Solid-State Rectifiers
The advent of solid state rectifiers—nowadays almost exclusively based on silicon bipolar junction technology—effectively caused the demise of valve rectifier systems, although for a short period, prior to the general adoption of semiconductor rectifiers, gas-filled rectifiers, such as the 0Z4, had been used, principally in car radios, in the interests of greater circuit convenience because, in these valves, the cathode was heated by reverse ionic bombardment so that no separate rectifier heater supply was required. The difficulties caused by the use of these gas-filled rectifiers were that they had a relatively short working life and that they generated a lot of radio frequency (RF) noise. This RF noise arose because of the very abrupt transition of the gas in the cathode/anode gap of the rectifier from a nonconducting to a conducting state. The very short duration high current spikes this caused shock excited the secondary windings of the transformer—and all its associated wiring interconnections—into bursts of RF oscillation, which caused a persistent 100- to 120-Hz rasping buzz called modulation hum to appear in the audio output.
The solution to this particular problem was the connection of a pair of capacitors, shown as C1 and C2 in Figure 5.1(a), across the transformer secondary windings to retune any shock-excited RF oscillation into a lower and less invasive frequency band. Sometimes these modulation hum prevention capacitors are placed across the rectifiers or across the mains transformer primary winding, but they are less effective in these positions.
With modern, low conduction resistance, semiconductor diodes, low equivalent series resistance (ESR) reservoir capacitors, and low winding resistance (e.g., toroidal) transformers, this problem can still arise, and the inclusion of these capacitors is a worthwhile and inexpensive precaution. The circuit layout shown in Figure 5.1(b) is the PSU arrangement used in most contemporary valve amplifiers. For lower voltages, a wider range of circuit layouts are commonly used, also shown in Figure 5.1.