Tube synthesizer revisited

Published on May 16, 2014 256byteram·133 videos

"I've been working on my tube synth again. It's a voltage controlled, monophonic synthesizer with a linear control input.

The circuit works a lot like a Theremin, though why it outputs a pulse waveform and not a sine wave is beyond me. It's probably the two oscillators interfering with each other.

It works with a reactance tube (a 6BA6, a remote cutoff pentode to give a musical scale from a linear input) modulating the frequency of an LC oscillator (a 6C4) by a few tens of kilohertz across the voltage range. The oscillator runs at a few hundred kilohertz so this frequency is mixed (in a 6AN7A) with a fixed oscillator (also in the 6AN7A). The difference of the two frequencies is extracted and sent to the amplifier. This is what you hear.

For the voltage controlled amplifier, the 6AN7A's screen grid's voltage is controlled to adjust the gain. The high frequency from the fixed oscillator is tapped off and amplified through a dual triode driving an IF transformer. The secondary winding of the transformer is put between the keyboard's lowest note and ground. This doesn't change the DC operation of the keyboard but it injects a high frequency sine wave into it. When a key is pressed, the sine wave is allowed through. The DC level is sent to a capacitor to control the pitch and the AC signal is sent to the other half of the dual triode and amplified through another IF transformer. The output of this is rectified and sent to a capacitor, which drives the screen grid of the 6AN7A.

Getting the thing to stay in tune is a real pain. It's RF so moving any component changes the operating frequency.

I'll draw a schematic up and post it if anyone wants a look."