Here are the specs I follow in my projects, from the 1940 ARRL Radio Amateur's Handbook:

Click on the chart to enlarge it.

This is information you will not find concisely and authoritatively anywhere else on the net. The 6L6 is designed for audio use. It is not a true pentode; it is a beam power tetrode, with special beam forming elements. Many amateur radio operators used the 6L6 in class C service for transmitters over the years because it was inexpensive due to mass production, and it could be salvaged from scrapped TV or audio equipment. Almost all of the tube data in commonly available receiving tube handbooks is for audio use. It included service from single ended class A to push pull class AB2, but it did not have the specs for class C RF operation. As a result, many of the handbook designs did not take full advantage of the 6L6 in RF class C operation.

The chart I provide at the beginning of this article shows a 6L6GX, which is likely unavailable as NOS these days. I have found the Sovtek 6L6WXT+ a sturdy version which seems to work in my gear using my table data. While it does not have the RF low loss base, it is very rugged. Use it in the Heathkit AT-1 or a home brew rig. It also makes a great modulator tube for medium power rigs like the Globe Scout and Multi Elmac AF-67.

Compare the Sovtek specs to the 6L6GX here:  https://www.newsensor.com/pdf/sovtek/6l6wxt-sovtek.pdf
The tube chart base shows it as a true pentode. That ain't so. It is a beam power tetrode. It does have impressive specs though.
Buy it here: https://www.thetubestore.com/sovtek-6l6wxt


Here are a few of the sources currently on the web that DO have some information about class C operation:

• http://n4trb.com/AmateurRadio/RCA_Ham_Tips/issues/rcahamtips0603.pdf
  This page is dated December 1946, an early source. The power output given for a standard 6L6 here (28 W) is slightly less then the 6L6GX (30 W) in the table above. The 6L6 specs listed here are:
  • Max Plate Voltage: 400 V
  • Max Screen Voltage: 300 V
  • Grid bias: -125 V
  • Max Plate Current: 100 mA
  • Max Screen Current: 12 mA
  • Max Grid Current: 5 mA with 100K resistor
  • Max Plate Dissipation: 21 Watts
  • Max Screen Dissipation: 3.5 Watts
  • Max Power Out: 28 Watts, based on 70% efficiency, the most optimistic figure for Class C operation (I settle for 65% Max)
  • Max Frequency for full power: 10 MHz (reduce power above this frequency)
  • Maximum Ratings are absolute maximums not to be exceeded under any condition of operation
  • Grid bias calculator mU factor (approx): 8
  • Cgp: 0.4 pF
  • Cin: 10 pF
  • Cout: 12 pF
  • Specs for the 6L6's little brothers, the 6V6GT and 6AQ5 are included in this article. The 6V6 is only good for 11 Watts up to 10 MHz, like the 6L6. Surprisingly, the 6AQ5 is good for 54 MHz and 11 Watts Output. This suggests the 6AQ5 RF driver use in the Viking II is a pretty good choice. The chart also lists the 6AG7 as good for 7.5 Watts out up to 10 MHz. This page is a good discussion of the new "Ham" ratings or ICAS, specifically with precautions for pentodes and tetrodes, which will not take the abuse triodes can shake off. Screen dissipation is an important consideration, ignored in the Valiant, for instance. None of these tubes show color under safe operating conditions, even ICAS ratings. Properly operated 811s do not show color either, although even modern manufacturers do not observe that in the horsepower race. Keep in mind that you have to raise the power 4 times to get an S unit of change. Running tubes above ICAS ratings is not logical.
• https://www.w8ji.com/designing_ham_transmitter.htm
    W8JI, always an authoritative source, points out the problems inherent in using a single resistor from the plate supply for screen grid voltage. This deficiency disturbs the grid biasing problem, and is a must read for anyone using the 6L6 in class C for an RF amplifier. This may be necessary in AM Plate Modulated service, but in CW you can benefit from feeding the screen from a separate lower voltage supply.
• https://tdsl.duncanamps.com/show.php?des=6l6
    Duncan Amps is a generally reliable resource that does include some class RF and C specs. But it needs some explanation. The RF, C line is probably correct except for the power of 50 watts, which is way too high. Similarly the AM class C. The problem is evident in the Va or anode voltage of 750 or 600 volts, clearly extrapolated from 807 specs. The base and socket of the 6L6 will flash over at anything in excess of 450. There was a 6L6GX with a low loss base, which could maybe be pushed to 500 V with a ceramic socket, and supposedly had better RF performance due to shorter internal leads. However, those were only produced during WW2 and were quickly supplanted by the 807 which used a plate cap to overcome the flashover problem and provided some shielding in the base to improve RF stability. The Duncan Amps RF, AB1 spec is presumably for two tubes. It is unrealistically high power output and anode voltage. This spec appears to be extrapolated from the 807. The 6L6 is not a "junior 807". It is the grandpa. The Duncan Amps B spec is a configuration of a pair of 807s, driving the g2 or screen and tying it to the g1 control grid via a 20K resistor to turn it into a triode. The OUTPUT of 120 watts for even a PAIR is too ambitious for a 6L6. Further, this configuration is common for an 807 in class B audio service. Whether that will work in RF applications is not well documented in contemporary literature. I would advise extreme caution employing any of the data on this particular page using plate voltage above 500 Volts. Further, a triode connection will definitely require neutralization at ANY RF frequency. To verify my conclusions on the maximum ratings for a 6L6, you only need to refer to the top of the Duncan Amps page where it rates maximum Va plate voltage at 360 VDC. I prefer to operate tubes within their manufacturer's ratings for safety and reasonable lifespan.
• http://www.nj7p.org/Tubes/SQL/Tube_query.php?Type=6L6
    NJ7P provides manufacturer's data from RCA RC-13 manual dated 1937. This information is reliable and uses Va just below 400 VDC. The expected output power is 17 W for a class C CW rig, and 11 W for a class C AM rig using a 6L6. The later versions of the 6L6 had more plate dissipation and could be pushed a little harder. Remember that you have to double the power output to move the other guy's S meter half an S unit. Shortening the life of your output tube hardly seems worth using it outside its specs, given the cost of the tubes these days.

Another problem with the 6L6 is that it is designed as an AUDIO tube. The common varieties use inferior insulation on RF in the base. The leads from the tube elements to the base are long enough to cause instability, which was cured in the 807 to some extent by internal shielding, and mounting the tube socket below the chassis one inch or so. Most competent 6L6 rigs of the 1930 - 1940 era used a variety of tricks to make the 6L6 work on any band above 80 meters. One way was to connect it as a triode and neutralize it; but that resulted in lower power output and required more drive. Another way was to use the standard grid drive, tetrode mode and use it only as a doubler on frequencies above 80 meters; this was used in the famous Heathkit AT-1 and its cousins, and it also resulted in less output. The AT-1 struggles to make 12 Watts output, even with the matching antenna tuner. Another way to get around the 6L6 instability issues was to use it in grounded grid, which got good power efficiency, but had high complexity in interstage coupling networks employing plug in coils. W6SAI Bill Orr, in one edition of the Radio Handbook, manages to get a 6L6 RF amplifier to work on 6 Meters, probably just to prove he could. The rest of us probably do not possess the engineering credentials or tenacity of Bill Orr. He looks great in the cape and leotards too.

In the end, a 6L6 rig is a great nostaligia project, but an 807 is a better choice from an engineering standpoint, based on RF stability issues. It made sense if you were building a Novice rig for 80 and 40 meters, today even 30 meters. You could build an 807 rig for 160 through 15 meters using two tubes and no neutralization, with the convenience of band switching. The Johnson Adventurer and its clones are good examples. The 5933 or stubby 807W has shorter internal leads, and probably better stability at higher frequencies. However, its larger base will not fit some commercial transmitters; the sockets are mounted below the chassis to provide some shielding of the grid leads from the plate circuitry usually found on the top of the chassis near the plate cap. This sub chassis mounting also provided some cooling air around the glass envelope. The 5933 will not fit through the chassis hole. The 6146 family is similar in construction, and an improved design. Do not use the 5933 at the 6146 specification levels.

That having been said, there is one 6L6 transmitter project I have been looking at for some time. It is in the 1958 ARRL handbook on page 180. You can also download the January 1957 QST if you are an ARRL member, to see it. The final 6L6 amplifier runs straight through, not frequency doubling, on all bands. It uses a 6AG7 oscillator (can be modified for VFO) and parallel 6L6s running 75 watts input on 80, 40, 20, and 15 meters. It can be built for CW as well as AM operation. It uses a pi network output and neutralization. The grid current for a pair of 6L6s is 6 mA. The Plate voltage is 400 V. The maximum cathode current for CW is 230 mA. The screen voltage runs around 200 V. For AM an appropriate reduction in power is required. You probably should dial it back some on 20 and 15 meters too. If you are considering building a 6L6 rig, I would suggest you give this one a try. Even though this article is published in 1957, the techniques it uses were known to better builders before 1950, so it should qualify for the AWA Linc Cundall contest.

One more thing: the 5933 is NOT a KT66. The KT series is completely different, a special audio design family (KT88 etc) that is not an exact branch from the 807 family. The "KT" stands for Kinkless Tetrode. This refers to a tube characteristic graph which shows the screen current characteristics as the plate voltage swings at low voltages from a push pull audio transformer. This KT66 design trick reduced distortion and instability in audio amplifiers. The 807 family, including the 5933 and the 6146 does not have the kinkless feature, and screen dissipation specs may be exceeded if you try to use them in circuits designed for a KT66. Do not consider a swap of KT66 for 807 or 5933 in either direction. Once this kind of misinformation propagates on the net, it is nearly impossible to exterminate. For a discussion of the heritage of the 6L6 and KT series see: https://en.wikipedia.org/wiki/6L6 For historical information on the 807, see: https://en.wikipedia.org/wiki/807_(vacuum_tube)

There are some necessary tricks with the 807 to stabilize it as an RF amplifier. This involves the usual Plate parasitic choke made from a parallel 50 to 100 ohm 2 watt resistor and a few turns of wire. You may also find a 47 to 100 ohm resistor in the control grid is helpful, even in audio service; I have seen 1K resistors at that location in AF service as modulators. In addition, you often find a 68 ohm resistor at the screen grid. Think of this screen resistor as a "positive resistor" to cancel out the "negative resistance" caused by the "kink" in the screen characteristic curve. A "negative resistance" can be used to build an oscillator, as Heathkit does in their Tunnel Diode Grid Dip Oscillator. See this for an explanation: https://www.nostalgickitscentral.com/heath/73-index/articles/Tunnel%20Dipper-12-64.pdf Keep the leads on all these parasitic or self oscillation circuits as short as possible. The handbooks and popular literature featuring better designs (The Radio Handbook by W6SAI) should give you guidance. I have nearly a full collection. You should have at least one Radio Handbook from the dates of 1959 to 1969, to obtain true enlightenment.

I may provide some links to those techniques later. I will not burden you with a bunch of thetas and betas. The 6146 also has this property, and occasionally needs similar help. I have seen rigs using two 6L6s, multiple 807s, and multiple 6146s, such as the Valiant. The 1958 ARRL handbook mentioned earlier shows a CW/AM rig with two 6L6s and neutralization. Multiple tubes require more challenging parasitic suppression. The 1625, a 12 volt filament variant of the 807, was routinely used on 80 meters by emerging SSB operators in combinations of half a dozen or more tubes in an amplifier, because it was so cheap as surplus. My opinion is that if you need more than two output tubes, you need to pick a different tube. But that opinion is shared by other crusty old buzzards, based on experience.

This discussion would not be complete without a discussion of a proper tube socket. If you are using the Sovtek tube at the 500 V rating, a ceramic socket is a non negotiable essential. This also gives lower loss at RF. Use a standard style with ears and mounting screws, not the kind that has no ears and mounts through a single large hole with a snap ring. Or if you are lucky enough to find a larger Johnson ceramic socket, use that, but sub mount it on standoffs long enough to provide sufficient arc distance from the button rivets on the socket to the metal chassis. Also, use shrink sleeve on both the screen and plate contacts, covering all the socket pin and the wire up to 1 inch. This avoids the type of arcing that is commonly seen in the Heathkit Apache on the base of the 6CA7 modulator tubes (which run at 700 VDC). You can also use high voltage dope similar to the now obsolete Glyptal. I have employed these techniques when repairing guitar amplifiers to correct that flaw. Once a plastic or bakelite socket has carbon tracks on it, it must be replaced, preferably with a ceramic socket. In severe cases, I have installed short shrink sleeve "leggings" on the Apache 6CA7 tube pins, with glyptal paint around them on the phenolic tube base; these leggings have to be short enough to allow fully seating the tube in the socket. This trick will not work on 6CA7s that already have carbon tracks from arcing; I discovered this when working on guitar amps with 6L6s. I have seen Apaches changed to 6146 modulators (similar to the B&W 5100) that solved this problem. The 6CA7 does a nice job putting out 35 watts per channel in the Dynaco stereo 70 amp; pushing the 6CA7 to 700 V and 60 Watts output like the Apache does is not so smart. I admit that one Sylvania tube handbook gives data for the 6CA7 at those levels, but the practical experience shows the problems that result. I speculate that this arcing also contributes to Modulation transformer failure in the Apache. This is what happens when people do not read specs and ignore "Maximum Ratings". Modern education seems to be more IC related, if it even discusses discrete devices or hardware at all any more. Despite what you may have learned in college, read the specs, and be aware that a SPICE model does not necessarily tell you everything you need to know because its only as good as the modeling assumptions. You could learn a lot from a "dummy", to paraphrase a seat belt commercial. https://www.adcouncil.org/Our-Campaigns/The-Classics/Safety-Belt-Education

73, Vince, Larry, and Janis AB2RA

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