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.
Antique Wireless Association - Old Timer's bulletin
Volume 30 issue 4 page 17 by Patrick Dowd
Volume 31 issue 1 page 33 by Patrick Dowd
Volume 31 issue 4 page 27 by Philip Taylor
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.
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.
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!".
I think it is good to bring such model examples here, giving the brands, year and linking to the models with schematics. We might later even have a full story of the different systems of tuning indicators - either as link to other articles in our forum or as a post here. Indicators for tuning are an old story and can include, normal voltmeters, shadow tuning indicators (Schattenzeiger, Abstimmanzeiger), glow-tube (glow-tubes), magic eye (magic eyes, tuning eye, kathode ray tuning) or even Luminous quartz resonators (see post 7 in English), nixie tubes and dekatrons etc. - or the simple bulb with special transformer. I write those terms for getting results with the SEARCH ;-)
Joe Sousa shows us a very interesting experimental bargraph for a model with modern neon indicators which can be bought as new products.
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.
The question for a tuning aide was always there but the superhet was even more dependent. Superheterodynes started only in 1931/32 to be more and more dominant. Many different approaches arrived the market - until the magic eye swept them away in 1935/36. The most used system shortly before were - both in Europe as in the US - the neon indicators, perhaps followed by normal instruments (meters) or "shadow indicating instruments" probably better named as shadograph (USA, Philips).
Saturable core reactors as tuning aid were used by some. Example: In 1932/33 Grigsby-Grunow used it in their Majestic model 300 and Majestic model 320. They called it "Synchro-Silent Tuning" or "Reactance Resonance Indicator". Something extracted from Majestic page 3-19 in Rider's volume III: "This is a reactor with three windings on three legs respectively of the iron core. The windings on the two end legs are connected in series with the pilot lamp, while the winding on the center leg is connected in series with the plates of the RF, 1st Detector, and IF tubes. An electrolytic condenser is connected so as to shunt the center winding. If no station is tuned in, a relatively large plate current will flow through the center winding. This saturates the iron core so that the reactance of the two outer windings is quite low, and considerable current therefore flows through the pilot light. When a station is tuned in, it operates the G-4-S Automatic Volume Control tube so that an automatic bias voltage is built up across Resistor R-9. This bias voltage is, in turn, impressed upon the control grids of the RF, 1st Detector and IF tubes. When this bias is impressed on these amplifier tubes, the normal AVC action the pilot light is therefore reduced, causing the pilot light to dim."
Joe Sousa showed us that General Electric (GE) used a slightly different method for a device they called Colorama. They used it at the same time the magic eye arrived and therefore it was used only in a few models like Colorama E-91 - up to Colorama E-125 ( I believe). I asked Joe if he can outline this a bit in an article and introduce those models - but that I will introduce the models if he can't. Later we will link some here.
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.