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The comparison between traveling-wave-tube circuits of different geometries can be facilitated by the use of phase velocity, group velocity, and stored energy as parameters. For a given stored energy per unit length, lowering the group velocity with respect to the phase velocity increases circuit impedance (and, hence, gain), increases attenuation, and narrows the band. A filter-type circuit consisting of pillbox resonators is shown to be much inferior electrically to a helix. Expressions are presented showing the effect of gap length in filter-type circuits consisting of pillbox resonators, and the attenuation is calculated for such circuits.