In this work we consider the experimental studies that were conducted using a transparent (without microwave absorber) travelling wave tube (TWT) with helical slow-wave structure. The difference between used in experiment TWT and the standard one is that the input signal is supplied to the modulating grid positioned near the cathode. The used in our experiments transparent TWT worked in crestatron regime. The experimental results show that the scheme with feeding the input microwave signal to the modulating grid, located near the cathode has a significant advantage, which is the high value of the gain, and efficiency of the TWT amplifier and the expansion of operating frequency band. It has been established that the maximum value of electronic efficiency for the version in which the input signal is fed to the modulating grid positioned near the cathode is 56% compared to 43% for option in which the input signal is fed to a retarding system Technical efficiency is 73 ÷ 75%. An increase of the bandwidth to the values Δf/f ∼ 1.5 ÷ 1.7 was determined.