Metasonix TS-21 modulator

[astouffer]

This post isn't about amplification per se, but still pertains to tubes. I have been playing around with tube circuits for an analog synthesizer using the articles at http://www.cgs.synth.net/tube/index.html

The section on beam modulators describes a 6AR8 type tube to produce some interesting audio effects. I found the owners manual for the Metasonix modulator at http://www.metasonix.com/TS21man.doc and it describes the operation of the circuit. What is giving me problems is figuring out how pulse width modulation is done with a dual triode. To me it sounds like an asymmetrical multivibrator with some type of adjustment. I tried doing this by varying one of the grid resistors but the pulse width did not seem to change much. Can anyone shed some light on this?