The write-up/article below, is around 90 plus years old Maybe it will give you some added insight into this tube.
How are your voltages at the rectifier tube pins, with the 280 tube pulled? Also set your variac to where you read 115 Volts, at the primary, with all tubes pulled. It seems your Variac may very well be putting out higher voltages than you are setting the Variac for.
Rectifier Tube 280
THE 280 is a full -wave rectifier tube for use with receivers requiring a maximum rectified voltage of 350 volts or less and a current of 125 milliamperes or less, or a maximum rectified voltage of 400 volts with a maximum current of 110 milliamperes. It has a five volt filament winding requiring a current of two amperes. CHARACTERISTICS OF THE 280 RECTIFIER Filament voltage 5.0 Filament current, amperes 2.0 AC voltage per plate (RMS) 350.0 Max. load milliamperes for 350 volts on plates 125.0 AC voltage per plate (RMS) 400.0 Max. load milliamperes for 400 volts on plates 110.0 Maximum overall length, 5 5/8 inches. Maximum diameter, 2 3/16 inches. Socket, standard UX. The power transformer used with this tube should have two secondary windings, one of high voltage for the plates and one of low voltage for the filament. The high voltage winding should be accurately center -tapped so that both plates get the same voltage. The filament winding need not be center- tapped but a center -tap is desirable if the tube is to deliver a high current. If the load current is taken off on one side of the filament that side will be operated at a higher temperature because more of the rectified current will flow through this side. When a center -tap is used the load current is divided equally between the two legs of the filament and the heating will be uniform. Line Voltage Variations . The primary should be wound for 115 volts but should prefer- ably have taps for 107.5, 115 and 122.5 volts so that compensation may be made for line voltage fluctuations. If the primary is not tapped, it should be wound for about 115 volts and a variable resistance should be put in the primary circuit to cut the voltage down to that required in case the line voltage should rise above 115 volts. The actual effective voltage across the high voltage secondary depends on the DC voltage desired, and may vary from 220 to 400 across each half of the winding, that is, from 440 to 800 volts across the entire winding. In Fig. 1 is shown a typical rectifier circuit incorporating the 280 tube, together with the filter circuit and the voltage divider. The average output characteristics of such a circuit are shown in Fig. 2. These curves show the variation in the DC output voltage with changes in the load current for different values of RMS input voltage per plate of the rectifier tubes. The output voltage is as measured at V in Fig. 1. The full lines are for condenser input to the filter and the dotted lines for choke input. The difference between these inputs is indicated on the diagram in Fig. 1. These curves may be used to estimate the maximum output voltage when the current drain and the input voltage are known. Suppose, for example, the effective AC voltage per plate is 350 volts and we wish to draw a current of 100 milliamperes. The 350 -volt curves, tell us that the voltage is 345 volts at V, with condenser input, and that it is only 250 volts with choke input. The curves do not take account of the drop in the choke coils so that the useful voltage will be less. Suppose the total resistance of the two chokes is 400 ohms. If the current is 100 milliamperes the drop will then be 40 volts. This leaves 305 volts for the case of condenser input and only 210 volts for choke input. Condenser Requirements As indicated in the rectifier circuit, the capacity of the first condenser is 4 mfd. If the capacity of this condenser is increased, the voltage will be slightly greater, and if the capacity is less the voltage will be less, but changes in voltage with the condensers ordinarily used are so small that they may be neglected. The capacities and resistances associated with the voltage divider are not specified because they would depend on the circuit with which the rectifier is to be used. This is especially true of the resistances. It may always be stated that the capacities of the condensers should be as high as practicable and that not less than one microfarad should be used in any position. In selecting condensers it should be remembered that the voltage may rise to the peak of the voltage applied to each plate of the rectifier tubes and that the condensers must stand this voltage for brief periods. The peak voltage is 1.41 times the RMS voltage, so that if the applied voltage per plate is 400 volts, the voltage across the condensers might rise to 564 volts. This would occur when the load is removed without turning off the power to the transformer. Even when there is a bleeder current which remains when the load is removed, the voltage will rise, although not to the same value. This rise in voltage is indicated on the curves in Fig. 2. At 10 milliamperes, for example, the voltage across the first condenser, when the voltage per plate is 400 volts, RMS is 510 volts. Only the first three condensers will be subjected to the maximum voltages. Surges This voltage rise does not take surges into account, but only the rise due to regulation. When there are surges the voltage may rise to much higher values instantaneously, and surges will occur when there is a sudden change in the current. For these reasons the voltage rating of the condensers should be at least twice as high as the highest steady operating voltage. Moreover, care should be taken that the power is always turned off before any change is made in the circuit, such as removing a tube or changing the position of some lead on the voltage divider. It would seem that the condenser next to the rectifier tube would be subject to the greatest stress, but in most instances it is the third condenser that breaks down first, if the three con- densers across the entire line are of the same rating. This rup- ture, no doubt, is due to high voltages generated in the amplifier due to static crashes and other disturbances of a similar nature, and is superinduced by the heating of the condenser by the con- stant flow of normal signal current through it. While it is customary to put the two filter chokes in the posi- tive side of the line, as is done in Fig. 1, they could also be placed in the negative side. This is sometimes done in commercial sets. Or one could be placed in each side of the line, and this method, too, has been adopted by some manufacturers. When the load current is high it is common practice, especially when push -pull amplification is used, to tap the current for the power tubes from the junction of the two chokes. This diverts the larger part of the load from the second choke so that it becomes more effective in filtering the remaining current.
Ralph
