Audio synthesis via vacuum tubes/Keyboard controller for tube synthesizer

The keyboard controller for tube synthesizer compensates for the variable response of the individual thyratron tube used in the tube VCO instead of the more familiar voltage divider network.

Background
Classic analog synthesizers commonly achieved equal tempering with one of two schemes: either a linear-response VCO (or VCOs) with a keyboard attached to a voltage divider network with exponential scaling; or, more commonly, a VCO with an exponential-to-linear voltage scaler circuit, and a keyboard with a linear divider network. Both schemes have their weaknesses. The former, when an LFO or other modulation voltage was added to the VCO input, did not offset in equal tempering. Also, the resistor divider often required unusual high-precision values. The latter scheme was easy to do, If one could control the temperature drift endemic to the exponential scaler (it usually relies on base-emitter exponential response to do the conversion, which drifts a great deal with temperature). The thermistors used in early exponential scalers are now difficult to obtain.

These problems do not apply directly to our tube synthesizer. Instead, we have a different problem: our very simple VCO is temperature-stable, but has a response that varies semi-randomly with the individual thyratron tube used. So, we not only have to use a non-linear divider on the keyboard; we have to use one that can be adjusted for the individual thyratron. If we want to use two or more VCOs in parallel, we have to match the thyratrons for pitch-to-voltage response curves.

The Trautonium never had this problem--it used a ribbon controller, with no fixed keys. We could always do that with our tube circuits. Refer to the classic Electronotes newsletter for a number of suitable circuits. However, if you insist on having an equally-tempered keyboard, we must use a divider made of trimpots.

How it works
Figure 1 shows how this is done. Normally, the output voltage of the divider drifts up to the supply voltage (you can use +12 or +15), because none of the keys are down and the voltage is not divided. When a key is pressed, the voltage goes to a lower level. By appropriately adjusting the trimpots, the output voltage can be set to driven a tube VCO to follow equal tempering. (It should be obvious that you adjust the pots starting at the leftmost key, i.e., the lowest-pitch key.)

We follow the buffer IC1a (which can be any inexpensive single-supply quad op-amp) with a sample-and-hold, Q1. IC1b is used as a comparator to detect the drop of the divider voltage from the +V, and raises its output. This is the gate signal. It triggers the S/H, and may be used to operate any conventional ADSR or otherenvelope generator to feed the VCA. Q1 can be any inexpensive n-channel junction FET, such as the commonly-used 2N3819 or 2N5486. (Obviously, +v should come from a well-regulated supply. Less than 20 mA are needed by this design.) The pitch bend/adj pot shown on the output buffer IC1D is optional--novices would be smart to just ground that point from the 10k resistor, and leave this pot out. It complicates tune-up. Note that it will not give a full pitch-control range on the lowest few keys. If you desire a true up-or-down pitch bend, this would be more easily done with the master-tune pot in the anode of the thyratron.

Our controller circuit will give about 2 1/2 octaves of control range on the tube VCO, from a few hundred millivolts above ground to nearly +V. This should be enough for most experimental music applications. (Getting a greater range out of the thyratron VCO is an advanced subject, which we will not get into at this time.) The AGO keyboard sold by PAiA Electronics can be used with this circuit. It has 37 keys. We can only use about 28, so remove the unused keys after the keyboard is installed in your intended cabinet, and cover the open areas with aluminum or plastic plates. I have built a few such circuits, and can assure you that they work very well.

Alternatively, if you already have an old analog controller keyboard and want to convert it to drive the tube VCOs, just substitute the trimpot divider for the existing voltage divider. This keyboard can be used to drive any VCO, tube or transistor, linear, exponential, what-have-you. And it can produce any desired scale; equal-tempered, just, microtonal. Simply drive the VCO desired and adjust each trimpot (while holding down its key) for the desired pitch. Finally, MIDI control of your tube circuits. You can NOT use a conventional MIDI-CV converter, due to to difficulty of converting the (usually linear) CV output to the voltage the tube VCO would like to see. So, just replace the keyboard in Figure 1 with a JW Electronics MIDI-to-Parallel converter. Its relay outputs are open-collector, and perform the same function as the keyboard by shorting the required divider point to ground. I have used an MTP-1 for this successfully. Its major advantages are that it is less expensive and easier to use than other MIDI-parallel interfaces now available. (And yes, there is nothing to keep you from wiring the MTP-1 AND the keyboard to a single controller. All they do is ground a point on the divider. So, as long as one is not being used, the other can control the synth.)

Warning
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.

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