Telmatronics are developing Groovetube - a synthesiser with a plasma tube, making sounds that can be expressively controlled with a magnet.

The machine is configured as a valve pre-amp, with a telmatron plasma tube as current source. The plasma sound produced in the tube affects the output, but is not simply 'noise in the pipes' as introduced audio signals draw current from the plasma and so directly affect the instability, the output being the interaction between them.

The synth can be controlled with external signals or with its own onboard control system. This page will be updated with demos and details of specification/performance as the project continues.


Groovetube machine

Our latest demo video:

Groovetube has evolved as a working prototype, with many controls added and removed to test functionality along the way. This process is now complete, with a more sensible control layout designed for the first production machines.

The fluorescent tubes used by this machine are due to be phased out soon, so we've been securing adequate supplies for the short term. Longer term, we aim to develop our own tubes designed for specific sound output - an exciting prospect that will take some time. First experiments are now being constructed.

Working Spec

The machine uses a nominal system voltage of around 350 VDC to drive a cold cathode discharge inside a 9" T5 fluorescent tube, with RF coils used for auxiliary excitation. Power supplies for the tube and excitation circuits are regulated by preamp valves, which are well-suited to handling high voltages, and valves are also used for the main audio circuit. The telmatron systems are optically controlled, and the RF system has an additional control which allows relative plasma stability to be altered.

Control system:
The synth is controlled by CV/gate and modulation signals, that can be generated internally for stand-alone operation or using an external control system. Externally-generated audio can also be introduced. Onboard control uses a simple 8-step sequencer to run preset CV sequences and gate patterns, and generates a timing signal for synchronising with other equipment. Two digitally-controlled LFOs are used to create signals for modulating plasma voltage and excitation, allowing good control over frequency and phase which significantly affect the plasma sound produced. 

Release details

In early 2023 we made Groovetube available for pre-order in complete and kit form as part of a Kickstarter campaign, which ran until March 23rd 2023.

We are currently focussed on production of the first batch of instruments, which will ship in the first part of 2024. After this we will open for further orders - in the meantime, you can register an interest here.

We have also promised to post schematics and construction notes - this will happen in due course when time allows.

This article was updated on February 4, 2024