Output circuit

The output circuits tie the pitch and volume circuits together into a complete Theremin.  The critical functional component is a voltage-controlled amplifier (VCA) with its gain set by the DC control voltage generated by the volume control circuits.  The VCA amplifies or attenuates the signal produced by the pitch circuitry and produces the familiar Theremin output.  I also added an output buffer to reduce the Theremin’s overall output impedance.

The VCA circuit itself somewhat resembles a differential long-tailed pair, but doesn’t really behave like one.  Credit for its design goes to Art Harrison and his 126 Vacuum Tube Theremin.  I tried a few VCA designs of my own conception, but in the limited time I had remaining I couldn’t come up with anything as simple and reliable as Art’s VCA.  The signal from the pitch circuitry is fed to the grid of U6A through a large coupling capacitor.  The DC voltage from the volume circuitry also appears at the grid of U6A and sets its bias point.  When the volume control voltage is most negative (with your hand far away from the antenna), U6A is mostly cut off and doesn’t conduct much of the pitch signal.  Likewise, when the control voltage nears zero volts, U6A conducts and the pitch signal makes its way through the connected cathodes of U6A and U6B to the output amplifier.

If you’re wondering what the large resistor R20 is for, think about the high output impedance of the pitch circuit’s RC filter and the low output impedance of the volume circuit’s half-wave rectifier.  Without R20, the pitch circuit will be loaded down by the volume circuit and you won’t hear any output from the Theremin.  I found this out the hard way after we had fully assembled the GT Theremin.  Every functional component had worked correctly on its own (the pitch circuit, volume circuit, and output circuit), but something went afoul when everything was connected together.  Eventually I discovered the problem and added R20 to fix it.  My care in keeping the schematics tidy and on separate pages worked against me by helping me to completely overlook the obvious problem with connecting the volume and pitch circuit outputs directly together.  The large value of R20 has a fairly noticeable effect on the responsivity of the volume control, since it limits the slewing rate of U6A’s grid.  I’ve chosen a compromise value which I believe yields good results, but you can tweak it somewhat if you want to play with the volume response.  Don’t reduce the value too much or you’ll defeat its purpose entirely.

U3B is another cathode-follower current amplifier.  It sets the GT Theremin’s output impedance at a bit under 2.2 kΩ.  It buffers the output signal, though it’s not completely necessary to include.  I added it mainly because the mixer only uses one half of its 12AU7A triode, and it seems a shame to leave the other half unused.  I was a little concerned about the possibility of some of the RF signal in the mixer coupling to the output buffer by virtue of co-habiting the same tube.  However, the plate-to-plate capacitance between sections in these small twin-triodes is fairly small (on the order of 2 pF), and I haven’t observed any problems with this arrangement.

The output is physically connected to a standard 1/4″ mono phone jack.  No trickery here.  I also added a simple muting switch, which I believe to be an invaluable feature for a device capable of producing such annoying sounds.

Power supply

The power supply is, in principle, very straightforward.  However, it presented me with several unexpected difficulties that I wish to discuss in some detail.  I wanted to keep the power supply all tube-driven along with the rest of the GT Theremin.  I also got hooked on the idea of using an 0A3/VR75 glow regulator tube very early on (that’s the big tube in the photo below; the glow actually appears a good deal brighter to the eye).  Why?  I’d be lying if I gave any other reason than “it looks really cool”.  Both of these requirements made the design more challenging to implement, but I’m quite pleased with the end result.  However, there’s no reason my power supply design couldn’t be replaced with a much simpler diode bridge based circuit so long as it can provide about 20 mA on the 75 V rail.

The power supply consists of a low-voltage heater supply circuit and a high voltage plate supply, affording a nominal 6.3 VAC and 75 VDC, respectively.  The low-voltage circuit must be capable of supplying 2.5 A for the vacuum tube heaters.  This is accomplished very simply by using a 6.3 VAC center-tapped transformer wound for just this purpose.  Some audio purists prefer to use a DC heater supply to avoid any possibility of a 60 Hz tone coupling into the signal path via the heaters.  Fortunately I adhere to no such doctrine and lack golden ears anyway, so this simple design is perfectly adequate for the heater supply.  Note that the 12AU7 and 12AX7 triodes can use a 12 V or 6 V heater supply, so care must be taken to properly connect the heater and heater center tap pins such that the tubes heat up sufficiently.

The high voltage source is a 220 VAC center-tapped toroidal transformer (that’s 110 VAC between the center tap and each side of the secondary).  It isn’t necessary to use a toroidal transformer here; it was merely what we could source on short notice, though it does have the added benefit of looking pretty cool.  This transformer has a secondary winding voltage significantly higher than the 75 V required from the high-voltage supply.  This is due to the fairly large voltage drop over the vacuum diodes, as well as the necessity that the supply be capable of producing an initial 110 VDC to strike the glow tube (more on that later).  The transformer must also be center-tapped in order to implement a full-wave supply.  Those of you familiar with semiconductor full-wave supplies may immediately think of a bridge rectifier.  However, allow me to draw your attention again to the aforementioned large voltage drop over the rectifier tubes: around 10 V as opposed to ~0.7 V for a semiconductor diode.  In retrospect, I think a half-wave rectifier would have been perfectly adequate for the GT Theremin and could have saved us a lot of trouble in hunting down a transformer.  Regardless, U7 is a 6X4 full-wave rectifier tube (datasheet) whose function is self-explanatory.  C30-32 filter the half-wave rectified power signal and produce a DC value.  The only reason I used three capacitors instead of a single larger-valued one is because these were on-hand at the time.  As with all the capacitors in the GT Theremin, care must be taken that the filter caps are rated for a higher voltage than the roughly 150 VDC peak that they might see.  The caps I used were rated for 500 V, though this is over-kill.

Fuses F1 and F2 do the typical fuse thing, no surprises there.  Don’t omit them unless you want wires or resistors to end up serving their purpose.  Both fuses must be slow-blow type, otherwise the initial current rush from a cold start will melt them.  The metal case of the Theremin absolutely must be connected to an earth ground by a three-prong power cord.  Once again, the high voltages used in this design are nothing to be trifled with; it’s far better to trip a circuit interrupter than to electrocute the performer.

The string of LEDs was included entirely for visual impact and is completely optional.  They do have the side benefit of quickly discharging the high-voltage supply when the mains power is removed, but this could also be accomplished with a bleeder resistor.  Note that the current drawn by these LEDs (about 10 mA) was included in the calculation of the very-important series dropping resistors R26 and R27.  If the LEDs are not present, either the power supply dropping resistors must take a larger value, or a ~7.5 kΩ bleeder resistor must be added in the LEDs’ stead.

Now to explain what that weird 0A3 glow tube does and what the lonely silicon diode is for.  The 0A3/VR75 (datasheet) is a gas discharge (or “glow”) tube which functions as a simple shunt voltage regulator akin to a Zener diode.  Instead of operating on Zener breakdown, however, the glow tube is filled with an inert gas (neon in this case) which will sustain a fairly constant voltage drop over a limited range of current once partially ionized (the spontaneous emission of light is a side-effect often used in “neon” signs).  The minimum ionization voltage is higher than the regulated voltage (110 V versus 75 V in this case), so the power supply must be capable of briefly supplying this “striking” voltage to start the glow tube.  Glow tubes were typically used as part of voltage references in precision test equipment.  The high voltage supply in the GT Theremin doesn’t actually require regulation, but I was determined to use this very neat looking piece of glass regardless.

However, I did not realize until late in the design that the tube’s negative differential resistance makes it keen on oscillating when connected in shunt with a large capacitance, such as the electrolytic decoupling caps used throughout the signal circuitry.  This configuration is known as a relaxation oscillator and is how a lot of high-power RF sources work (using a Gunn diode rather than a gas-filled tube), but is a very undesirable feature in a power supply.  I was running up on deadlines by the time I discovered this oscillation issue, but was still far too stubborn to simply remove the 0A3 tube.  Therefore, I used a single silicon rectifier diode to break up the relaxation oscillator circuit.  It works like a charm, and the diode is small enough for me to discretely hide my shame beneath some layers of heat shrink tubing.

Resistors R26 and R27 drop the DC voltage from the filter capacitors to the appropriate 75 V at an average quiescent current.  The reason I used two resistors instead of one is merely because we had no 3 watt resistors on-hand.  Remember to be conservative about power dissipation limits when paralleling power resistors since the current will be unevenly split through real non-identical resistors.  One last note about the 0A3 is that it includes a pair of jumper pins intended to be used to cut power to the high-voltage supply if the glow tube is removed.  This is a good safety precaution, and my power supply circuit implements it.  As configured, the regulated 75 VDC rail provided by this circuit can supply a maximum current of about 25-30 mA, which is more than sufficient.

« Previous: Circuit details part 1 | Next: Construction »