Audio synthesis via vacuum tubes/Metasonix TS-21 hellfire modulator

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The Metasonix TS-21 hellfire modulator is unlike any electronic audio processor you have tried before. So, we recommend reading this manual carefully and thoroughly before starting. Experimentation with the TS-21's controls is encouraged! You can't damage the TS-21, no matter how "wrong" you hook up the controls because it is all-tube. The following is taken directly from the original owners manual. Only some parts of this document will apply to any home constructed units. Please note that Metasonix is not available for troubleshooting help.

Beginner's quick reference

circuit diagram of the TS-21 Hellfire Modulator. Click through for a larger version.

1. Plug the TS-21 into a source of 100-120V AC power. It uses about 25 watts. (Check the label on the back if the unit is wired for 240V, attach 220-240V power.)

2. Plug a signal source into the AUDIO INPUT jack. A line-level signal is best, though it will also accept dynamic microphone or electric guitar.

3. Attach the AUDIO OUTPUT to an amplifier or mixing board's high-impedance line-level input. A mono cable is OK. TS-21 produces a stereo signal at all times to hear the stereo signal, use a 3-wire (ring/tip/sleeve) 1/4" cable to go to a stereo mixer. Please use the volume control in the amp or mixer to control the listening level. (Stereo headphones are especially useful in understanding the TS-21's sounds when first starting out.)

4. Set the controls of the TS-21 as follows:

  • INPUT LEVEL - full counterclockwise
  • PWM MANUAL - full counterclockwise
  • PWM BYPASS - down
  • RANGE - down
  • BEAM SCREEN - 12 o'clock position
  • L1 - full counterclockwise
  • L2 - full counterclockwise
  • SPEED - doesn't matter

Turn on the power. Allow the unit to warm up for a few minutes.

5. Play the signal. Adjust the INPUT LEVEL and the volume controls in the amp or mixer to get a comfortable signal level. (If the amp/mixer has a clipping level indicator, keep the TS-21 output level below clipping indication.)

6. First, try adjusting the BEAM SCREEN control. Observe how the waveform and its "tone" change. (You might hear a slight crackle at around the 3 o'clock position this is normal for the circuit you do not have a bad TS-21!) Set the BEAM SCREEN to the 4 o'clock position.

7. Then try adjusting the PWM MANUAL control. As it is turned up, you will hear the tone get "spikier". At around the 12 o'clock position, it will break into a howl of self-oscillation. Keep the control below this point. (Note: this point varies with the input signal level and the INPUT LEVEL setting.)

8. Now try the beam tube's L1, L2 and SPEED controls. Turn the L1 and L2 controls to the 12 o'clock position. You should hear the signal switching back and forth. (It sounds very "sloppy" and weird this is normal.) Adjust the L1 and L2 controls to set the switching effect the signal distortion will change also. The SPEED control and L1/L2 will interact to control the speed of switching.

9. Now set the RANGE switch to the up position. It will interact with all the beam tube controls and produce all kinds of unexpected effects.

How it works

The TS-21 hellfire modulator contains two independent effects circuits.

The first one is the PWM circuit. This circuit was taken directly from a 1960 textbook on pulse and switching circuits using vacuum tubes. Its behavior is so peculiar that we just had to make an audio waveform processor out of it!

The PWM is actually a very crude monostable multivibrator. Because it uses positive feedback, it belongs to a class of designs known as "metastable" circuits. Varying the PWM MANUAL control causes the switch point of the PWM circuit to change. It only responds well to sharply-defined waveforms, such as square waves or sawtooth waves. The output pulses tend to be short and rich in harmonics but also might have noise or bursts of oscillation riding on them. There are some settings which will cause the PWM to burst into oscillation, whether a signal is going into it or not. It can produce something resembling "VCO sync" effects with certain settings and input waveforms.

The second circuit is the beam modulator. This is based on an obscure tube, which (so far as we know) has never been used in audio equipment. The 6AR8/JH8/ME8 was originally intended as a balanced modulator for use in TV sets, as the chroma detector! It is a very, very crude form of balanced modulator it literally uses electrostatic deflection to switch the input signal between two anodes, while also providing some signal gain. It's not a perfect balanced modulator, as it does pass some of the input signal to the output (in other words, it doesn't completely suppress the carrier signal). However, its electrical behavior is so strange and un-solid-state-like that it can be used as a sound effect.

The beam modulator in the TS-21 is wired to a built in LFO. This LFO can be disabled if the modulation effect is not wanted, by turning down both the L1 and L2 controls.

It causes the beam to switch back and forth between two anodes, which then go to the output. That's right this is a stereo modulator. You can insert either a mono 1/4" plug into the AUDIO OUTPUT jack, and get a mono signal. Or you can insert a stereo plug, and get a stereo effect. The beam tube switches the signal back and forth between the two outputs. Plus, the beam tube has something solid-state lacks a screen grid. By adjusting its voltage, we get both gain control, and we get variable distortion of a very unusual nature! You have to hear its effect to understand. The beam tube is very non-linear in most settings, so it can be used as a distortion effect. At some positions, the beam screen control causes waveform "folding" roughly similar to that seen in the Serge waveshaper modules but with tube-style nonlinearities.


The controls on the left of the TS-21 control the input preamp and PWM.

  • INPUT LEVEL - just what it says controls input volume. The input goes into a pentode preamp, which provides lots of gain.
  • PWM MANUAL - this is the PWM's most important control its setting both controls the width of the pulses, and allows the PWM circuit to burst into oscillation if turned past the 12 o'clock point, going in the clockwise direction. You really need to adjust this control carefully, while listening to the results, to find out what the PWM circuit does. It can be a "touchy" control. Note: The PWM circuit works best with waveforms having sharp discontinuities, such as square or sawtooth waves. Although the TS-21 can be used to process "smooth" signals from sources such as guitar or voice, it works best with synthesizer waveforms. The setting of the INPUT LEVEL also is critical for PWM operation.
  • PWM BYPASS - switches the PWM circuit out of the signal path, when switched UP. Then, only the pentode preamp and beam modulator are in the signal path.
  • RANGE - switches the signal into different input electrodes on the beam tube. Try experimenting with the two settings to get a feel for the results. When the RANGE switch is in the up position, the L1-L2 controls have to be turned up somewhat to get a signal.

Also on the left is a VCA CV input, which lets you control the gain of the pentode preamp and use it as a VCA. If using this, the gain of the circuit is limited, requiring a fairly "hot" input signal to get interesting results. Note that this does not affect the output there may be some noise in the output, internally generated by the PWM and beam tube, even if this CV is at zero volts. Use of a post-TS-21 noise gate is recommended if you intend to use it as a tube VCA.

The controls on the RIGHT of the TS-21 panel deal with the beam modulator.

  • BEAM SCREEN - controls the screen-grid voltage to the beam modulator tube. Turn it fully counterclockwise and the tube is fully cut off, with no signal reaching the output. Turn it to the 12 o'clock position to fully turn the signal output on. Turn it past the 12 o'clock point to vary the distortion. Various settings of this knob cause different waveform "folding" effects. You should experiment to fully understand it.
  • L1 and L2 - these adjust the level of the built in LFO which is applied to the deflection electrodes in the beam tube. They also interact with the LFO to control its speed slightly. Try setting them to the 12 o'clock position to start, and increase them slowly until the deflection effect is heard.
  • SPEED - controls the rate of the built in LFO. Turn the control to approximately the 10 o'clock position to obtain LFO rate control. From this point to the fully-clockwise position, the LFO speed is adjustable from approximately 1 Hertz to about 5 Hertz. NOTE: this control interacts with the L1 and L2 controls.

Also on the right is a BEAM/PWM CV input. This input modulates both the PWM MANUAL setting and the BEAM SCREEN setting, and is in parallel with the settings of these controls. This input is capacitively coupled, so an LFO or other fast changing control voltage is required to get usable effects. An audio signal may also be used to drive it this signal should be very "hot" (we recommend at least 10 volts p-p) to get good results.

The audio input is high impedance (250k ohms). It can be driven with any line-level source. The outputs are 10k ohms impedance, and should be used only with high impedance line level equipment inputs (such as are found on most modular and pre-patched analog synthesizers). The VCA CV input is high-impedance and is usable with an input voltages from 0v DC to +5vDC or greater. The BEAM/PWM CV input is 10k ohms and should be driven only from a low impedance control voltage output.


When mounting the TS-21 in an EIA 19" rack, note that it produces some heat. It will not get as hot as a guitar amp but convection cooling must be allowed for. We recommend at least ½" space above and below it to allow airflow. The TS-21 uses a 0.5 amp, 5x20 mm fast-blow fuse. This fuse should not blow under normal circumstances.

Obviously, you should keep the TS-21 cool and dry. Moisture can ruin it instantly.

The tubes in the TS-21 will probably last for many tens of thousands of hours. They are being run conservatively. Nevertheless, should you need to replace any of them, this is the replacement guide:

Warning: the TS-21 is full of high voltages. If you are not experienced in working with tube electronics, leave the following to a competent servicer!

Unplug the unit and let it sit for at least 15 minutes, to give the capacitors time to discharge.

Remove the top cover eight small Phillips-head sheet metal screws. Be sure to replace the cover and screws when done.

The tubes are as follows. Be careful not to break any wires when removing or inserting a tube.

V1 is a 6AL5 or 6097. It is the plate-voltage rectifier tube that power the other circuits.

V2 is the pentode preamp. Types from the 6AK5/5654 family work here. You can also use the 6AU6/6BA6/5749 family. There is a long list of tubes which will work in this socket. We recommend 6AK5 types as a safe choice (unless you are really crazy and want to experiment with "tube rolling" in that case, you are on your own! Be sure to get the right pinout tubes, or you will invalidate the TS-21's warranty!).

V3 is the PWM tube. We recommend a 6AQ8, 6BQ7, 6BK7, 6BZ7, 6BZ8 or similar type. (Some types that will work in the socket, such as 6DJ8/6922 or 6CG7, may not work as pulse generators as well as the 6AQ8/6BQ7 family, which have really awful electrical characteristics making them interesting for the TS-21!)

V4 is the beam deflection tube. 6AR8, 6JH8 or 6ME8 will work here. The 6ME8 will sound different from the other types, as its voltage gain is lower.

V5 is the LFO for beam switching. It is the same type as V3. You can try different tubes here, but as it's only used as a simple oscillator, you won't notice any tonal differences.


This circuitry is intended for the more advanced builder. Because high voltages are used, a shock hazard exists. We do not recommend that the novice DIY musician try to construct this module. Some experience with tube electronics is highly recommended.

All these projects and designs should be considered dangerous if not lethal if not used safely. When working on projects based on these designs, use extreme care to ensure that you do not come into contact with mains AC voltages or high voltage DC. If you are not confident about working with mains voltages, or high voltages, or you are not legally allowed to work with mains voltages, or high voltages, you are advised not to attempt work on them.


Readers are permitted to construct these circuits for their own personal use only. Eric Barbour retains all rights to his work.

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