Valves and their Habits
By H Peters from Practical Television August 1965
A small television picture tube type M14-170W.
Introduction
Since the original notes on 'Valves and their Habits' appeared in Practical Television in February 1961 a new generation of enthusiasts has emerged. At the same time the industry has produced a new set of valves.
This revised version of the original article includes all of the valves previously listed, even if some of them are no longer in common use, together with information on the latest types. The fact that a valve is mentioned in the list does not necessarily mean that it is certain to go wrong, or for that matter that it will not develop symptoms far removed from those given below.
Double valves, particularly the triode pentodes, still appear to be the most troublesome. They are naturally more difficult to construct and, as the entire valve is discarded when one half breaks down, their chance of failing is consequently twice that of a 'single' type.
Their versatility can indeed be their own downfall, since equipment designers often use them in a variety of unuusual circuits quite removed from their original purpose.
Emergency Replacements
From time to time 'emergency replacements' are mentioned in the text. These are valves with the same basing (base type and pin connections), usually the same heater current, and only slightly different characteristics. You will not find them given as equivalents in valve characteristic manuals but they have proved themselves capable of keeping receivers going at such critical times when new valves of the correct type are not available.
BY100
Perversely enough our valve list begins with a diode. The BY100 silicon diode is to be universally found in modern dual-standard sets. Due to its size and price it may be used with advantage to replace the HT rectifier in earlier receivers. Its main disadvantage is that it is too efficient and, if insufficient care is taken to absorb the switching-on surge, damage can result to the electrolytics and the on/off switch. The HT line may rise in excess of the specified voltage and a limiting resistor should be incorporated to absorb the excess.
DY86, DY87
These two 1.4 V EHT rectifiers have similar characteristics to the EY86. Their heater wattage is the same as the U26, for which they make an emergency replacement with shortened life. The only difference between the DY86 and the DY87 is a silicone treated glass envelope on the latter for better insulation in humid conditions.
EB91. (6D2, D77. 6AL5)
This sturdy little 6.3 V 0.3 A double diode has been used in detector, limiter, discriminator, and interlace circuits right from the beginning of post-war TV.
It has the habit of lighting up very brightly on switch-on, but this does not affect its performance. The commonest fault is heater-cathode leakage, causing hum in varying degrees. This may sometimes be cured in, series heater circuits by removing the beater to the low potential end of the chain.
An alternative is to construct a 'cold' EB91 using two OA71, crystal diodes on a seven-pin adaptor plug. Unequal halves only appear to affect line flywheel discriminator circuits of the balanced type and a simple check is to short the input anode and cathode together.
If the line whistle appreciably alters in pitch the valve is conducting unequally and should be replaced.
EBF80. EBF89
Two, 300 mA double diode pentodes frequently encountered in IF stages. Main fault is intermittence, usually stimulated by a gentle tap. Both have the same base connections and are interchangeable in an emergency, although the EBF80 is straight and the EBF89 is variable μ.
EF91, (6F12, 8D3, Z77, 6AM6)
This is a good reliable valve used in almost all early TVs but nowadays superseded by the EF80 or similar valves. As an RF amplifier it works well. Also as a mixer or IF amplifier it is efficient (even if it reads 'low' on test) but tends to give trouble if it is fitted into the video amplifier or sound output stages where it is being run pretty hard. The symptoms of low emission in the video amplifier stage are similar to a failing CRT.
ECC81 (12AT7)
This valve is a medium impedance double triode employed as a frequency changer in five-channel receivers. The oscillator section usually ceases to function first, although the heater remains intact.
ECC82 (12AU7)
A double triode valve of low impedance with many applications such as frame and line multi-vibrators, AGC circuits, and driver stages in amplifiers. They prove, most troublesome in multi-vibrator stages, the symptoms being variation of the line (or frame) speed when tapped or the time-base running fast and difficulty of bringing into lock by the hold control.
It also contributes to the variable contrast when it is faulty in the AGC gating stages where one half is sometimes strapped as a diode. If only this section is faulty then an OA71 crystal can be tried as a substitute to save replacing the valve.
ECC83 (12AX7)
The third valve in the trio of double triodes is high impedance and, although normally encountered in audio equipment, is frequently used in discriminator stages of line time-bases. If you can only carry one of these three double triodes in the tool kit make it an ECC82.
Note
Tapped heaters are used on the above three valves (ECC81/82/83). In 0.15 A circuits connect to pins 4 and 5) in 0.3 A circuits join pins 4 and 5 and connect to their junction and pin 9. In the latter case one half of the heater sometimes blows prematurely, the symptoms being that the remainder of the chain is still lit but the other half of the faulty valve lights up much brighter than the rest.
ECC84
This is the 6.3 V, parallel heater version of the PCC84 cascode RF amplifier (see PCC84).
ECF80
This is the 6.3 V parallel heater version of the PCF80 triode pentode (see PCF80).
ECL80 (6AB8, LN152)
One of the earliest triode output pentodes (6.3 V 0.3 A), the ECL80, has a medium impedance triode and an output pentode capable of 1.4 W output (class A). Despite both sections having a common cathode this versatile valve is used as line and frame multi-vibrator, frame and sound output, frequency changer, etc.
A favourite combination is to use a pair, coupling their triodes as frame multi-vibrator and using the pentodes as sync separator and frame output.
Common fault symptoms with ECL80 valves
Line multi-vibrator: the line speed varies when tapped. Line frequency is too high to be brought into lock with the hold control. Watery verticals. Frame multi-vibrator: the hold control needs constantly readjusting, the speed varies if tapped, the picture 'judders' vertically. Frame output: lack of height, fold over at the, bottom of the raster.
Sound output: Distortion and self-oscillation (but check the cathode bias decoupling capacitor first). Mixer/local oscillator: the slow-heating triode section causes the sound and picture to arrive with a 'bang' after the rest of the set has warmed up. Coincidence detector (flywheel sync): produces oscillating verticals-picture waves about snake fashion.
Shortcuts
The sound output and sync separator stages are the least troublesome as far as these valves are concerned and the ECL80s in these two stages may sometimes be successfully changed over with those in the more sensitive parts of the set.
In series heater circuits a PCL83 can be used as an emergency replacement provided that valve holder pins 3 and 7 are joined, as they usually are.
EF80
Another reliable valve which will give good results in IF stages even after many years use. Although the characteristics are the same the internal appearance of EF80s may vary considerably. A bright spot appearing near the top or bottom of the e1ectrode assembly sometimes denotes inter-electrode shorts.
In series heater circuits this valve can be used as an emergency replacement for 30F5, EF85, 6F19, 6F23. The pin connections are the same even though the characteristics are slightly different.
EF85
The variable μ companion to the EF80, this valve is commonly used as the first vision or common IF stage. Being variable μ its gain can be controlled over a wide range by the vision AGC line and, if the circuit is carefully arranged, the AGC voltage can also vary the grid to cathode capacity. In this way the input circuit tuning can be varied according to the signal strength so that the overall response is peaked towards the visio carrier on a weak signal and is progressively flattened to give a wide passband on a strong one. This arrangement 'automatic bandwidth control' has been in use for some years on one make of receiver.
A common fault is grid emission or inter-electrode leakage. This results in a positive voltage being fed to the AGC line and produces negative pictures.
EF183, EFI84
The two frame grid successors to the EF85 and EF80, giving double the gain with good design. The EF183 is variable μ and the EF184 is straight. The basing remains the same as the EF80, which you can use as an emergency replacement, but you will notice the difference. Well made valves, giving little trouble.
EH90
Seven-pin heptode found in the audio stages of dual-standard sets where it serves as locked oscillator detector on 625/FM and AF amplifier on 405/AM.
EL81
6.3 V equivalent of the PL81 used in parallel heater circuits (see PL81).
EY51
A wire-ended EHT rectifier with a 6.3 V 90 mA heater which is normally fed from a well-insulated winding on the line output transformer. This is the original EHT rectifier, which in 1948 made the AC/DC technique possible in the television set and which is still in extensive use today. During the years its peak inverse voltage rating has been raised to 17 kV in order to cope with larger tubes and higher final anode potentials and its physical size has been slightly reduced.
General symptoms of its failure are blackening around the glass just below the bell and a picture, which 'blows up' in size as the brightness control, is advanced (the latter fault may also be due to a displaced ion trap magnet). The high voltages present when running make the valve difficult to check by conventional means and the filament colour provides a useful means of ascertaining the operating conditions by comparing it against the filament colour of a similar valve run from a 6 V battery.
In many cases a dim filament will indicate a low-line output stage and it is not always appreciated that unless the line output stage is working correctly the EY51 may not light up at all.
An EHT short or excessive current drain by the CRT will also dim the filament and this property is useful to enable the brightness control to be set to a working level before adjusting the ion trap magnet when a new tube is fitted. The author found that if the brightness control is set to just cause the valve filament to dim slightly a bright raster is ensured when the ion trap passes its correct position.
If the filament is too bright, short life usually results, and this may be overcome by fitting a small resistor in series with the heater lead to absorb some of the surplus. A 22 Ω or 27 Ω carbon resistor, 0.25 W, is usually adequate.
EY86
The EY86 is the plug-in version of the EY51 and has a slightly higher filament current and higher Peak Inverse Voltage rating. These improvements make for easier servicing and better results with the wider angled tubes.
The fault symptoms and general notes on the EY51 apply equally to the EY86. Early productions of EY86 valve had no apparent gettering (the metallic patch on the glass being absent) but current EY86 valves are gettered in the conventional way.
In a certain make of receiver the EY86 suffers a short life and if continual replacements are required a special replacement type TY86 should be used.
EL38
This is a 6.3 V octal based line output valve with similar characteristics to the PL38.
OA70, OA71
Two devices which if carried in a tool kit will cover most applications likely to be encountered. The OA70 will replace any vision or sound detector diode and the OA7l will replace most limiter or AGC diodes.
PC86
UHF frame grid mixer/oscillator triode, the universal mixer valve in modern UHF tuners.
PC88
UHF frame grid RF amplifier, companion to the PC86 in UHF tuners. Opinion is divided on the advisability of replacing either of these two types without returning the tuner for realignment. It would appear that in strong signal areas the valves may be replaced in situ.
PC95, PC97
Two frame grid seven-pin RF triodes found in the smaller new VHF tuners. The PC95 is the variable μ valve of the pair but in other respects they will interchange.
PC900
An improvement on the PC97 but with different base connections.
PCC84 (30LI, 7AN7, B319)
The PCC84 is a double triode cascode RF amplifier with 7.0 V 0.3 A heaters for receivers operating on Bands I and III. Its commonest fault symptoms are low gain and a grainy picture, and in fringe areas have been superseded by the PCC89 and 30L15 valves.
These vales have the same base connections and the same heater current but their mutual conductance is double that of the PCC84, providing a considerable increase in gain, particularly on Band III. Unfortunately it is not possible to use these later types as direct replacements for the PCC84 or its equivalent without alteration to the tuner unit.
The author has found that in many cases a 30L15 will, however, give greatly improved results by merely retuning the small trimmers usually positioned at the top of the tuner or on the individual coil biscuits.
PCC85
300 mA version of the ECC85 to be found in FM VHF radio tuners in combined receivers.
PCC88
The original 'frame grid valve'. Has a wasp waist very reliable. Has the same base as the PCC85 and will interchange.
PCC89
A frame grid double triode cascode RF amplifier wiih 7.0 V 0.3 A heaters (see PCC84).
PCC189
Variable μ version of the PCC88 with the same base and heater details.
PCF80 (30C1, 8A8)
This 9.0 V 0.3 A heater triode pentode was primarily intended as a mixer on Bands I & III tuners but was subsequently used for almost everything else.
A low triode or one, which has been overloaded, may usually be recognised by brown patches on the glass opposite the hole, which is halfway up the triode anode side. Intermittent faults can usually be observed by giving the valve a gentle tap whilst running.
Other fault symptoms are poor signal-to-noise ratio, which indicates a pentode fault, or a weak Band III (or no Band III but Band I satisfactory) indicates a triode fault.
Line multi-vibrator: The line timebase runs fast and will not lock and the line-hold control at the extreme end of its range. The line speed may vary when tapped or during an evening's viewing.
Sync separator-half line oscillator: When used in this circuit pulling on picture (cogging), false line lock and line timebase runs fast may result if the valve is faulty. A negative picture may also occur (if AGC is mean level).
Video amplifier/cathode follower: In this type of circuit a red-hot anode or red-hot G2 electrode may occur. This is not usually due to a faulty valve but to a heater to cathode breakdown on the CRT although the valve's emission will be impaired if the set is run for a while in this condition (see also PCF82).
The concluding part appeared in the September 1965 edition of Practical Television and we would be delighted to add this second part if a copy was available.