“To fully understand why this requires the use of a rectifier with a separately heated cathode, it is only necessary to draw the equivalent circuit of a power supply”…”Since there is no completed d.e. circuit, no d.c. current flows; no flowing current means no voltage drop across [transformer winding resistance and choke resistance]. Therefore the entire 550 volts appears across the filter capacitor, the open circuit for d.c.”
“This is exactly what happens when there is no separately heated cathode in the rectifier tube. A rectifier with a filament type cathode, such as the 5Y3G or the 5U4GB conducts at the moment the amplifier is turned on. The output tubes have not had the time to heat their cathodes so as to start conducting. Since there is no current being drawn from the high voltage power supply for the first half to three-quarters of a minute, (while the output-tube cathodes are warming up) there is no current flowing through the power supply and rectifier tube. No flowing current means no voltage drop in the rectifier tube and power transformer. All the high voltage then appears across the electrolytic capacitor, and this may damage it.”
“The solution to this problem is to prevent the rectifier from heating up fast and setting up a high voltage before the output tubes start conducting. A slow-heating separate cathode will provide this necessary time delay to protect the electrolytic capacitors, while the output tube cathodes are heating up. This will prevent the initial excess voltage surge, protecting the capacitor.”
“Tubes like the 5V4G provide this feature. However, for higher current applications, as in high power amplifiers, either two 5V4G’s are necessary or the European GZ-34 made by Mullard and Amperex, may be used.”
Source: M.Horowitz, “High-Power Audio Amplifiers”, Audio, March 1958, link
“It is advantageous for rectifier tubes to be indirectly heated because this prevents the breakdown voltage rating of the input capacitor from being exceeded when the set is first turned up. “
“The transformer resistance, Rt, must not be less than the minimum value quoted in the rectifier tube data. This transformer resistance may be determined as follwos. The resistance, Rp, of the primary and the resistance of one of the secondaries, Rs [in the case of center-tapped transformer], are measured. The transformer resistance is Rt=Rs+n^2 Rp in which n is the winding ratio (voltage ratio) between one secondary and the primary. If the value found is below the minimum value given in the tube ratings, a resistor with a value of Rx=Rt_min – Rt must be connected in series with each rectifier plate. Each resistor carries half the direct current and half the ripple current so that its power rating should be based on a current of 1.2 times the total d.c. delivered by the rectifier. As an alternative scheme, the two resistors may be replaced by a single one of the same value connected in series with the cathode of the rectifier. In this case, the required power rating should be 2.4 times the total d.c. The input capacitor should be abble to handle the ripple current which is approximately 1.4 times the total d.c. delivered by the power supply.
Source: D. Saslaw, “Audio Designer’s Handbook (Part II)”, Audio, February 1960, link