This is a tube based Voltage Controlled Amplifier / timbral gate. While this module basically operates as a VCA, it does add a degree of distortion to the signal. How much distortion depends on how hard it is driven. Add feedback and it begins to oscillate, synchronizing to the incoming signal to some extent. All this while running on a standard synthesizer power supply, with no extra heater supply required.

A demonstration of feedback oscillation and synchronizing on the Panther Euro version of this module.

A little on how it works.

There are several distinct sections to this circuit. First to consider is the heater supply. The tube chosen is the Russian sub-miniature 1J24B (1SH24B, 1Ж24Б). It operates on low voltage, and uses a mere 13ma at 1.2 volts for its heater. In other words, the heater uses less power than the average LED. Power for the heater is derived from the negative rail, by passing the current through first the heater, then some current limiting resistors with LEDs in series with them. Two resistors have been used to keep the dissipation within the resistors to appropriate levels. The LEDs are also using a little of the waste power. More info on this is given alongside a photo below. The two 1N4148 are merely an insurance policy. They are to prevent the heater seeing any more than around 1.5 volts if an incorrect resistor is installed, or a similar mishap occurs.

The top left of the schematic is a very standard summing amplifier/mixer with unity gain. It's output is AC coupled to the grid of the tube.

The bottom left of the schematic is a very standard summing amplifier/mixer with a gain of around 2. It's output is fed through a "de-thump" lag circuit and into a second stage, also with a gain of around 2. This part of the circuit is the CV mixer. It converts the expected 0 to 5 volt CV signal into something appropriate for driving the second grid of the tube by both amplifying it and adding an offset voltage.

The core of the circuit is of course the tube itself. It can be viewed as running on a 30 volt single rail supply, as its cathode is referenced to the negative 15 volt rail instead of 0 volts. It is a mixture of a triode wired pentode and a multiplier, though sans the usual automatic cathode bias, due to the use of a directly heated cathode.

The final section of the circuit is simply a load-independent, non-inverting buffer/gain stage.