807 vacuum tubes

807 vacuum tubes DEFAULT

Postby doug houston » Nov Wed 28, 2007 6:31 am

RCA made several different theater amplfiers that used PP-Parallel 6L6's. was lucky enough to get one before they went into outer space with price.

They also made lots of their 70 watt theater amplifiers using 6146 in Push-Pull. Mighty good sounding. I have 3 of them. Indeed, the plates of the 6146's could be a big hazard, but you use ceramic insulated plate caps on them, or you're just goosing lady luck!

A comment on Williamson amplifiers. They were all the rage when i was in college, and I made my first one nfrom the Stancor sheets that were mentioned above. Fantastic sounding, too. A couple of years later, I had a record playing on the Williamson while working on an AM set on the bench. Well, whadya know, a station was playing the same record on it as I was playing on the Williamson! And what else; when I stopped playing the record, the station did, too! Then later, I bought my first E-V speaker system, and built a Heath W5M amplifier to use with it. Not long into using it, I noticed a lack of treble. The tweeter was burned out. E-V replaced the tweeter free of charge for me. I dumped both Williamsons, and replaced the W5M with one I made, using a 6AS7 output. No more blown tweeters after that. The burnt child fears the Williamson!

I haven't checked it yet, but isn't an 815 a couple of 6L6's in the same jug? I have an Ampex tape recorder that uses an 815 for the bias and erase oscillator, and I've often wondered what the two tubes inside it are. never thought to pull out the little RCA books and find out. It looks tempting for an audio tube.

But, going back to the 6AS7 amp that i built out of an RCA tube manual from about 1952, It has as clean an output as anything I've ever heard. Pure Class A.

doug houston
Posts: 2904
Joined: Jan Thu 01, 1970 1:00 am
Location: Ortonville, Michigan


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The 807, a Vacuum Tube for the Ages

Like the Enigma, the HX-63 was an electromechanical cipher system known as a rotor machine. It was the only electromechanical rotor machine ever built by CAG, and it was much more advanced and secure than even the famous Enigmas. In fact, it was arguably the most secure rotor machine ever built. I longed to get my hands on one, but I doubted I ever would.

Fast forward to 2010. I'm in a dingy third subbasement at a French military communications base. Accompanied by two-star generals and communications officers, I enter a secured room filled with ancient military radios and cipher machines. Voilà! I am amazed to see a Crypto AG HX-63, unrecognized for decades and consigned to a dusty, dimly lit shelf.

I carefully extract the 16-kilogram (35-pound) machine. There's a hand crank on the right side, enabling the machine to operate away from mains power. As I cautiously turn it, while typing on the mechanical keyboard, the nine rotors advance, and embossed printing wheels feebly strike a paper tape. I decided on the spot to do everything in my power to find an HX-63 that I could restore to working order.

If you've never heard of the HX-63 until just now, don't feel bad. Most professional cryptographers have never heard of it. Yet it was so secure that its invention alarmed William Friedman, one of the greatest cryptanalysts ever and, in the early 1950s, the first chief cryptologist of the U.S. National Security Agency (NSA). After reading a 1957 Hagelin patent (more on that later), Friedman realized that the HX-63, then under development, was, if anything, more secure than the NSA's own KL-7, then considered unbreakable. During the Cold War, the NSA built thousands of KL-7s, which were used by every U.S. military, diplomatic, and intelligence agency from 1952 to 1968.

The reasons for Friedman's anxiety are easy enough to understand. The HX-63 had about 10600 possible key combinations; in modern terms, that's equivalent to a 2,000-bit binary key. For comparison, the Advanced Encryption Standard, which is used today to protect sensitive information in government, banking, and many other sectors, typically uses a 128- or a 256-bit key.

A photo of the inside of the HX-63.In the center of the cast-aluminum base of the HX-63 cipher machine is a precision Swiss-made direct-current gear motor. Also visible is the power supply [lower right] and the function switch [left], which is used to select the operating mode—for example, encryption or decryption.Peter Adams

A photo of a series of rotors with numbers on them.  A total of 12 different rotors are available for the HX-63, of which nine are used at any one time. Current flows into one of 41 gold-plated contacts on the smaller-diameter side of the rotor, through a conductor inside the rotor, out through a gold-plated contact on the other side, and then into the next rotor. The incrementing of each rotor is programmed by setting pins, which are just visible in the horizontal rotor.Peter Adams

Just as worrisome was that CAG was a privately owned Swiss company, selling to any government, business, or individual. At the NSA, Friedman's job was to ensure that the U.S. government had access to the sensitive, encrypted communications of all governments and threats worldwide. But traffic encrypted by the HX-63 would be unbreakable.

Friedman and Hagelin were good friends. During World War II, Friedman had helped make Hagelin a very wealthy man by suggesting changes to one of Hagelin's cipher machines, which paved the way for the U.S. Army to license Hagelin's patents. The resulting machine, the M-209-B, became a workhorse during the war, with some 140,000 units fielded. During the 1950s, Friedman and Hagelin's close relationship led to a series of understandings collectively known as a “gentleman's agreement" between U.S. intelligence and the Swiss company. Hagelin agreed not to sell his most secure machines to countries specified by U.S. intelligence, which also got secret access to Crypto's machines, plans, sales records, and other data.

But in 1963, CAG started to market the HX-63, and Friedman became even more alarmed. He convinced Hagelin not to manufacture the new device, even though the machine had taken more than a decade to design and only about 15 had been built, most of them for the French army. However, 1963 was an interesting year in cryptography. Machine encryption was approaching a crossroads; it was starting to become clear that the future belonged to electronic encipherment. Even a great rotor machine like the HX-63 would soon be obsolete.

That was a challenge for CAG, which had never built an electronic cipher machine. Perhaps partly because of this, in 1966, the relationship among CAG, the NSA, and the CIA went to the next level. That year, the NSA delivered to its Swiss partner an electronic enciphering system that became the basis of a CAG machine called the H-460. Introduced in 1970, the machine was a failure. However, there were bigger changes afoot at CAG: That same year, the CIA and the German Federal Intelligence Service secretly acquired CAG for US $5.75 million. (Also in 1970, Hagelin's son Bo, who was the company's sales manager for the Americas and who had opposed the transaction, died in a car crash near Washington, D.C.)

Although the H-460 was a failure, it was succeeded by a machine called the H-4605, of which thousands were sold. The H-4605 was designed with NSA assistance. To generate random numbers, it used multiple shift registers based on the then-emerging technology of CMOS electronics. These numbers were not true random numbers, which never repeat, but rather pseudorandom numbers, which are generated by a mathematical algorithm from an initial “seed."

This mathematical algorithm was created by the NSA, which could therefore decrypt any messages enciphered by the machine. In common parlance, the machines were “backdoored." This was the start of a new era for CAG. From then on, its electronic machines, such as the HC-500 series, were secretly designed by the NSA, sometimes with the help of corporate partners such as Motorola. This U.S.-Swiss operation was code-named Rubicon. The backdooring of all CAG machines continued until 2018, when the company was liquidated.

Parts of this story emerged in leaks by CAG employees before 2018 and, especially, in a subsequent investigation by the Washington Post and a pair of European broadcasters, Zweites Deutsches Fernsehen, in Germany, and Schweizer Radio und Fernsehen, in Switzerland. The Post's article, published on 11 February 2020, touched off firestorms in the fields of cryptology, information security, and intelligence.

The revelations badly damaged the Swiss reputation for discretion and dependability. They triggered civil and criminal litigation and an investigation by the Swiss government and, just this past May, led to the resignation of the Swiss intelligence chief Jean-Philippe Gaudin, who had fallen out with the defense minister over how the revelations had been handled. In fact, there's an interesting parallel to our modern era, in which backdoors are increasingly common and the FBI and other U.S. intelligence and law-enforcement agencies sporadically tussle with smartphone manufacturers over access to encrypted data on the phones.

Even before these revelations, I was deeply fascinated by the HX-63, the last of the great rotor machines. So I could scarcely believe my good fortune in 2020 when, after years of negotiations, I took possession of an HX-63 for my research for the Association des Réservistes du Chiffre et de la Sécurité de l'Information, a Paris-based professional organization of cryptographers and information-security specialists. This particular unit, different from the one I had seen a decade before, had been untouched since 1963. I immediately began to plan the restoration of this historically resonant machine.

People have been using codes and ciphers to protect sensitive information for a couple of thousand years. The first ciphers were based on hand calculations and tables. In 1467, a mechanical device that became known as the Alberti cipher wheel was introduced. Then, just after World War I, an enormous breakthrough occurred, one of the greatest in cryptographic history: Edward Hebern in the United States, Hugo Koch in the Netherlands, and Arthur Scherbius in Germany, within months of one another, patented electromechanical machines that used rotors to encipher messages. Thus began the era of the rotor machine. Scherbius's machine became the basis for the famous Enigma used by the German military from the 1930s until the end of WW II.

To understand how a rotor machine works, first recall the basic goal of cryptography: substituting each of the letters in a message, called plaintext, with other letters in order to produce an unreadable message, called ciphertext. It's not enough to make the same substitution every time—replacing every F with a Q, for example, and every K with an H. Such a monoalphabetic cipher would be easily solved.

A rotor machine gets around that problem using—you guessed it—rotors. Start with a round disk that's roughly the diameter of a hockey puck, but thinner. On both sides of the disk, spaced evenly around the edge, are 26 metal contacts, each corresponding to a letter of the English alphabet. Inside the disk are wires connecting a contact on one side of the disk to a different one on the other side. The disk is connected electrically to a typewriter-like keyboard. When a user hits a key on the keyboard, say W, electric current flows to the W position on one side of the rotor. The current goes through a wire in the rotor and comes out at another position, say L. However, after that keystroke, the rotor rotates one or more positions. So the next time the user hits the W key, the letter will be encrypted not as L but rather as some other letter.

Though more challenging than simple substitution, such a basic, one-rotor machine would be child's play for a trained cryptanalyst to solve. So rotor machines used multiple rotors. Versions of the Enigma, for example, had either three rotors or four. In operation, each rotor moved at varying intervals with respect to the others: A keystroke could move one rotor or two, or all of them. Operators further complicated the encryption scheme by choosing from an assortment of rotors, each wired differently, to insert in their machine. Military Enigma machines also had a plugboard, which swapped specific pairs of letters both at the keyboard input and at the output lamps.

The rotor-machine era finally ended around 1970, with the advent of electronic and software encryption, although a Soviet rotor machine called Fialka was deployed well into the 1980s.

The HX-63 pushed the envelope of cryptography. For starters it has a bank of nine removable rotors. There's also a “modificator," an array of 41 rotary switches, each with 41 positions, that, like the plugboard on the Enigma, add another layer, an unchanging scramble, to the encryption. The unit I acquired has a cast-aluminum base, a power supply, a motor drive, a mechanical keyboard, and a paper-tape printer designed to display both the input text and either the enciphered or deciphered text. A function-control switch on the base switches among four modes: off, “clear" (test), encryption, and decryption.

In encryption mode, the operator types in the plaintext, and the encrypted message is printed out on the paper tape. Each plaintext letter typed into the keyboard is scrambled according to the many permutations of the rotor bank and modificator to yield the ciphertext letter. In decryption mode, the process is reversed. The user types in the encrypted message, and both the original and decrypted message are printed, character by character and side by side, on the paper tape.

A photo of the paper tape inside the HX-63 machine.While encrypting or decrypting a message, the HX-63 prints both the original and the encrypted message on paper tape. The blue wheels are made of an absorbent foam that soaks up ink and applies it to the embossed print wheels.Peter Adams

A photo of the rotors and the keys numbered 1-9 below it.  Beneath the nine rotors on the HX-63 are nine keys that unlock each rotor to set the initial rotor position before starting a message. That initial position is an important component of the cryptographic key.Peter Adams

To begin encrypting a message, you select nine rotors (out of 12) and set up the rotor pins that determine the stepping motion of the rotors relative to one another. Then you place the rotors in the machine in a specific order from right to left, and set each rotor in a specific starting position. Finally, you set each of the 41 modificator switches to a previously determined position. To decrypt the message, those same rotors and settings, along with those of the modificator, must be re-created in the receiver's identical machine. All of these positions, wirings, and settings of the rotors and of the modificator are collectively known as the key.

The HX-63 includes, in addition to the hand crank, a nickel-cadmium battery to run the rotor circuit and printer if no mains power is available. A 12-volt DC linear power supply runs the motor and printer and charges the battery. The precision 12-volt motor runs continuously, driving the rotors and the printer shaft through a reduction gear and a clutch. Pressing a key on the keyboard releases a mechanical stop, so the gear drive propels the machine through a single cycle, turning the shaft, which advances the rotors and prints a character.

The printer has two embossed alphabet wheels, which rotate on each keystroke and are stopped at the desired letter by four solenoids and ratchet mechanisms. Fed by output from the rotor bank and keyboard, mechanical shaft encoders sense the position of the alphabet printing wheels and stop the rotation at the required letter. Each alphabet wheel has its own encoder. One set prints the input on the left half of the paper tape; the other prints the output on the right side of the tape. After an alphabet wheel is stopped, a cam releases a print hammer, which strikes the paper tape against the embossed letter. At the last step the motor advances the paper tape, completing the cycle, and the machine is ready for the next letter.

As I began restoring the HX-63, I quickly realized the scope of the challenge. The plastic gears and rubber parts had deteriorated, to the point where the mechanical stress of motor-driven operation could easily destroy them. Replacement parts don't exist, so I had to build such parts myself.

After cleaning and lubricating the machine, I struck a few keys on the keyboard. I was delighted to see that all nine cipher rotors turned and the machine printed a few characters on the paper tape. But the printout was intermittently blank and distorted. I replaced the corroded nickel-cadmium battery and rewired the power transformer, then gradually applied AC power. To my amazement, the motor, rotors, and the printer worked for a few keystrokes. But suddenly there was a crash of gnashing gears, and broken plastic bits flew out of the machine. Printing stopped altogether, and my heartbeat nearly did too.

I decided to disassemble the HX-63 into modules: The rotor bank lifted off, then the printer. The base contains the keyboard, power supply, and controls. Deep inside the printer were four plastic “snubbers," which cushion and position the levers that stop the ratchet wheels at the indicated letter. These snubbers had disintegrated. Also, the foam disks that ink the alphabet wheels were decomposing, and gooey bits were clogging the alphabet wheels.

I made some happy, serendipitous finds. To rebuild the broken printer parts, I needed a dense rubber tube. I discovered that a widely available neoprene vacuum hose worked perfectly. Using a drill press and a steel rod as a mandrel, I cut the hose into precise, 10-millimeter sections. But the space deep within the printer, where the plastic snubbers are supposed to be, was blocked by many shafts and levers, which seemed too risky to remove and replace. So I used right-angle long-nosed pliers and dental tools to maneuver the new snubbers under the mechanism. After hours of deft surgery, I managed to install the snubbers.

The ink wheels were made of an unusual porous foam. I tested many replacement materials, settling finally on a dense blue foam cylinder. Alas, it had a smooth, closed-cell surface that would not absorb ink, so I abraded the surface with rough sandpaper.

After a few more such fixes, I faced just one more snafu: a bad paper-tape jam. I had loaded a new roll of paper tape, but I did not realize that this roll had a slightly smaller core. The tape seized, tore, and jammed under the alphabet wheels, deeply buried and inaccessible. I was stymied—but then made a wonderful discovery. The HX-63 came with thin stainless-steel strips with serrated edges designed specifically to extract jammed paper tape. I finally cleared the jam, and the restoration was complete.

One of the reasons why the HX-63 was so fiendishly secure was a technique called reinjection, which increased its security exponentially. Rotors typically have a position for each letter of the alphabet they're designed to encrypt. So a typical rotor for English would have 26 positions. But the HX-63's rotors have 41 positions. That's because reinjection (also called reentry) uses extra circuit paths beyond those for the letters of the alphabet. In the HX-63, there are 15 additional paths.

Here's how reinjection worked in the HX-63. In encryption mode, current travels in one direction through all the rotors, each introducing a unique permutation. After exiting the last rotor, the current loops back through that same rotor to travel back through all the rotors in the opposite direction. However, as the current travels back through the rotors, it follows a different route, through the 15 additional circuit paths set aside for this purpose. The exact path depends not only on the wiring of the rotors but also on the positions of the 41 modificators. So the total number of possible circuit configurations is 26! x 15!, which equals about 5.2 x 10 38. And each of the nine rotors' internal connections can be rewired in 26! different ways. In addition, the incrementing of the rotors is controlled by a series of 41 mechanical pins. Put it all together and the total number of different key combinations is around 10600.

Such a complex cipher was not only unbreakable in the 1960s, it would be extremely difficult to crack even today. Reinjection was first used on the NSA's KL-7 rotor machine. The technique was invented during WW II by Albert W. Small, at the U.S. Army's Signal Intelligence Service. It was the subject of a secret patent that Small filed in 1944 and that was finally granted in 1961 (No. 2,984,700).

Meanwhile, in 1953, Hagelin applied for a U.S. patent for the technique, which he intended to use in what became the HX-63. Perhaps surprisingly, given that the technique was already the subject of a patent application by Small, Hagelin was granted his patent in 1957 (No. 2,802,047). Friedman, for his part, had been alarmed all along by Hagelin's use of reinjection, because the technique had been used in a whole series of vitally important U.S. cipher machines, and because it was a great threat to the NSA's ability to listen to government and military message traffic at will.

The series of meetings between Friedman and Hagelin that resulted in the cancellation of the HX-63 was mentioned in a 1977 biography of Friedman, The Man Who Broke Purple, by Ronald Clark, and it was further detailed in 2014 through a disclosure by the NSA's William F. Friedman Collection.

A man stading front of shelves filled with electronic devices.After a career as an electrical engineer and inventor, author Jon D. Paul now researches, writes, and lectures on the history of digital technology, especially encryption. In the 1970s he began collecting vintage electronic instruments, such as the Tektronix oscilloscopes and Hewlett-Packard spectrum analyzers seen here. Peter Adams

The revelation of Crypto AG's secret deals with U.S. intelligence may have caused a bitter scandal, but viewed from another angle, Rubicon was also one of the most successful espionage operations in history—and a forerunner of modern backdoors. Nowadays, it's not just intelligence agencies that are exploiting backdoors and eavesdropping on “secure" messages and transactions. Windows 10's “telemetry" function continuously monitors a user's activity and data. Nor are Apple Macs safe. Malware that allowed attackers to take control of a Mac has circulated from time to time; a notable example was Backdoor.MAC.Eleanor, around 2016. And in late 2020, the cybersecurity company FireEye disclosed that malware had opened up a backdoor in the SolarWinds Orion platform, used in supply-chain and government servers. The malware, called SUNBURST, was the first of a series of malware attacks on Orion. The full extent of the damage is still unknown.

The HX-63 machine I restored now works about as well as it did in 1963. I have yet to tire of the teletype-like motor sound and the clack-clack of the keyboard. Although I never realized my adolescent dream of being a secret agent, I am delighted by this little glimmer of that long-ago, glamorous world.

And there's even a postscript. I recently discovered that my contact at Crypto AG, whom I'll call “C," was also a security officer at the Swiss intelligence agencies. And so for decades, while working at the top levels of Crypto AG, “C" was a back channel to the CIA and Swiss intelligence agencies, and even had a CIA code name. My wry old Swiss friend had known everything all along!

This article appears in the September 2021 print issue as “The Last Rotor Machine."

To Probe Further

The Crypto AG affair was described in a pair of Swedish books. One of them was Borisprojektet : århundradets största spionkupp : NSA och ett svensk snille lurade en hel värld [translation: The Boris Project: The Biggest Spy Coup of the Century: NSA and a Swedish genius cheated an entire world], 2016, Sixten Svensson, Vaktelförlag, ISBN 978-91-982180-8-4.

Also, in 2020, Swiss editor and author Res Strehle published Verschlüsselt: Der Fall Hans Bühler [translation: Encrypted: The Hans Bühler Case], and later Operation Crypto. Die Schweiz im Dienst von CIA und BND [Operation Crypto: Switzerland in the Service of the CIA and BND].

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The classic 807 beam tetrode valve is unusual in still being designated as current production in the 1960s and having a five pin base.
The parameters given are for single ended use. When used in a class AB1 as a push pull pair with 600 volts on the anode an output power of 65 Watts was normal.
These valves were much used in medium power amateur radio transmitters for high level modulation. In this use the output transformer, called a modulation transformer, would be impedance matched to the anode circuit of the RF power valve. The audio power would increase or decrease the amplitude of the RF envelope.
The 807 was also a workhorse of the HF bands. As a transmitter power amplifier (shown in the lower table) operating in class C producing 40 Watts of RF, it would need a pair of 807's in push pull to generate the 20 Watts of audio required to fully modulate the valve. Other modulation systems operating on the screen grid would require much less audio drive but with less RF output.
The classic envelope has a maximum diameter of 47 mm and a length, excluding the UX5 base pins, to the tip of the top cap of 128 mm.
References: Data-sheet & 1040. Type 807 was first introduced in 1938.


Pin Connections


Absolute Maximum Operating Conditions
Beam Tetrode


Absolute Maximum Operating Conditions
Beam Tetrode
Thanks to Frank Philipse for supplying the above PDF datasheet.
Updated April 03, 2015.
shape:classic construction:pinch type:beam.tetrode age:1930.1940 base:ux5 heater:6.3v pins:5 pin:1.h pin:2.g2 pin:3.g1 pin:4.k pin:5.h pin:tc.a
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807 (1625) Vacuum Tube Audio Amplifier With Triad Output Transformer \u0026 AM Transmitter

6L6 Tube / Valve: 6L6G & 807

- the 6L6 valve or tube represented a milestone in vacuum tube technology along with its sibling the 807 used in many radio transmitters.

History of the Valve / Tube Includes:
History overview     Early discoveries     Fleming's oscillation valve     de Forest's Audion     Development of the basic idea     R-type valve     6L6 valve    

The 6L6 vacuum tube or thermionic valve was one of the most iconic valves ever produced.

The 6L6 first appeared before the Second World War, and was widely used in many applications, especially in audio amplifier, finding particular favour in guitar amplifiers.

A development of the 6L6 was the 807 - a valve that took the basic 6L6 design, but used a top cap connection for the anode to enable it to be used in RF power applications.

6L6 history & introduction

The 6L6 valve design was undertaken in the mid-1930s. At this time Phillips had introduced its pentode valves and patented the idea. As the performance of triodes was far exceeded by that of tetrode and pentode valves, RCA developed and introduced the 6L6. The 6L6 was a beam tetrode and by adopting this format, it enabled them to circumvent the pentode concept.

Image of 6L6 and its derivative the 807 valve / tube
6L6 & 807 Valves / Tubes

Up until the early 1930s all valves that were manufactured were triodes, i.e. they had a cathode, grid and anode. It was discovered that additional grids could be added to significantly enhance the performance of the basic triode and therefore valve manufacturing companies sought to offer the best performance by introducing valve designs with additional grids.

The disadvantage of the tetrode was that it used an additional grid with a relatively high potential on it to attract electrons away from the cathode area towards the anode. The downside of this was that the electrons gained sufficient energy that they often bounced off the anode and this gave rise to a negative resistance kink in the characteristic and this lead to instability in the circuit. This effect was known as secondary emission.

The 6L6 valve incorporated a number of new elements into the design:

  • Critical distance:   One of the concepts introduced into the 6L6 valve was that of the critical distance. A British engineer named J. Owen Harries is believed to have discovered that there was a critical distance between the anode and screen-grid in a valve. At this distance the efficiency of a power tetrode was maximised and the secondary emission from the anode was also reduced.
  • Beam tetrode:   The other main concept used within the 6L6 valve was that of the beam tetrode technique. In this, the concept behind, what was often termed the Harries valve, was further refined by EMI engineers Cabot Bull and Sidney Rodda. They used a pair of beam plates, connected to the cathode, which directed the electron streams into two narrow areas on the anode. This also acted like a suppressor grid to redirect some secondary electrons back to the anode reducing secondary emission and eliminating the kink in the tetrode characteristic curve. The Marconi Osram Valve Company, MOV marketed valves using this technology under the family designator, KT indicating the term 'kinkless tetrode.'
Image of 6L6 beam tetrode with a glass envelope
6L6 beam tetrode valve

The launch of the 6L6 is believed to be the first commercial production beam tetrode when it was launched in 1936. Although MOV had made a pre-production batch of their KT40 at least a year earlier it had not been a success and full production was not started and the design was abandoned. Apparently the company said it had been too difficult to assemble.

The 6L6 valve rapidly accelerated the electrons and it used very short electron paths to provide its high level of performance. However it still possessed a small kink in its characteristic and this mean that it did not provide the highest audio fidelity at high power levels.

Once the concept of the beam tetrode had been provide to work within a production environment by the 6L6, MOV re-engineered their designs to produce the KT66. This was very similar to the 6L6 valve but had a larger cathode and shorter and fatter anode. In addition to this, electrons were not accelerated as quickly and the paths were longer. This resulted in there being almost no kink in the curve. Accordingly the KT66 was the preferred choice for audio enthusiasts.

6L6 variants

The original 6L6 valve was contained within a black metal tube or casing. Later the 6L6G had an all glass envelope - this improved the radiation cooling of the anode.

Other versions of the 6L6 valve included the 6L6G, 6L6GA, 6L6GB, and the final version which was the 6L6GC. All these had glass envelopes.

807 beam tetrode valve developed

The 6L6 was particularly liked for audio applications, but it was not suitable for high power RF. By bringing the anode out through a top cap in the glass envelope. This overcame the issue of the 6L6 where high transient voltages on the anode when operating in class C could cause a flashover between pins 2 and 3 on the octal base.

Image of an 807 valve normally used for transmitting applications - note the anode top cap connection
An 807 - note top cap for anode connection

The 807 beam tetrode was used in many medium power transmitters. It was widely used in the Second World War. The British Army No 19 Set used one 807 valve in the final RF power amplifier stage, and the add on linear power amplifier used two 807s.

Image of military version of the 807 designated the ATS25 - note the ceramic base
ATS25 - military version of 807

6L6 specifications & parameters

6L6 Characteristics & Parameters
Heater voltage Vh 6.3V
Heater current Ah 900mA
Max anode voltage Va 350V (500V for 6L6GC)
Anode dissipation 30W
Screen voltage, Vs 250V
Anode resistance, ra 33kΩ
Mutual conductance, gm; 5.2

6L6 Valve Base Connections
Pin Number Connection
1Not connected
2Heater, h
3Anode, a
4Screen grid, g2
5Control grid, g1
6Not connected
7Heater, h
8Cathode, k & beam forming plates

NB: the 6L6 uses an octal valve holder / base.

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Radio history timeline     History of the radio     Ham radio history     Coherer     Crystal radio     Magnetic detector     Spark transmitter     Morse telegraph     Valve / tube history     PN junction diode invention     Transistor     Integrated circuit     Quartz crystals     Classic radios    
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Vacuum tubes 807


807 (1625) Vacuum Tube Audio Amplifier With Triad Output Transformer \u0026 AM Transmitter


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