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Expo convertor with tubes.

No warning this time.

This circuit is not using high voltage levels at all!


The purpose of this page is to show one of my lab experiments. It describes a novel exponential convertor which employs a vacuum diode as its nonlinear element. Its not a musical module, but could be a subcircuit of one.


The circuit

It is almost a textbook circuit. With the notable exeption that the diode is reverse biassed. There is still a small current even when the cathode is more positive than the plate. This is due to the energy distribution of the emitted electrons, some of them have enough energy (in eV) that they are able to overcome the field between cathode and plate. (The grid is connected to the plate to form a diode from a triode, so these two electrodes are really acting as the plate here.) If this is strange to you, consider that marbles can run uphill, when they have sufficient momentum. The law for this 'up-hill current' (its called the "Anlaufstrom" in german literature, I don't know the english word for it) is given in the picture. Like with the bipolar exp circuit Io (or Ia(0V) if you will) varies with temperature. Also the temperature voltage Et is dependant on the heater current, so the situation is pretty much that of a bipolar circuit. Yet I think this circuit is interesting enough. Since the cathodes are held at a (pretty much fixed) high temperature, no compensation for Io was done. With a dual diode (triode or whatever) you could use a compensated scheme. The main restriction is that in order to ensure that the plate potential is fixed it must be connected into a virtual ground, or at least into a low-z input. The driver must also have quite low z-out to ensure the cathode is really at the intended potential, thus you mustn't use the usual scheme of a trimmer at the output of the CV summing stage. Then the currents are much smaller than in the case of the bipolar expo circuits which can sink up to 1-2mA, 100-times more. But suitable circuits such as the CCO core of my VCO3 allow operation at low currents. (In the case of the VCO3 low leakage was one of the main goals.) There is one thing I forgot to draw: the heater which must be grounded. Which also explains why the plate is connected to the inverting input of the opamp. I feared insulation problems, and capacitive coupling.

The tubes

I've used a Tesla E83CC for my experiments since it showed the best tradeoff Io vs. filament current when I measured. (And the two sections could be paralleled.) Nearly every vacuum tube has this effect, but the current considerably varies. The 6N1P showed approx. 20A, I measured 1µA for the 6J1Bs that I used in my VCA. A large rectifier tube showed 0.3mA, but at the price of higher heater current.

Further perspectives

The first idea that comes to mind is using this circuit in addition to a normal expo convertor to archeive a "linear detune" feature. It allows to control the tiny currents that are required, the tubes with smaller Io would be a good choice. With a opamp current reflector it could be possible to interface this circuit to a CCO core which has the integrating cap to +15V. I didn't specifically test the tuning curve for this circuit, just used it over some 2-3 octaves range.