In systems like the magnetron or klystron, this is accomplished with another resonant cavity. Oliphant's group at Birmingham worked on developing two promising devices, the klystron and the magnetron. The klystron, tube waveguide, is a beam tube including at least two apertured cavity resonators. In the reflex klystron the electron beam passes through a single resonant cavity. To reinforce the bunching, a klystron may contain additional "buncher" cavities. High-powered klystron amplifier tubes can provide hundreds of kilowatts in the UHF range. A gyro-klystron is an amplifier that functions analogously to a klystron tube. Helical waveguides have very nonlinear dispersion and thus are only narrowband (but wider than klystron). Amplification was achieved by exciting a ruby comb with a 12.0 gigahertz klystron. Devyatkov followed this with a simpler, single-resonator device (a reflex klystron). This was essentially a conventional two-cavity klystron with one cavity removed. The simplest klystron tube is the two-cavity klystron. Modern semiconductor technology has effectively replaced the reflex klystron in most applications. The reflex klystron (as it was later called) had just been developed by Nikolay Devyatkov.