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Thermionic Valves - Electron Tubes - Vacuum Tubes - Lest we forget

Updated on July 12, 2013

Thermionics - The Birth of Analogue Electronics

Instantaneous long distance wireless communication was made possible by Thermionics. The Electron Tube or Thermionic Valve reigned supreme for about fifty years before semiconductor technology began to erode its position of dominance. This hub looks back over thermionics, lest we forget.

Philco People's Set, 1933. For the purists: the knobs are not original; they are from a 1950s Ferguson Radiogram.
Philco People's Set, 1933. For the purists: the knobs are not original; they are from a 1950s Ferguson Radiogram. | Source

Thermionics in Retreat

The 50 great years of thermionics were from 1920 to 1970. During this period, thermionic valves were at the heart of radio, and later, television production, transmission and reception, audio amplification and public address, radar and radio telescopy. Valves also made possible early tape recording and mainframe analogue and even digital computers.

Towards the end of this era, semiconductor technology (transistors) began to replace thermionic valves in most 'routine' applications, though in specialised areas valves hung on for many years, most notably in image gathering (TV cameras) and displaying (TV picture tubes). Today, they have all but disappeared, except in Geiger-Muller tubes (geiger counters) and a few hi-fi audio applications where their continued use owes more to fashion than function.

What's inside a valve?

The typical valve is a glass envelope enclosing a vacuum, or as close to a vacuum as can be achieved. Inside are a number of electrodes, accessed (from outside) by wires that pass through the glass and terminate in connection pins in the valve's base.The electrodes are:

  1. the cathode - this is heated by an electric filament (indirect heating) or may actually be the filament (direct heating). When heated, the cathode emits electrons into the surrounding vacuum.
  2. the anode - this is maintained at a positive potential (with respect to the cathode) and attracts and collects the emitted electrons, causing a current to flow.
  3. the grid (or grids) - this is a wire mesh or helix through which the electrons pass on their way to the anode. The flow of electrons is controlled by the grid potential.


conventional symbol for a triode valve
conventional symbol for a triode valve | Source


internal construction of a triode valve
internal construction of a triode valve | Source

The drawing on the right is the symbol for an indirectly heated triode valve, as used in circuit diagrams. A triode is a valve with three electrodes, cathode, anode and grid. The heater is not classed as an electrode. The symbol for a diode valve (two electrodes) is the same, but without the grid. The symbol could give the impression that the electron flow is from the bottom to the top of the valve, but in fact, the internal construction is cylindrical, with the cathode in the middle. The filament (heater), not shown, is inside the cylindrical cathode. Electron flow is radial, outwards from the cathode, past the grid, to the anode. Apart from early experimental devices, the only valves that work axially instead of radially are television camera and display tubes.

How do valves work? - the basics

1. The Diode Valve

The cathode is heated to a dull red heat. This causes negatively charged electrons to leave the surface. These liberated electrons are then accelerated by the radial electrostatic field, crossing the vacuum to land on the (positive) anode. Electron flow is cathode to anode. If the polarity is now reversed, i.e. the anode is made negative with respect to the cathode, no current flows. Why not? Because electrons leaving the cathode are driven back to the cathode surface by the reversed electrostatic field direction. Also the anode, being cold, does not emit electrons, so there is no source of electrons for reverse current flow. The diode is therefore a device that passes current in only one direction. It has two main applications in radio receivers:

  • Rectifying Diodes are used in power supplies to convert mains alternating current (AC) to direct current (DC) to feed the rest of the receiver circuitry.
  • Detector Diodes are used to extract the audio signal from the received amplitude modulated (AM) radio frequency (RF) signal.

2. The Triode Valve

In a triode valve, electron flow is still from cathode to anode. However, they must first pass the grid, which is held at a negative potential with respect to the cathode and therefore tends to repel electrons back towards the cathode. This negative grid voltage is called grid bias. So, how do electrons get past the grid? The fact is, electrons don't just 'fall off' the cathode; they come flying off with a wide range of velocities. The slower ones will be turned back by the grid, but the faster ones will manage to pass through the grid into the pull of the anode. In the language of Physics, their initial kinetic energy must be sufficient to overcome the potential barrier of the grid. The grid acts like a tap (faucet) to control the cathode to anode flow. A small change of grid voltage can result in a large change in anode current. This is the principle of amplification. Triode valves have three main applications in radio:

  • Triodes can amplify low level audio signals to drive headphones or loudspeakers.
  • Triodes can amplify low level RF signals from an aerial, prior to diode detection (as described above)
  • Triodes can function as 'local oscillators', to produce the continuous RF wave signal required in superheterodyne receivers. (To be explained!)

The Triode as an Amplifer

The next circuit diagram shows how a triode valve can be configured for use as an amplifier of small a.c. signals, e.g. the output of a microphone.

a triode a.c. amplifier
a triode a.c. amplifier | Source

The DC conditions - Electron flow is from ground through RC to the cathode, then from cathode to anode, and through RA to the positive supply rail (+V). The voltage drop across RC causes the cathode to be slightly positive with respect to the grid which is held at ground potential (0 volts) by RG. (No current flows in RG). The anode resistor RA is chosen so that the voltage drop across it is about half of the supply voltage. For example, if the supply voltage is 200V and the anode current is 1 mA, we could have RA = 100KΩ making VA 100V, and RC = 10KΩ, making VC 10V. With the grid at 0V, the grid bias is effectively -10V.

Signal (AC) amplification - The input signal Vin is applied to the grid via a decoupling capacitor (to ensure the DC conditions are not disturbed). A small increase in grid potential causes a large increase in anode current. As this current flows through RA, the voltage drop across RA increases causing the anode potential to fall. The output signal voltage Vout thus follows the input Vin, but the small fluctuations are amplified, and the sense is inverted (see sketch). Notice the bypass capacitor across RC. This couples the cathode to ground for AC (signal frequency) and prevents the gain reduction that would otherwise occur if the cathode voltage were allowed to follow the input signal.

Radio frequency amplification

The same basic circuit can be used to amplify radio frequencies, but then it is normal to replace the anode resistor RA with an inductor, called an RF choke. This is a coil of wire usually wound on a piece of ferrite. At audio frequencies and below, its reactance is low so no signal voltage develops across it, but at radio frequencies its high reactance gives rise to a high signal output. Taking this a stage further, using a parallel tuned circuit as the anode load produces an amplifier that is highly selective of one particular frequency. This principle is used for tuning a radio receiver to a particular station.

Valves with more electrodes

The triode is the fundamental 'active device'. The diode cannot amplify or produce signals; the triode can do both. It was the triode that made possible radio transmission and reception and audio amplification. But triodes have their limitations, and these led to the development of multi-electrode valves.

The Tetrode

One problem with the triode valve is the capacitance between the anode and the control grid. As the anode signal is inverted, the effect of this capacitive coupling is to reduce gain, especially at high frequencies. In some cases, when using reactive anode loads, this capacitive coupling can result in instability and oscillation. The solution is to place a second grid, called a screen grid between the control grid and the anode. This grid is held at high potential but is decoupled to ground by a capacitor. The effect is to screen the control grid from the anode and therefore eliminate the undesirable effects described above. Tetrodes therefore provide higher gain at high frequencies.

The Pentode

Unfortunately, the tetrode introduced a new problem of its own. When electrons strike an anode at high velocity, sometimes they 'dislodge' other electrons. This is called secondary emission. In a triode, these secondary electrons are recollected by the anode, but in a tetrode, some of them are collected instead by the (positive) screen grid. This causes a kink in the graph of anode current vs anode voltage, in some cases even a downturn of the curve, giving a region of negative dynamic resistance. Negative dynamic resistance results in instability or oscillation (the dynatron effect). To counter this, a third grid is added, closer to the anode, and held at a low potential. This has the effect of reflecting secondary electrons back to the anode, preventing their take-up by the screen grid. This extra grid is called the suppressor grid. In terms of performance and stability, the pentode is generally considered the 'standard' valve.

Special purpose valves

Triodes, tetrodes and pentodes can all be considered general purpose valves. Additionally, several valves were designed for very specific purposes, especially for radio receivers. The aim was often to reduce the number of physical devices by mounting two or even three electrode sets in a single envelope. This was particularly prevalent in the UK, where radio manufacturers paid a certain amount of duty per valve to the exchequor. Most of the compound valves combined some of the functions of oscillating, mixing and detection as required in superheterodyne receivers. Some of the commoner specials were the double-diode-triode, the triode-hexode, and the pentagrid.

ECH2 Triode-Hexode Valve
ECH2 Triode-Hexode Valve | Source

Some valve miscellany to finish

In no particular order:

  1. Cathodes are doped with thorium to increase electron emission at relatively lower temperatures. Before this discovery, the 'bright cathode' valves ran much hotter.
  2. To improve (harden) the vacuum during manufacture, after the bulb is sealed, a substance called a 'getter' is vapourised inside (by electromagnetic induction). The getter captures residual ions and deposits itself (and them) on the inside of the glass envelope.
  3. Cathodes wear out eventually becoming unable to source enough electrons for the required anode current. This is called cathode stripping.
  4. Some valves have a 'cap' terminal on top as well as the pins in the base. This is the control grid. It is led out through the top to reduce capacitance with other electrodes.
  5. Some valves are entirely encased in a metalised conducting 'paint' which is connected to chassis ground. This is to prevent RF radiation to adjacent circuitry, not to screen the valve from outside interference.
  6. Valves can become microphonic - airborne vibrations (sound waves) if passed on to the electrodes, especially the grid, can result in an output signal. This is why if you tap a radio valve a sound may come from the speaker.
  7. Valves are not better than transistors in guitar amps. It's simply that a lot of classic tracks were cut on valve amps and people are nostalgic for that particular type of distortion!
  8. Valves are immune to EMP (electromagnetic pulse) as produced by certain nuclear weapons. This led to valves' extended deployment in some emergency broadcast facilities in nuclear bunkers.

Enough is enough

I could carry on forever about thermionics. What has kept my interest alive is my small but select collection of 1930s wireless sets which I keep in good working order and listen to regularly. This hub has barely scratched the surface of a fast fading but fascinating technological field. The Golden Age of Wireless might just have been our finest achievement as a creative species.

Thank you for reading!


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    • wandererh profile image

      David Lim 8 years ago from Singapore

      You reminded of the days when I was young. I'd peer into the back of a tv set and look at the dull glow of these valves.

      God, I'm getting old!

    • Paraglider profile image

      Dave McClure 8 years ago from Kyle, Scotland

      You had TV? That's not old ;)

      Thanks for the read!

    • wandererh profile image

      David Lim 8 years ago from Singapore

      I remember the TV. It was always breaking down and we spent a fortune on repairs for it. And the heat it generated probably contributed to global warming. ;)

      Well, I'm 45. Not really old I guess but definitely not as young as I used to be.

    • Paraglider profile image

      Dave McClure 8 years ago from Kyle, Scotland

      We got our first TV just after Queen Elizabeth's coronation. I was less than a year old at the time, so I can't claim to remember its arrival. That would have been 1953. We're all getting on :)

    • profile image

      ColdWarBaby 8 years ago

      A bit on the technical side for me but interesting nonetheless.  It does inspire me to ask a question. 

      I’ve recently finished a short book which I obtained from the internet in PDF format.  “REPORT FROM IRON MOUNTAIN: ON THE POSSIBILITY AND DESIRABILITY OF PEACE” With introductory material by Leonard C. Lewin

      The Dial Press, Inc. 1967, New York

      Library of Congress Catalog card Number 67-27553 Printed in the U.S.

      The book is considered by most to be a hoax although some of Lewin’s contemporaries claim it is not, even though the author “admitted” to it.

      At any rate, at one point in the book it is stressed quite vehemently that all great inventions and art, throughout history, are a direct result of war.  I have read similar opinions but have never done adequate research to either confirm or deny their validity.  My instinct tells me that this position is not only wrong but preposterous.  While it may be true that many great discoveries were seized by governments and perverted to the cause of war, I do not believe that those who made them or did the research were motivated expressly by the thought of war and weapons in their quest for knowledge.

      As to art, I fail to see the connection between war and The Mona Lisa.

      What’s your take on this version of progress in general and do you know if it applies directly to the subject you cover in this Hub?

    • Paraglider profile image

      Dave McClure 8 years ago from Kyle, Scotland

      Hi CWB - First, thanks for reading. Thermionics and wireless in particular are very close to my heart - I moved seamlessly from electric trains and slot cars to second hand valve wireless sets when I was about 12, and no doubt that was the interest that moved me to join the BBC. So these old valves directed my life.

      I think valves and wireless developed from each other, in many ways, and the bulk of the development was in the 1920s, after the Great War. By 1930, the theory and practical pieces were all in place, and the rest was mainly refinement in performance, reliability, stability etc. Radar was the main war application of thermionics, and certainly that was war driven. There was also work on miniaturisation (laughable now, but 'suitcase transmitters' for spies!) But towards the end of WW2, the British government realised that they couldn't talk to the people because most of the pre-war domestic wireless sets had packed in, and there was an urgent need to replace them -

      A standard circuit design for a reliable simple receiver was produced, and even a standard plywood no-frills cabinet, and all the manufacturers were instructed to mass produce these so-called "Wartime Civilian Receivers" or "Utility Sets". I've still got one.

      But back to the original question, the pioneering work by people like Bell, Fleming, Eddison - none of that was primarily war effort. Neither was the early TV work by Baird and Marconi. These guys were all that breed of inventor-scientist, in the Faraday tradition, that you'd have had to manacle to the wall to stop them creating. So, I suspect Mr. Lewin is just being provocative to sell a book.

      You mention the Mona Lisa. Leonardo also invented the helicopter, in principle, but didn't have materials that could build it. That wasn't for war either.

    • profile image

      ColdWarBaby 8 years ago

      Thank you Parglider. You've largely confirmed by opinion. (notice I didn't say "belief"!)

    • prettydarkhorse profile image

      prettydarkhorse 7 years ago from US

      I agree with you Dave, the wireless tech is a great achievement for all of us, for convenience too,

      thanks for the information again, I read it again twice, but it is ok,

      Have a good day Dave, Maita

    • Paraglider profile image

      Dave McClure 7 years ago from Kyle, Scotland

      Hi Maita - this hub proves that I'm not writing here for the money. 90 year old technology attracts very few clicks! Thanks for being one of them :)

    • Sufidreamer profile image

      Sufidreamer 7 years ago from Sparti, Greece

      Sorry PG - I am stalking you through Hubtivity today - you are visiting some interesting places!

      Wonderful Hub - your passion, love and admiration shines through and you have explained things so simply and concisely. It was an interesting journey through history!

    • Paraglider profile image

      Dave McClure 7 years ago from Kyle, Scotland

      Most welcome! I only get to 'play' with my old wireless sets when back in UK, because they're too big and fragile to travel the world. "London calling..."

    • profile image

      Andy 7 years ago

      Hello - I'm a guitarist who uses valve amplifiers. Was just browsing to understand the construction/workings of valves. In this modern age of physical modelling where digital code has taken the audio world by storm, it is still comforting to hear and "feel" directly the overwhelming evidence that valve amplifiers translate the nuances of my fingers/guitar picking technique in a way physical modelling has not yet managed to achieve. Long live the ECC83 and the EL34 (and the gZ34 and EL84)

    • Paraglider profile image

      Dave McClure 7 years ago from Kyle, Scotland

      Andy - the guitar is an analogue device so is bound to sound most authentic with analogue amplification. I play too, and for acoustics with pickup I actually prefer the sound I get using a solid state keyboard amp (Peavey). Very clean and sounds exactly like the unplugged guitar. But for electric, something like a Vox AC30 takes a lot of beating.

    • profile image

      Peter Enmore 6 years ago

      Love your hubs Paraglider! And Andy, my brother says the same thing about his guitar amps.

    • Paraglider profile image

      Dave McClure 6 years ago from Kyle, Scotland

      Thanks Peter :)

    • profile image

      mmmmmmmmmmmmm 6 years ago

      how can valves be used in computers to make decisions?

    • Paraglider profile image

      Dave McClure 6 years ago from Kyle, Scotland

      they're not used in computers any more, because they take up too much space. Computers use semiconductor components.

    • profile image

      Raymond Kinman 5 years ago

      This was so interesting! One of the other commenters mentioned that it reminded him of his childhood and when he used to peer into the tv set.

      Well it takes me back a few years too, when us kids would gather up my fathers electronic and electrical odds and ends and try to make something out of them.

      Yes today's technology is exciting, but I still marvel at the inventions of yesteryear, like the thermionic valve, electron tubes and vacuum tubes. They brought excitement to my world as a little guy.

    • Paraglider profile image

      Dave McClure 5 years ago from Kyle, Scotland

      Hi Raymond - my story is much the same. I used to buy 2nd hand wireless sets at jumble sales and try all sorts of experiments with the parts (with varied success, of course!)

    • profile image

      rick91 4 years ago

      In the 50s I made valve amplifiers from parts bought in the 'wireless' shops in Edgware Road, such as 'Henrys'. Usually I used circuits from books by a Mr Camm. Then took the amp to school to experiment with like-minded boys in the science lab at lunch-times, with the connivance of the Physics teacher. happy days!

    • Paraglider profile image

      Dave McClure 4 years ago from Kyle, Scotland

      Hi Rick - you must have a few years on me. In the 50s I was more interested in electric trains! In the 60s I was experimenting with valves and (germanium) transistors. I remember Henrys adverts in PW and PE magazines and bought from them often, by mail order. Thanks for the visit :)

    • profile image

      Charlie Evans 4 years ago

      I was just wondering. I have an old valve amplifier but I can't use it as the power adapter from my laptop (macbook pro) causes a horrible noise. Is there anything I can do to stop it?

    • Paraglider profile image

      Dave McClure 4 years ago from Kyle, Scotland

      Could be tricky. It sounds like RF interference from the switched mode power supply, but it's impossible to be sure without seeing it. Do you really have to use them both at the same time?

    • profile image

      karen Jane Nicholls 4 years ago

      Can you explain the parts of the wireless radio in very simple language? we are looking at an artifact in school for technology study. The red valve you show earlier goes through a metal box that looks like a dolls house bookcase and one stands between twin metal towers. I can identify the light and the speaker. There are also some glass tubes. one is like a dutch stack inside doll and one is cylindrical. There are two black metal 'valves'? There is a semi circle set of fins that move with the tuner. It is a Radion with a Rola Speaker. We really want the features and their function.

    • Paraglider profile image

      Dave McClure 4 years ago from Kyle, Scotland

      The valves do all the hard work. They amplify the radio signal, change its frequency and amplify it some more, separate out the audio signal, amplify that and use it to drive the loudspeaker. The metal towers are tuned transformers, used to separate wanted from unwanted signals and move the signal forward from one valve to the next. The semi-circle set of fins is a variable capacitor, used (together with some wire coils) to select which part of the radio frequency spectrum the wireless is tuned to receive. Hope that helps?

    • profile image

      karen Jane Nicholls 4 years ago

      Thanks, you rock!

    • eugbug profile image

      Eugene Brennan 20 months ago from Ireland

      This is a bit of nostalgia, I remember the old valve radios and TV we used to have and how in those days they often had wood casings to make them look like pieces of furniture. My uncle used to fix TVs as a hobby, so we were always getting a new one as parts failed or a valve aged. I think when I was young also, the radio was called the "wireless"! The TV took ages to come on as valves heated up, and when switched off there was that bright dot in the center of the screen as the cathode in the CRT cooled down. The one advantage of valves is you didn't have to worry about static electricity damaging the electronics. I always try to avoid touching pins in connectors in case this happens! (or ground myself first)

      A good guide for those who are interested in the history of electronic technology and voted up!

    • Paraglider profile image

      Dave McClure 20 months ago from Kyle, Scotland

      Thanks eugbug - Thermionics was very much a technology that people could understand and learn to mend and experiment with. Gradually this has become less and less possible with solid state and hard coded firmware. Something gained, something lost, I suppose.

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