the transformer's primary inductance. At zero input signal, the nonlinear RL filter has a cutoff frequency of 3 Hz. This combination of cutoff frequencies, wrapped inside a 2030dB frequency loop, is unstable. Williamson tried to suppress it with a compensation network, also serving as a smoothing filter. The transformer's nonlinearity also improved stability: at high signal currents effective inductance of the primary increased, causing a decrease in cutoff frequency and a rise in phase margin. The simplest solution was to spread apart cutoff frequencies of the RC filters, provided that the output transformer conforms to the Williamson specification. For example, the 1952 Ultralinear Williamson by David Hafler and Herbert Keroes had these frequencies set at 1.3 and 6 Hz. Precise analysis at ultrasonic frequencies is impossible due to the asymmetry of the phase splitter stage, and unknown parasitics and nonlinearities of the output stage. Depending on the chosen analysis model, open-loop response can be roughly approximated with a combination of either four or five low-pass filters. Different authors used different approaches and estimated somewhat different cutoff points of these filters, but in each case at least three of four or five cutoff frequencies were dangerously close to each other, which was a certain sign of instability. Williamson, again, fixed the problem with an RC compensation network, but even then phase margin remained dangerously low. DIYers had to tackle oscillations themselves: some added shunting capacitors to the screen grids, others tweaked layout and wiring, or deliberately narrowed the amplifier's bandwidth, negating the benefits of the original circuit. Component problem The Williamson amplifier was very sensitive to the quality and parameters of passive components and valves. Carbon and composition-type resistors generated excessive noise and caused harmonic distortion; American valves used as substitutes for the British types specified by Williamson, could not match their performance. Williamson warned that the KT66 has no direct substitutes, and should be preferred over any alternatives. Amateurs who copied the Williamson amplifier were unable to identify and fix its critical weak points. An amateur armed with an analogue multimeter could "see" infrasonic oscillations by watching the instrument needle, but fixing high-frequency issues required an oscilloscope with bandwidth of at least 1 or 2MHz bandwidth. In the 1950s bandwidths of many commercial oscilloscopes were too narrow for the task, and even these models were too expensive for the DIYers. Articles by professional engineers dealing with analysis and fine tuning