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Vacuum Tubes

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1.1.1       The Vacuum Diode

De Forest one day sucked the air out of an old glass bulb one day. It had just been laying around and had a two terminal filament of wire with another piece of metal placed next to it. He connected the filament wires to a battery and watched the cosy dull red glow from its metal wire filament. Then he thought about that forgotten third terminal – why not connect that to a battery also and see what would happen. To his surprise, actually it made no difference – the filament glowed pretty much the same – but then he placed his ammeter in the current path to the third terminal.


What De Forest found was a slight twitch on the dial – how, he asked in silence of his lab, could current travel those cold empty distances of space? The third terminal was not connected, and a vacuum separated it from the filament. But when the filament glowed, De Forest could see the ammeter needle twitch.



Although I was not there at the time, I guess De Forest must have felt some sense of adventure in all of this. For some reason the hot metal filament was releasing electrons into the local vacuum and these were being attracted to the third electrode that carried a positive, attracting charge. In this way a small but measurable electric current flowed from the negative battery terminal, launched itself into the vacuum of space inside De Forest’s glass bulb, got collected by the third terminal which had a positive attracting bias and then flowed back through De Forest’s ammeter to the battery’s positive terminal.


In those small quiet moments De Forest grasped the implications alone and imagined what if the filament was hotter, or if more positive voltage was available to attract even more electrons. Or even, what if the polarity was reversed – would those pesky electrons be repelled by the negative charge on that third terminal and be sent hurtling safely back home?


De Forest tried a crack at all of this, and the vacuum diode was born.


Soon everyone wanted one of these devices. The vacuum diode became a “one way” device, i.e. it would pass current in one direction but block it in another. This allowed it to convert alternating current “AC” signal into direct current “DC” signals. In two particular ways this invention was timely,


The 1920’s etc had adopted AC power for house-by-house distribution. This was fine for light bulbs and allowed convenient voltage transformation, but how could people charge up that old Henry Ford motor-car battery when it went flat? AC was no good, but if a vacuum diode could covert it to DC then people could be off to a racing start
AM Broadcast had just begun – at first primitive spark gaps but more was soon to follow. The vacuum diode was the first jump-start to technology into the land of amplification and oscillation. 


Hence the good old vacuum “diode” came into focus in our history at a very convenient time.


1.1.2       The Vacuum Triode – Part 1

People had been playing around with static electricity even before the time of the Egyptians. Sparks seen and felt when rubbing a cat’s fur were commonplace. They had all done the two “orange pith” on a string trick, watching them stay apart when static electricity was around. The repulsive nature of electrons were evident even to them, so why put them into commodity product’s like the then day vacuum tube?


I guess the simple answer is that when there’s gold in them darn hills, why then someone’s got to mine it. Could a diode be made to do more than just rectify – could static repulsion be used to make it return a mother-load from even small incoming signals. At this point perhaps a short diversion is justified,                  A Short Diversion Into Coherer Technology

The ocean going ships of the time had very little way to communicate situations of distress and call for rescue when needed. The most they could do is play semaphore games to people back on land if they could be seen, or make loud fog horn calls if they could be heard. However people such as Nikola Tesla had been experimenting with coils that make high voltage discharges, and wanted to send power throughout the world based on resonant transmission line effects between the planets surface and its atmosphere. Marconi wanted to do the same but on a smaller scale. His interest was to make a big spark in one local wire loop recreate a smaller spark in another remote wire loop. His experiments sparked a revolution. The birth of radio communication screamed with flashes of light, the smell of ozone and loud crackling noises.


What Marconi showed was that information, not just power, could be transmitted at a distance “over the air”. Just as those electrons in De Forest’s vacuum diode did soon to follow. So what if a big boat had some massive big spark generator – surely a remote person monitoring could see some tiny flash if trouble came and raise an alarm? Well sadly no. The sparks fading too quickly with distance and the second remote antenna couldn’t outreach the semaphore eye.


Some people thought about this and reasoned that the distance of the receiving spark gap was the problem. Perhaps there are transactions on this subject? How could the gap be made smaller so that the tiny sparks could fly across and make their mark?


I forget who the bright spark was but someone came up with the idea of filling a glass tube with rusting old iron filings. Either she or he attached electrodes to each end. When a RF pulse came along some of the filings would arc and short, allowing a DC current to pass.



The “coherer” was actually quite sensitive at detecting RF signals but it could only do it once. This made it a “1 bit detector”. The operator, however, could tap it loose after the received headphone click and it would be ready for action again. Actually very little has changed from this “reset” approach today, except that we prefer not to have to reset all the time.


The simple mechanical “coherer” evolved with self resetting versions but never would have the ability to recover speech transmissions. Morse Code was its domain and it was firmly there to stay. Or so it thought – until De Forest’s vacuum diode came along.


Soon after the diode came the triode and the poor coherer was quickly doomed.


1.1.3       The Vacuum Triode – Part 2

The coherer was simple and cheap, but lacked sensitivity and always needed a good tapping. The vacuum diode was far more effective in detecting RF signals and was self resetting to boot. However it was still “deaf” by today’s standards and needed something to boost the signal prior to rectification. Further the maritime spark generator transmitters seriously polluted the RF spectrum and people sought to see them banned in favour of a cleaner signal source. This is were the triode valve sought is first entry point – its electrons could be controlled by static charges and currents could be made to follow the application of voltage



At the time this was a remarkable discovery. Since the grid was negatively charged it drew no current as the electrons were repelled by it. This hindered, and eventually could prevent them from reaching the positively charged Anode. Consequently almost zero power was required to control the Anode current. Since this was able to operate at a very high Anode voltage, the potential so generating significant power variation in a load resistance from an infinitesimal Grid control signal power quickly lit the imagination of many. Finally the amplification device that people had long dreamt for had arrived.


Two subsequent pioneers, Hartley and Collpitts soon discovered that the vacuum triode could be used to generate continuous sinusoidal output signals. Their “oscillators” consisted of a vacuum triode, a resonant tuned feedback circuit and a DC supply. Unlike the broad output spectrum produced by maritime spark gap transmitters, these oscillators produced a pure single frequency tone. They, and others experimented with many variations on the same theme, and soon high power, clean signal sources were available. Now that many people could occupy the same common spectrum at different transmit frequencies, the use of spark gap transmitters was soon outlawed.


1.1.4       The Vacuum Tetrode and Pentode

Despite the magnificent break-though performance of the vacuum triode it was not without its limitations. Even given improvements in vacuum technology that allowed anode voltages up to ~3000 V to be supported, it was still an inefficient amplifier. The effect of the grid voltage on electron flow was also influenced by anode voltage. High Anode voltages attracted more electrons from the Cathode filament and increased the Grid voltage needed to stop their flow. This form of “internal negative feedback” reduced the amount of voltage gain that the early vacuum triode could achieve. In addition, even Grid voltages of –zero volts could only support a finite Anode current, and this Anode current reduced as the Anode voltage fell. This restricted the maximum Anode voltage output swing, and so DC to signal output power was compromised.


At one stage in the vacuum triode history, people began to experiment with positive grid voltage control. Although grid current was drawn and input drive power was increased, useful power gain was still possible and output efficiency gains were obtained. The positive grid voltage attracted electrons, far more so than would otherwise have reached the Anode. These new tubes promised to be the solution.


Such positive grid control devices emerged for a short time until the penny dropped – why not place this positive attracting field on a separate grid? The main control grid could then remain negatively charged and so draw no current. This second “screen grid” could draw the electrons forward and hurtle them towards the Anode. More importantly, once pass the Screen Grid the Anode voltage would capture them as effectively with low Anode voltage as with high.



A second unexpected improvement resulted. The close proximity between Grid and Anode in the vacuum triode resulted in significant Grid-Anode feedback capacitance. In the early AM Tuned Radio Frequency (TRF) broadcast receivers, several valves were successively lined up to amplify weak incoming signals. In order to achieve high power amplification and selectivity, high impedance resonant coupling circuits were needed. However the internal feedback capacitance of the triode resulted in self-oscillation. Some compensating “neutralising” circuits were needed, requiring delicate adjustment at each received frequency. The Tetrode however placed an isolating “screen grid” between the two terminals, and the several pF coupling seen in the vacuum Triode was reduced to less than 0.02 pF.


However even these improvements did not come at a cost. The vacuum Tetrode suffered from an effect called “secondary emission”. At certain low Anode voltages the screen grid appeared positive to its lower potential and attracted electrons from it. The electrical consequence was a region of negative resistance in the Anode voltage versus current transfer curve, leading to audio distortion and the potential for instability at large output voltage excursions.


A simple solution was soon found. These unwanted Anode electrons could be re-attracted by the inclusion of a third grid between the Screen grid 1 and the Anode. This Screen grid 2 was kept at zero potential and the resulting 5-terminal structure was called the vacuum Pentode.

1.1.5       Multiple Grid Devices and Special Configurations

As previously mentioned, early AM broadcast radios used TRF technology, in which multiple RF amplifying stages were cascaded up the point of RF to Audio AM demodulation. Each of these RF stages used tracking RF tuned circuits to pass a wanted signal frequency and reject others.


This approach was very much in vogue by the early 1930’s, but required much skill in tuning, especially as “ganged” multiple sets of tuning capacitors were not readily available and each tuned circuit had to be simultaneously tuned to a given radio station for it to be heard. In earlier vacuum Triode designs, additional “neutralisation” trimmers had to also be adjusted for each RF stage to retain stability. Consequently, once set up, people seldom ventured to the misery of changing to another radio channel.

Note: Some very popular approaches to the “pure” TRF receiver remained popular. The most notable was the single valve “regenerative grid leak detector” receiver for AM. This used the diode rectification properties of the control grid to demodulate the AM signal, and either a Triode, Tetrode or Pentode to amplify the audio signal. Since RF was present on the grid, RF also appeared at the Anode. This was feed back to the input tuned circuit as controlled positive feedback. This allowed much higher selectivity and sensitivity to be obtained over a simple diode detector, and only required one tuning control and one feedback control to be effective. This dual frequency processing capability was also exploited in the “Reflex” circuit, in which a given valve would amplify at RF, have the signal demodulated to audio, and then amplify this audio signal a second time.


TRF technology remained well into the 1950’s, especially for VHF communications. A simple but accidental discovery revealed the “Super Regenerative Receiver”. This consisted of a supersonically gated RF oscillator tuned to the incoming frequency. Sensitivities of less than a few microvolts were achieved using a single tube, and selectivity still exceeded that of the single tuned circuit used. However the presence of a loud background hiss, for which many people labeled it as a “rush box” eventually saw it made redundant by the Superhetrodyne, or “superhet” receiver.

















Because the technology relating to oscillators had been previously established (Hartley and Collpitts), some people wondered if these could not be used somehow in a receiver. After all, people were very familiar with the audio beat note, or howl, that arose from a mistuned AM broadcast receiver. Only Armstrong had the mad genius to connect the observation with improvements to receiver design. He made use of the non linear properties of amplifiers to convert energy from one part of the radio spectrum to another.


Armstrong found that if a valve amplifier was supplied with a weak RF input signal, and if an additional large amplitude sine wave was also applied, then the valve would create outputs at sum and difference frequencies. This realisation was to be the champion idea behind almost all current receiver (and some transmitter) architectures.



Armstrong called these new frequencies “Intermediate Frequencies” or IF. Several important advantages were evident.


The receive frequency could be adjusted by tuning just one oscillator circuit
A single RF input tuned circuit was sufficient to reject one of the “image” frequencies  or
All main amplification could occur at a single fixed IF frequency
Gain at Low frequencies was much easier to obtain than gain at RF
Frequency selectivity was also easier to obtain with fixed, low frequency IF’s

This, to Armstrong, was the best thing since sliced bread, and he knew he had a winner. This superhet technology made VHF and UHF reception possible, and inexpensive. Only two high frequency devices were needed to process RF, everything else could work down at much lower, and easy to process frequencies. Still, some drawbacks existed. The method of introducing the LO signal required a common electrode, and this not only caused oscillator loading (and frequency pulling) effects, but also allowed a sizable LO signal to leak out to the antenna, causing potential interference to other spectrum users. Armstrong needed a better mixer device.


Various configurations were tried, ranging from injecting the LO signal into the Cathode, or Screen grid 1 and even Screen grid 2. All three methods gave good results and the combination of Screen grid 1 as a “virtual Anode” and the Cathode even allowed a simple oscillator to be constructed inside the mixing valve. However such configurations were never optimum in terms of convenience.


Soon people trigged that the LO signal could be injected into any of the valve’s grid terminals, and that sum and difference frequencies would be generated, they began to see the process of frequency conversion as a multiplication function. Voltage on one grid could control the gain of the device to a signal on a different grid. This quickly led to the generation of a special mixer valve called a Hexode (6 terminals) and Heptode (7 terminals).


These new mixer valves used control grid 1 for RF signal input and the “suppresser grid 3) for LO injection. A forth grid was also added after grid 3 to further attract electrons by adding additional positive charge. However the ugly head of “secondary emission” reared once more, so a 5th suppressor grid was added, connected to ground. This completed the final evolution in valve mixer technology, or so it seemed.


All mixer structures so far were single ended or “un balanced”. For simple receivers this was fine, but what if maximum dynamic range was needed. A push pull or “balanced” approach could at least add superior even order mixer term rejection. In the early low density RF environment non linear effects in valves was somewhat unimportant, but as the number of transmitters increased, and their signal powers, “spurious responses” in the mixing process started to become a big concern.


As mentioned previously, sum and difference frequencies are generated by the mixing process. However products relating to combinations of RF and LO harmonics also occur. These allow unwanted frequencies to be received in places where they should not be.


For example, let’s say we want to receive an AM radio station at 1.4 MHz using an IF frequency of 500 kHz. We could use an LO frequency of 1.9 MHz or 0.9 MHz. Let’s choose this higher option. The second harmonic of the LO signal is 3.8 MHz, and the third harmonic of an incoming RF signal at 1.4333.. MHz is 4.3 MHz. This combination will produce the same IF output frequency of 0.5 MHz. A signal as close as 33.3.. kHz from the wanted signal could elicit a spurious response!


A few solutions were proposed – all based on the idea of push pull topologies that could provide second order cancellation. One particular mixer tube had two Anodes whose electron streams where switched by between them by a common but inverted LO signal. But by now the semiconductor revolution had begun, and the dinosaur cult of the valve was poised to a break point of imminent failure.




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