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Valves Galore

Jack Darr*, Wireless World, April, 1958.
    
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And some notes (mostly Sour) on series-heater circuits.

Although series-heater radio receivers are common in America, the low mains supply voltage has until recently prevented the use of this technique for TV receivers. Now that suitable power rectifiers and capacitors are available (voltage-doubling for the HT line is universal) series-heater TV receivers are becoming common. This article deals with some of the servicing problems that Americans are meeting for the first time.

Among some of the more entrancing features of Life In These United (more or less) States is the opportunity of repairing some of the weird circuits used in the manufacture of radio and TV receivers. Probably the principal factor in the design of much of our mass-produced equipment is economy of construction. This accounts for the Printed Circuit, anathema to most service technicians, but beloved of the development men and accounting departments because of their cheapness of manufacture. Another is the Series Heater Circuit.

This arrangement, identical with that used in the older strings of Christmas-tree lights, was developed to eliminate the need for a mains transformer. Of late, it has resulted in the introduction of some of the most weird heater-voltages imaginable! The technician comfortably puttering around in a new set, accustomed to such sensible types as the 6BQ6GT (a very popular line-scan output tube), now finds himself staring in horror at such cute little numbers as 12BQ6GT, l4BQ6GT, 17BQ6GT, 19BQ6GT, and even a 25BQ6GT. These are accompanied by such items as the 12AX4 and 19AU4 for dampers (efficiency or boost diodes). Perhaps because of its retiring disposition, always hiding shyly in the box screening the line-scan output transformer, the EHT rectifier has somehow escaped notice, and remains the old and faithful 1B3! It is quite heartening to the technician to find at least one valve (tube to us!) in the set with a number he has seen before! (and is likely to find in his tube-kit!).

In other parts of the set, the same confusion prevails. IF stages, in the Good Old Days, used such types as the 6AU6, a sharp-cut-off pentode, with perhaps a 6CB6 in the first stage, for AGC purposes. Now, upon lifting the screens from the IF tubes, we find a group of such Little Monsters as 3CB6, 4BZ6, 5AU6, 7AU7 etc. Oops! Pardon me. Im a bit confused; the 7AU7 is not found there, but is a replacement for our old familiar friend, the 6SN7. It has a large family, too; 14AU7 etc. etc.

Sync separators, clippers etc are inhabited by another strange tribe; 5U8s, 5AN8s, 7AW8s, 3BN6s, while audio output and other chores are handled by 5AQ5 and 5BK5. Vertical oscillators find themselves crowded into one bulb with the output triode in the 6CM7. This novelty combines one 6J5 and one 6SN7, or half of it, all in one tiny bottle!

In the tuner, to use a native expression, they have gone simply 'hawg-wild!' Replacing the old, familiar and highly efficient 6BQ7-6BK7 et ak/i>, we find, if I may be permitted an atrocious pun, a perfect 'cascade of cascodes' 3BQ7, 4BK7, 5BZ8, 5CL8, 5AT8, 4BS8, 5CG8, on and on ad infinitum until one wonders where they get all the new letter combinations! Also prominent among such types are the new triodes - the 2BN4 is one. This is employed in a new type of tuner which is the only tuner giving less snow when the RF tube is removed from the socket! A 2AF4 is used in UHF tuners and was the source of much grief during its formative years; later production changes have ironed out some of the bugs, and they work very well now.

Mixer-oscillators, beginning with the old faithful 6J6, have changed to a triode-pentode combination, using a 6U8 in the beginning: these were soon re- placed by 5U8, 5AT8, 5CG8, 5BS8 et al.

The Reason For The Change

There was actually a bit of good reasoning behind the development of these odd-numbered valves. They do have two things in common; their filament current and their warm-up time. These are 600 mA and 11 seconds, respectively. This was done intentionally; the picture tubes all have 6 Volt heaters, with 600 mA current and (now) an 11 second warm-up time. (How they've overlooked them in the stampede to make odd-voltage tubes, I shall never know!) When used with the remaining tubes, this created quite a few rather unseemly series-parallel heater circuits. For instance, many sets used a dual-string arrangement, with 300mA tubes in each branch, meeting at the CRT heater for the first time, where the currents flowed together through its heater back to earth.

The difference in warm-up time created quite a bit of confusion and service difficulties in the older sets. Older tubes were quite individualistic about this parameter; they arrived at their operating temperature when and if they jolly well got ready, regardless of what was going on in other parts of the circuit. For instance, if the line-scan output tube got ready to go a bit early, no drive was available from the oscillator, if that worthy happened to be a bit slow in waking. This resulted in a severe overload in the anode circuit with occasional failures of fuses and much puzzling on the part of the service technician as to just what had caused the fuse to let go.

The first corrective measure applied to this problem was the inclusion of a special resistor in series with the heater string. This blob of thyrites had a negative temperature coefficient, which gave it a whacking great resistance when cold, up into hundreds of ohms, dropping to a quite low resistance when heated up. This unit was called an NTC resistor by the trade and several mouth-filling names by the advertising gentry (Magic Tube Saver, etc). It caused a delay in the warm-up time by limiting the current which could be drawn initially. This eliminated one very bad feature, the unequal loading, which caused some valves to suffer as much as a 400% overload while others were under-heating, resulting in amazingly short tube lives!

By making all of the tubes with identical current drains and warm-up times, the designers were able to string them all together like a tremendous Christmas-tree light string.

This controlled warm-up feature eliminated not only the resistor, but also speeded up the overall time required to get the thing into action. Using large NTC resistors it often required a minute and a half to get a picture on the screen; the new types will start the set to work within about 30 seconds. This makes the customer much happier; with the old types he often missed the opening commercial!

The Line-Connected Chassis and Shock Hazards

Another feature of the series-connected set is the need for using what is known among the setmakers as a line-connected chassis and among servicemen a 'Death-trap'. Instead of the familiar mains transformer and rectifier tube, we now find all sorts of arrangements, mainly using selenium or silicon rectifiers in half-wave and full-wave voltage doubler circuitry. The most charming feature of these arrangements is that they seem to require that one side of the circuit be connected directly to the AC mains (line). The inevitable result of this arrangement is that if the line plug is inserted a certain way, the full line voltage is present between the chassis and the nearest grounded object (which is generally the serviceman, but may be, on occasion, the customer!)

One delectable version of this was encountered recently by the writer while servicing one of these chassis. Touching the earthed antenna lead and the chassis gave a vicious shock. After reading a few appropriate verses from the Koran etc, the luckless wight reversed the plug and returned to his task, only to be unceremoniously removed from his stool and deposited on the shop floor when he again managed to complete the circuit between the chassis and antenna, this time with the elbow! Investigation (using a voltmeter) resulted in the shocking (oh!) disclosure that this little gem was hot - with the full line voltage to ground - in both positions of the plug!

All levity laid aside for the moment, a very serious problem does exist. This applies not only to the technician but also to the user. For economy again some of these sets have been housed in metal cabinets. Of course, in compliance with the Underwriters regulations, insulation is provided between chassis and cabinet. However there are some models in which this insulation seems to be entirely inadequate. Careless service procedure, owner tinkering etc can nullify even this slight degree of protection by allowing self-tapping screws, stray bits of metal or any other conductor to fall into the small spaces between chassis and cabinet, causing a short-circuit. Insulation which breaks up or fails when the chassis is removed and replaced, can cause the same trouble. Several accidents, including two deaths of set users, have resulted from this cause.

The serviceman shares to some degree the manufacturers responsibility for prevention of possibly fatal shocks, both by checking insulation when the set is serviced, and by warning the set user of the dangers inherent in use of the apparatus on damp concrete floors, or out in the yard, on damp ground, near grounded metal cabinets such as sinks or bath fixtures, etc. It is to be hoped sincerely that some safe solution to this hazard can be worked out very soon; after all, good customers are hard to find and we cannot afford to lose very many of them!

The Needle-in-the-Haystack Feature

There is one unmistakable characteristic of the series-heater set, displayed when a valve happens to reach the end of its useful life, and passes to its ancestors. Everything goes dead! No sound, no picture, no raster, no pilot light, 'no nuthin'. Of course, as a few unwary souls have discovered, there remains a nice unloaded potential of a few hundred volts of B+, floating about in the circuit. This is usually quite accessible at such points as the scanning yoke, the (exposed) rectifiers and especially at the new type of fuse used in most sets; all these are installed in the chassis with the hot end out! 'Werry Handy', as Sam Weller would say. Aside from this a silence and darkness, as of the grave, prevails.

This brings up a knotty problem. From past experience the technician knows that one of the valves has passed to that bourne from which none returneth, but the problem is Which One? With the old faithful parallel-heater sets, this was the simplest of operations; the extinct item was advertised by its absence of illumination. Even a faulty metal tube could be located in a moment or two by touching it; the dead one was cold.

No such bed of roses for our modern technician! His the task of isolating one dead tube from a string of possibly 18-20, all equally cold and silent. Of course, there is always the dogged method of beginning at one end of the string and testing each in turn until the dead one is located. This inevitably results in it being found at the other end of the string, no matter at which end tests were started. (Some technicians have tried a method of random selection, hoping to catch the bad one by surprise, as it were; this gives the same result.) This is a time-consuming process, and considering the almost entirely inaccessible locations of many tubes, a difficult one. Incidentally there is a firm belief current among American. technicians that the egg-heads at each factory have a special department, known as the 'Dirty-Trick' section, which specialises in unlikely and inaccessible locations for tube sockets! They love such didoes as locating the audio output stage just below the bell of the CRT, so close that the bottle must be bent sidewise to remove it. Between this and the back of the set is the exposed EHT rectifier and line-scan output tube which must be reached over to gain access to the AF output. These are either very hot or very charged, with results which may be left to the imagination. Undisputed champions in this category are the designers of a famous 1952 model, which had the EHT rectifier and both tubes in the tuner under the chassis, so that the chassis had to be removed from the cabinet to gain access to them. They are getting some stiff competition, however, from the designers of some of the new sets and almost lost the cup last year to the genius who mounted his tuner upside down, far up in the front of the cabinet, with the tubes hanging downward, accessible only through a very small square hole in the vertical chassis. He lost out at the last moment due to lack of co-ordination with his associates; someone carelessly mounted the tuner with only two small screws, so that it could be removed bodily and pulled out of the chassis, giving easy access to the tubes! (It is generally believed that next year this oversight will be remedied by riveting the tuner firmly to the chassis!)

This divertissement out of the way, we remain with the problem of locating the unknown dead tube! Some devices have been brought out in recent years which are very helpful. Chief among these is a wee 'Black Box' provided with tube sockets to match each type used in the set: 7-pin and 9-pin miniatures, octal etc. On the end, a CRT receptacle is provided. Inside, a small battery and pilot lamp are wired up to the heater terminals of each valve-holder so that the lamp glows when a good tube, or one at least having a reasonable degree of continuity, is inserted in the holder. An AC powered version includes a socket into which the cheater cord (Cheater-cord - a cable and fittings used to bypass the devices disconnecting the AC mains supply on removal of the receiver.) is plugged, and a small neon lamp with series limiting resistor, to give the same results.

Some thoughtful makers are pasting a 'heater-string layout' on the back of the cabinet or chassis. This shows a sketch of the chassis, with each tube indicated, and connecting lines showing just which tube is connected to what, etc. This is absolutely invaluable; by removing a tube in the centre of the string and measuring to each side of the line plug with an ohmmeter, the dead half of the string may be located. Further tests will narrow the number of suspects down until the culprit is found. One bewildered technician removed a tube, and took the measurements mentioned; much to his surprise each half showed continuity. Upon replacing the tube, the set still refused to light up. Repeating the tests gave the same result. After a little while, it dawned upon him that he had removed the dead tube (dammit) on the first try, and was holding it while he measured the resistance of the remaining perfectly good tubes. The next day, he encountered seemingly the same situation. 'Ah-ha'! he cried, checking the tube he had pulled out. To his chagrin, it was also good. Some little time later he discovered that the maker had thoughtfully included an auto-transformer in the design, so that he had continuity back to the line, no matter which side of the string he measured! He finally located the extinct unit by the old method of testing each one in turn. (What? Certainly it was the last one he tested! What did you expect?)

Aside from the difficulties of locating dead tubes, another utterly charming feature of the series-heater sets lies in the inability of the technician to disable easily any given stage, by pulling the tube. For instance, if the local oscillator is to be disabled while aligning a set, a duplicate tube with its anode pin clipped off must be inserted in the socket, to make up continuity for the heater string. To follow this procedure for the whole set would imply the destruction of a complete set of good tubes. Of course, the next set coming along would have different types, so there goes another set of perfectly good tubes!

Actually, although the writer may sound rather acidulous about the whole thing, and the schematic of the set may look as if someone had dipped a very active inchworm (measuring-worm?) in the inkpot and released it on the page, these little sets are marvelously efficient, and not too difficult to service. (With exceptions, of course.) In fact, we have grown rather used to the contrary little monsters, and don't mind them so much. With the proper techniques and equipment, one will, in time, become able to check them out almost as fast as the older types. The points about the shock-hazard still remain, however, and must be watched very carefully. All in all, it is rather like the old story about the man who hit himself on the head with a hammer, because it felt so good when he stopped! We rather enjoy it, because it feels so good when we stop.

Cheers.

* Jack Darr - Ouachita Radio-TV Service, Mema, Arkansas.

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