace and connection with an ohmmeter.

Mods and Substitutions

Say that your amplifier doesn't have tremolo, but you want to do some "trem-over-trem" experiments. Take another look at figure 1...notice that pin 5 is marked "VCO", for Voltage Controlled Oscillator. The chip designers included this function to allow control of the output frequency by an external voltage. I haven't tried it (no time), but I'm sure that applying a varying voltage here from a second LFO (or maybe from a swell pedal?) would do something suitably gnarly. Obviously, the LFO can also be extracted as a circuit fragment for driving vibratos, choruses, phasers, flangers and stereo panners. Have a field dayÖ

I hope you enjoy building the Tremulous Bear, and I welcome comments and questions at