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We have verified experimentally the theoretical predictions of our previous work (Belyavskiy, E.D. et al., IEEE Trans. Electron Devices, vol.48, no.8, p.1727-36, 2001) about the effect of electron beam alternating rotation on the parasitic backward-wave oscillation in the amplifier-type helix traveling-wave tube (TWT) under periodic permanent magnetic (PPM) focusing. We measured the backward-wave start-oscillation current as a function of the interaction length in a specially designed helix backward wave oscillator. The observed behavior is much closer to the predictions of our theory, than to theories that ignore the electron beam rotation. Our results can be used for improving the stability of the PPM-focused helix TWT with respect to parasitic backward-wave oscillation.