The Mystery of the Type 35Z5 Open Filament
Re-printed from Radio Circuit Hints, Volume One, pp. 56-60
Compiled and published by Sylvania Electric Products Inc.,
Copyright 1943, First Edition
Sam Service Man has been going around for some time now with mighty short finger nails and it really bothers him to no end, because those nails came in pretty handy to strip the insulation from wires when he left his wire strippers at home, but poor Sam has been scratching his head so often over those “doggone” open filaments that his nails just have no chance at all. Not that Sam’s head is extra hard, mind you, but those open filament problems have been so varied that as soon as Sam thought he had one licked, up “popped” another.
Sam says Mrs. So-and-So is having trouble with the pilot light popping out in her radio, and if he didn’t get there pronto to replace it the Type 35Z5 “popped” too; and in Mr. Whatsit’s AC/DC set there was trouble with the various tubes in the string which turned out to be an open filament; and he had to replace a lot of Type lN5s in AC/DC battery portables.
Let’s see if we can’t throw a little light on Sam’s filament difficulties, one at a time.
Figure 1 shows the circuit employed in using Types 35Z5 and 45Z5 in typical receivers. Types 35Z5 and 45Z5 are similar to other half-wave, close-spaced, rectifier tubes with the exception that the heater has a tap brought out for a pilot lamp at a point which is 7.5 volts from one end of the heater when rated heater voltage is applied and only heater current flows. Therefore, a pilot lamp can be placed across this tapped section which eliminates the necessity of a ballast resistor as is required to secure the pilot lamp voltage when using other type rectifiers.
However, in order to increase the brilliancy of the pilot lamp it is current design practice to connect the rectifier plate also to the heater tap so that the total plate current of the receiver will pass through the pilot lamp. This is why panel lamps flicker on strong signals because the plate current varies with the signal strength and naturally the amount of plate current passing through the pilot lamp will also vary.
Sam has often noticed that the pilot lamp in these receivers lit like a lighthouse when first turned on. This was due to the fact that when the tube filaments were cold, as they would be when first turned on, the current drawn by the series string filaments would be very high as the resistance of the filaments was low; but when the filaments become hot their resistance increases and the current is reduced, and as the pilot lamp is shunted across a section of the filament there is a large voltage drop across the pilot lamp, caused by the high current of the cold tubes, and it is this high voltage drop that can damage the pilot lamp and cause it to open. As soon as the tubes are warm this voltage decreases and the pilot lamp hardly lights until plate current starts to flow then the light comes up to normal brilliancy.
Now look at Figure 2 and see what would happen if that damaged pilot lamp was not replaced.
Remember that the pilot lamp acts as a shunt across a section of the rectifier filament and keep in mind that the total plate current of the rectifier also passes through this shunted section. Therefore it can be seen from Figure 2 that with a missing or burned-out pilot lamp the tapped section of the rectifier filament must complete the circuit for the plate current, carrying, in this way both the rectifier and filament currents.
The filament is designed to carry 150 mA. but the plate current is now added to this and as it is normally around 50 mA. we have a total of 200 mA. passing through the tapped section, thus subjecting the tube to a considerable power overload which will shorten its life. In other words, there is approximately 15 volts across a section that is designed to have only 7.5 volts. Now, should the receiver be turned off and on without allowing the tubes to cool, another problem presents itself which generally causes the filament of the tapped section to burn out. This problem is called “On and Off Cycling” and depends a great deal upon the capacity of the input filter condenser.
“On and Off Cycling” can be illustrated by the following example. Mrs. Smith had not replaced the pilot lamp in her receiver and one evening she thought she heard the telephone ringing, so she turned off her radio for an instant to make sure. The telephone was quiet so she turned on the set again – the set failed to work. The set did not work because when Mrs. Smith switched off the current the cathodes of the tubes were hot and had consumed all the voltage that was stored in the filter condensers. Then the next instant the current was turned on again and the rectifier cathode still being hot allowed current to flow to the filter condensers to become charged again. The current consumed in charging the input filter condenser is very high and the larger the capacity the higher it is. This current is plate current and must pass through the tapped section of the rectifier filament, thereby adding to the filament current and causing it to be high enough to burn out the filament. It often happens that this same charging current is high enough to exceed the peak plate current limit of the rectifier, thereby causing the cathode tab to melt. The melted cathode in turn may short to the filament or plate causing A/C voltage to be applied across the filter condenser which will certainly cause it to break down. To help prevent this difficulty, look at Figure 3.
All that is needed is the resistor “R” shunted across the pilot lamp. This resistor not only prevents the high voltage surge from damaging the pilot lamp when the receiver is first turned on but will protect the tapped section should the lamp be removed. Furthermore, it will help protect the tube from the high charging current drawn by the input filter condenser.
The value of resistor “R” is 300 ohms [5 watts] when a #40 or #47 pilot lamp is used. Should the receiver not use any pilot lamp then a resistor of 800 ohms [10 watts] must be shunted across the tapped section.