Tune-A-Lite
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Hits: 19212 Replies: 5
Tune-A-Lite (Tune-A-Lite)
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Joe Sousa
13.Feb.09 |
1
The following two photos show the I/V charactereristics of the main Anode/Cathode current for the Tune-a-lite 4 element neon tube used as signal strength meter in some Fada Radios as the Tune-o-graph. The 4 terminal Tuneon tube used in some Cossor radios works similarly to this tube. The curve sweeps were taken in a Tektronix 575 curve tracer. The cathode is grounded in all the photos. This first photo shows a low 130V trigger voltage for the main cathode that is approximately the same as the operating the voltage. This low trigger voltage is facilitated by the 100uA current flow through the small pilot cathode. The pilot cathode is seen glowing to the lower right of the main cathode. This tube has some darkening, indicating use. After about one hour of use, the same 5mA maximum current was enough to cover the entire main cathode with glow. The terminal numbering in the photos refers to the convention used with 4 pin tubes, with a clock-wise count starting at the lower left thick pin, when the two thick pins are at the bottom. I labelled the T terminal an anode, but a more generic designation of T-terminal is probably more appropriate.
This photo shows a greatly increased trigger voltage, up to 190V, because the pilot cathode is left open.
The small cathode eliminated the problem that the 2 terminal Tune-a-lite, and similar tubes, have of turning on suddenly, only when voltage is increased greatly after the glow is extinguished. The pilot cathode is present in all 3 terminal and 4 terminal tubes of this type.
The following sequence of photos shows the mysterious behaviour of the T terminal visible in these photos. The T terminal is covered with a glass sleve up to the top of T, where it is exposed to the long main cathode. As the cruve traces below will show, when the cathode glow is below the top of the T, which is to say, with less than 1mA of cathode current, the T terminal behaves as an open circuit. When the glow reaches the top of the T, the terminal starts conducting suddenly. If the load to the T is just the 500kOhms to ground that is common in the application circuit, the Voltage at the T is about 15V when the glow is even with the top of the T, and increases to 40V at full glow with 5mA of cathode current. Some of the large variation in generated voltage at the T terminal can be due to cathode poisoning that would develop in the cathode after decades of rest. The cathode poisoning increases plasma potential. I noticed that the glow grew longer after some time, and the postitive bias variation with with glow lenght was less pronounced. It is this positive voltage at the T terminal that is used in the audio mute function in some FADA radios from the 1930's. When the tube is in use, with the enabling switch under user control, the audio triode stage under control of the Tune-a-Lite is pre-biased below cutoff with a large positive voltage at the cathode, and the triode grid is grounded. So, between stations, the glow is below the T, and the audio triode remains cut off. When the glow reaches the top of the T, the positive voltage that is developed at the T, provides a bias to the grid of audio stage triode and brings it into conduction and amplification, thus stopping the muting. A strong station causes the glow to reach the top of the T. The curves below trace the I/V characteristic of the T terminal for a series of cathode currents up to 5mA. The T terminal appears to be open because the glow is below the top of the T:
The glow just reached the top of the T, and some conduction up to 80uA occurs for negative sweep voltage at the T.
Notice that current flows for negative and postive sweep voltages. The Cathode current is probably still below 2mA, but it is more than 1mA.
With 3-4mA flowing through the cathode, the T terminal starts to fire like a cathode, if drive negative, but in the application, the 500K load resistor to ground keeps it's conduction on the positive side of the curve, which behave more like a current source.
As compared to similar tubes with only 3 or 2 terminals, this 4 terminal tube requires that the main cathode be grounded for proper operation of the T terminal with respect to the audio triode stage under muting control. The internal glow adjustment in the 4 terminal device is done with a potentiometer at the Anode, while it may be done on either terminal of the 3 and 2 terminal tubes. The following circuit was originally done for the 4 terminal Tuneon used by Cossor, but is the same circuit that is used for the Tune-a-lite.
Source for this schematic: AWA-OTB Volume 31 issue 4 page 27 Note that the pin numbering shown in this schematic is for the Cossor Tuneon.
Summarizing the operation of the T terminal, it provides a positive high impedance current source into a 500K load when strong stations bring the glow past the top of the T. When the glow is below the T, no bias is developed, and the controlled audio stage remains off to prevent inter-station noise. Has anyone heard the muting action in operation? I only have the tube, I don't have a radio for this tube. If you need a replacement for this very rare tube, you may be able to improvise with a 3 wire IN-13 Russian analog bargraph Tube, or a 2 wire IN-9 version. This article link shows how to add one of these analog bargraph tubes to a common AC/DC 5 tube radio. Comments invited, -Joe Sousa -------------------References: Antique Wireless Association - Old Timer's bulletin Volume 30 issue 4 page 17 by Patrick Dowd |
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Dietmar Rudolph † 6.1.22
15.Feb.09 |
2
In the book "Ghirardi, Alfred, A.: Modern Radio Servicing, 1st ed., Murray Hill Books inc. N.Y.; 1935" in chapter 19, (pp.469 - 491) several "Silent Tuning (QAVC) Circuits" and "Tuning Indicators" are described. Here an OCR excerpt of the "Tune-a-Lite" is presented.
19 17. "Noise Gate" QAVC Systems.
There are many variations of the qavc systems just described, especially in the more recent commercial receivers in which noise suppression is effected in the audio circuit. One particular receiver which incorporates silent tuning employs a four element neon tube as both a resonance tuning indicator (which, indicates when the set is tuned to resonance) and noise suppressor, being more popularly known as a noise gate. No vacuum tube is required for obtaining the silent tuning feature with this system.
The circuit is shown in Fig. 19 24. Three of the four elements in the neon tube are used to indicate station resonance.
 FIG. 19 24. A circuit arrangement in which a special four element neon tube acts as both a resonance indicator and a noise suppressor or noise gate. Its noise gate action depends upon the fact that it causes a high bias to be placed on the first audio tube (making, it inoperative) when the receiver is tuned off resonance.
The voltage required for the operation of the tuning indicator is obtained from the drop across a resistor located in the plate circuit of the r-f and i-f tubes. Since an avc system is used in the receiver, the voltage drop across this plate resistor (which depends upon the plate current of the controlled tubes) will vary as the receiver is tuned to, or off, resonance. When the receiver is tuned to resonance, the voltage drop across the plate resistor will decrease because of the decrease in the r-f and i-f plate current, caused by the increase in control grid bias. As the voltage drop across the plate resistor decreases, the voltage between two of the neon elements increases, increasing the neon glow, which travels up the long electrode. (A detailed description of the action of this tube as a tuning indicator is given in Art. 19 23.) Electrode No. 4 has only the top portion of its length exposed to the discharge, the lower section being insulated by a glass sleeve. When the gaseous discharge rises up the long electrode, No. 3, and reaches the level of the exposed portion of the fourth electrode, a discharge takes place between the long electrode and the fourth electrode. This discharge develops a voltage across resistor R1. Since the first audio tube is biased to the cut off point by the drop across R2, the voltage developed across R1 reduces the high bias on the a-f tube and permits plate current to flow. Reception will then be normal. If the receiver is tuned off resonance, the light column developed in the long electrode of the neon bulb will fall below the exposed portion of the fourth element, and the voltage developed across resistor R1 will be removed. This will return the first audio tube to the high bias condition and the tube will not amplify. A switch is provided so that the "noise gate" action may be cut out if desired.
In the silent tuning control circuit employing a four electrode neon tube, shown at Fig. 19 24, any trouble which might develop to prevent the neon glow from extending up the long electrode above the exposed portion of the fourth electrode would result in an inoperative receiver when the switch is placed in the silencing position. This may be caused by a faulty neon tube (one in which the gas content has decreased), or by any failure in the avc circuit, since this affects the neon tube action.
19 23. The Flashograph.
An interesting application of the neon tube as an indicator of proper tuning is the Flashograph, or Tune-A-Lite. This device makes use of the fact that the greater the voltage applied to the terminals of such a tube, the higher is the column of light in that tube. In other words, the action is analogous to the action of a thermometer: the more heat applied to the bulb of a thermometer, the higher the column of mercury or alcohol in it rises.
 As shown in Fig. 19 29, the Flashograph consists of three (the older models had two) electrodes, one longer than the others, mounted in a thin cylindrical glass tube filled with neon gas. The neon gas is first ionized by the small potential between electrodes 1 and 2, the exact value of this bias being made variable between about 90 and 185 volts by means of the potentiometer R6. Resistor R5 is connected in series with this electrode to insure stable operation. The third electrode is used to maintain the tube in a state of ionization when the signal strength fluctuates over wide limits.
With no signal tuned in, the plate currents of V1, V2, and V3, (which may be r-f or i-f amplifier tubes under avc control) are high, and the voltage drop across R1 is high. This voltage "bucks" the voltage across R2, R6 and R3, so that the net voltage across electrodes 1 and 2 is small. The height of the ionized column in the tube is small as a consequence. But when the signal strength is above the avc threshold, the plate currents of the tubes decrease, the bucking voltage across R1 decreases, and the net voltage applied to electrodes 1 and 2 increases. Note that electrode 1 is taller than electrode 2; this means that the height of the ionized portion of the gas in the tube increases, depending upon the amount of voltage applied to electrodes 1 and 2. The tuning knob of the receiver should therefore be adjusted until the height of the neon glow is greatest.
If a receiver should be serviced in which the variation in height of the glow is small (or zero) as the set is tuned from station to station, resistors R2 and R3 should be checked for open circuit. An open circuit in either one will not affect the operation of the receiver to any marked extent, although the tuning indicating device will not work. Of course, the neon tube may have to be replaced at times.
Regards Dietmar |
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Joe Sousa
16.Feb.09 |
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Dear Dr. Rudolph, Thank you very much for greatly enriching this thread with the excerpt from Ghirardi. Today I went to "RADIO XL" antique radio fair in Westford MA-USA. I found a copy of the Ghirardi book and bought it inexpensively from Frank Bequaert at www.beqbooks.com for $18 (US). This book has a comprehensive collection of AVC and QAVC methods used in the 1930's. QAVC means Quiet Automatic Volume Control. The "Quiet" means muting between stations. This survey of AVC and QAVC methods is particularly interesting because it shows a very wide range of approaches that were tried before simple diode detection for AVC was used universally after the 1930's with few exceptions, even in communications receivers. Regards, -Joe |
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Ernst Erb
17.Feb.09 |
4
It is very nice to have everything together on one document which has been started by Joe Sousa with a brillant article, followed by Dietmar Rudolph with his well done enrichments. I think we should use also the terms/brands the different radiomanufacturers applied. The first neon tuning indicator of FADA was the two element Flash-O-Graph used in 1931 for the models Fada 48 (see the ad) and Fada 49 with four gang tuning condenser (Chassis KW), followed by the Model 65 from 1932 for $ 124.50 (lowboy without stretcher). The Rider schematic calls it Flash-O-Graph (not Tune-a-Lite), an add September 10, 1931, in the "Daily State Gazette" names "New automatic FLASHOGRAPH" for "the perfect tone quality!". |
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Ernst Erb
14.Jul.09 |
5
Tuning indicators are still a subject for solid state sets - which sometimes still show a magic eye - or tuning meters. Theses can also be banks of light-emitting diodes instead of the traditional meter. See an article about the different methods for tuning indicators before the Magic Eye. |
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Roy Johnson
14.Jul.09 |
6
The "Flashograph" described above using the controlled neon tubes differs from the original Fada Flashograph. This was a mechanical system which did not indicate signal strength or electrical tuning accuracy. It comprised a switch operated from notches in the circumference of the tuning dial operating a filament lamp. It can therefore scarcely fall within the scope of "tuning indicators" but is mentioned here for completeness of records. ============================================================== To expand, I can add a few more details about tuning indicators.
There are two devices named Flash-o-graph or Flashograph by FADA.
The principle of the first implementation is shown in the diagram below.
 The first use was in 1930 on the TRF receivers with the KA chassis – models 41, 42, 44, 46 and 47 and also on the KB chassis in models 81, 82, 84 and 86. Details are shown on the schematics for these models.
This system allowed a station to be tuned in with the volume control turned down – thus avoiding the disturbing inter-station noises. However it relied on a pre-set number of stations and seems to have been largely used in the New York area for local station tuning.
“The notches on the tuning disc were strategically positioned to conform with the dial positions of the most desirable stations. Naturally, some latitude for adjusting the dial scale, when the action of the pilot light and reception did not coincide, was allowed for. The set screw which fastened the tuning dial to the condenser gang was simply loosened and the condenser plates rotated slightly so that the two would jibe. In New York territory the position of the notches, and consequently the flashograph action, seemed to work out pretty well – with exceptions on, possibly, a few frequencies. In other territories the flashograph action was not quite as accurate” (Bernsley, 1936)
AUTOMATIC FLASH-O-GRAPH
The new FADA method employing true electrical indication was named the “Automatic Flashograph” in 1930/1931 and was introduced on the KW chassis used initially on models 48, 49 and 65.
This is the system normally referred to simply as the Flash-o-graph and used the linear neon (Tun-a-lite) as excellently described in the earlier posts.
SHADOWGRAPH
This was introduced as an alternative to the moving pointer milli-ammeters indicating plate/anode current from one of more of the IF tubes.
A vane was controlled by a coil as shown in the diagram below.
 The vane produced a variable shadow from a rear-mounted lamp onto a translucent screen. The mechanism proved rather too delicate in use and was soon superseded by electronic devices.
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