up until the 1960s sawtooth oscillators were also built with thyratrons. these were gas-filled triode electron tubes. these worked somewhat similarly to neon bulbs, the tube would not conduct until the cathode to anode voltage reached a breakdown voltage. the advantage of the thyratron was that the breakdown voltage could be controlled by the voltage on the grid. this allowed the frequency of the oscillation to be changed electronically. thyratron oscillators were used as time bases in oscilloscopes.