Tuesday, July 28, 2009

The 6L6 Beam Power Tetrode: An All-American Tube?

Made famous for its use as the output tube of the famed 1965 Fender Twin Reverb guitar amp, does the 6L6 beam tetrode really claim the fame as an all-American tube?

By: Ringo Bones

As the guitar amp that many electric guitar players view as the “soul-mate” of Leo Fender’s late 1950s iconic invention – the single-coil pickup equipped Fender Stratocaster – the 1965 Fender Twin Reverb guitar amplifier has over the years managed to inexorably sneak itself into posterity. But given the venerable guitar amp’s claim to fame of being as American as apple pie and baseball, does the output power tubes that made the 1965 Fender Twin possible – namely the 6L6 beam tetrode power tube – really claim the fame as an all-American tube? But first, let us examine this remarkable vacuum tube’s – or valve as they say in merry old England – history.

A guy called Schottky originally invented the screen grid-type tubes / valves called tetrodes while working for Siemens of Germany during the 1930s. Schottky’s work with tetrode-type tubes eventually lead to the creation of the 6L6 beam power tetrode and the KT66 tube. The 6L6GC tube was first introduced in1936 for a new generation of audio power amplifiers that produced high gain and lower total harmonic distortion with the help of the newly discovered principle of negative feedback. Amplifiers using the 6L6 tube became so popular over the years that it was eventually used in the 1965 Fender twin Reverb guitar amplifier that produces an 85-watt output using an all-tube driver stage. Often equipped with 12AX7 pre-amp tubes for the input stage and phase splitting duties and a12AT7 pre-amp tubes to drive the spring reverb section of the guitar amp.

As major American consumer electronic manufacturing firms made the shift from using vacuum tubes to solid state components like transistors and integrated circuits or ICs - which were reaching the point of becoming cheaper to manufacture than their thermionic counterparts. American audiophiles and electric guitar players who are still reliant on tube-based gear were left high and dry when it comes to acquiring replacement tubes. Some went to buying new old stock (NOS) US manufactured tubes, only to find out that these items slowly crept up in price as the years went by despite of their reliability and wide availability. Especially during the 1980s where everyone in the US seems to be starting their very own Heavy metal band, and you know how this kind of music is very reliant on tube-based electric guitar amplifiers when it comes to getting a good tone and feel.

Eastern Block countries during the eve of the fall of the Iron Curtain became a very important source of vacuum tubes for electric guitar and PA amplifier use almost overnight. Unfortunately, Eastern Block tube manufacturers – especially in China and Yugoslavia – still have a very steep learning curve to conquer in order to produce vacuum tubes that would equal the reliability and sound quality of American made tubes. It was only during the mid 1990s that Russian made tubes finally equaled their American NOS counterparts and China – and other Eastern European countries – had to wait till the very late 1990s or early 21st Century for their tubes to equal American made ones.

Those of us who don’t have more money than our common sense and/or were born a few years after Neil Armstrong and Buzz Aldrin walked on the Moon. It will be very unlikely that we’ll be using ultra-expensive American NOS versions of the 6L6 beam power tetrode tubes on our 1965 Fender Twin Reverb guitar amp. It is more likely – probably 99.9% - that we’ll be using Russian Edicron 6L6GC tubes if we want for our beloved 1965 Fender Twin to sound its best. Though a Chinese made Golden Dragon 6L6 will provide a little more power if we tweak the power amp circuit so that the cathode resistor voltage readings of the 6L6 tubes will rise to about 35 volts or so with reference to ground.

So is the 6L6 beam power tetrode tube an all American tube especially when used as the output tubes of an all American guitar amp like the 1965 Fender Twin Reverb? Well, it probably depends on how you define what constitutes being American anyway. Given that the American culture has been further enriched each time Americans indulge in the niceties of newly arrived ethnic groups then yes the 6L6 tube does pass muster as an all American tube. Whether it is made in post-Soviet era Russia or the People’s Republic of China. Especially the Chinese made ones, which fortunately haven’t managed to self-destruct during the Free Tibet concert or the Chinese made 6L6 tubes on guitar amps played in a Jetsun Milarepa fundraising. This tube is as American as apple pie, baseball or just “Rockin’ in the Free World”.


  1. From a design , development and application standpoint, 6L6 Beam Power Tetrode tubes / valves are indeed all-American. Considering that the development of the 1965 Fender Twin Reverb guitar amplifier involved - for all intents and purposes - true blue Made in the USA 6L6 Beam Tetrode power tubes / valves made by either general Electric or Sylvania.
    During most of the 1980s and the early 1990s, there was a trend of making tube-based guitar amps with the highest power output possible. I even remember reading from an 1992-era Guitar World and Guitar Player magazine article about newly-hired Iron Maiden guitarist Jannick Gers being unable to make his 230-watt Marshall rig more powerful despite having it "tweaked" by his favorite Marshall shop.
    By 1999 onwards, the trend shifted to tonality as noted by the introduction of Black Face era-style Fender Champ clones using single-ended 6V6 tube output stage.
    Sadly in my opinion, the 1965 Fender Twin fell somewhere in between even though in versatility terms I do think that this guitar amp has no equal.
    P.S. You should research more on Schottky's work on tetrode type tubes / valves back in the 1930s.

  2. It was Dr. Walter H. Schottky who worked at Siemens &Halske GMBH in Germany during World War I that led to the development of the Beam Tetrode type tubes / valves. China and Russia are now the leading provider of 6L6 tubes.

  3. As a guitarist and a hi-fi enthusiasts, here's my two-cents-worth of wisdom on the topic of the venerable 6L6 beam tetrode. a Sovtek 5881WXT can be used as a substitute provided you take precautions against parasitic oscillation.
    The Sylvania 6L6GC are famed as the best American NOS version of the 6L6 beam tetrode in electric guitar applications due to it's sweetness of tone. Though the Sylvania's microphonics can be a problem in comparison to lower-priced Svetlana and Sovtek 6L6 reissues. Has Electro-Harmonix stsrted making their own 6L6's?
    Sovtek 5881WXT can be a viable replacement for 6L6 beam tetrodes in most power amp applications, though tone-wise, Sovtek 5881WXT beam tetrodes has a tone (drynessof tone?) more suited to hi-fi amps than electric guitar amps like the famed 1965 Fender Twin Reverb or other similar reissues.

  4. Which is the correct spelling of the famed Iron Maiden guitarist in the No Prayer for the Dying album, is it Janick or Jannick? IN the Bruce Dickinson solo album (inspired by Prof. Richard Dawkins?) Accident of Birth, it is spelled Jannick while on Wikipedia and other guitar related sites, it is spelled Janick (one "n" only).
    The 1965 Fender Twin is probably one of the best guitar amps of all time... for alternative rock musicians!

  5. When it comes to electric guitarists preference to their chosen output power tube and electric guitar / pick up combination, here's a few generalizations that became established over the years:
    1) Heavy Metal guitarists prefer 6550 tubes on their amps,given that it can be swapped with an EL34 without any mods.
    2) Classic Rockers prefer EL34 and the humbucking pick up of a Gibson Les Paul for that boosted mid sound.
    3) Indie rockers prefer EL84 (expensive NOS Mullards?) for uniqueness of tone.
    4) Classic Rockers who prefer the single coil ping of a Fender Strat often use 6L6 beam tetrodes - especially NOS American made ones for that scooped in the mids voiced sound.
    5) Seasoned Blues and Jazz players with P-90 and older humbucker equipped guitars also gravitate to American NOS 6L6 tubes.
    Though Indie Rockers tend tomix and match.

  6. 6L6 Beam Tetrodes have gained widespread use in the electric guitar player's world thanks to the iconic 1965 Fender Twin. The 6L6 has a scooped / depressed mid sound, like the tube is equipped with a very transparent sounding Loudness control. The use of 6L6 tubes in hi-fi is quite rare though, the only amplifier that I know using this tube is the CR Development's Romulus Integrated Amplifier.
    The 6L6 tube is the preferred choice of single-coil pick up users like the Fender Stratocaster, although I never heard a humbucker-equipped Gibson Les Paul played through a 6L6-equipped 1965 Fender Twin amp, the sonic results could be interesting.
    EL84 tubes - especially those produced by Mullard - are probably the best tubes I've heard so far. Too bad they are now about impossible to get hold of. Speaking of the 6550 tubes used by Heavy Metal guitarists, I think this tube - after hearing it in action on Audio Research equipped ultra powerful and ultra expensive tube amps in hi-fi shows is too dry for my tastes when it comes to electric guitar amplification use.

  7. What is this Professor Richard Dawkins inspiring Iron Maiden's Bruce Dickinson to compose his best solo effort called Accident of Birth? I do like the album and the Oxford professors views. Did Professor Dawkins also inspired Ned's Atomic Dustbin to write the song "The Selfish Gene"?
    Speaking of the 6L6 Beam Tetrode, it is probably the soulmate of the iconic 1965 Fender Twin.

  8. Have you ever wondered why GE 6L6 tubes with reinforced anodes specially built for the US Air Force back in the 1950s are way better than GE's standard consumer tubes at the time? I wonder if GE's decision to reinforce the anodes of their vacuum tubes for the USAF is to make less prone to grid overheating or for - gulp! - sound quality reasons?

  9. Thanks to electric guitar players, the 6L6 vacuum tube has been in continuous production - for economically viable reasons - since 1936.