[FloridaBoy]

By Roger Modjeski owner of RAM Labs

The 6L6 is a favorite tube of mine but sadly has not captured the attention of the audio community. It appears to be a bit of a black sheep in the flock of popular output tubes. What I like about it is never mentioned so I will. It has the lowest filament consumption vs. plate dissipation of all the power tubes. It was also the first "beam" tube and the original sketch of the internals has become the most often pictured of the internals and electron path of any tube on this planet.

With that introduction lets set a few things straight that I have read here.

1. It is nothing like an EL-34 which is a traditional pentode with a loosely wound screen grid. Its little brother is the EL-84 which is of the same construction.

2. The KT series does indeed stand for Kinkless Tetrode but the 6L6 never had kinks. The tubes that had kinks were the early tetrodes with a screen and no supressor grid. They were awful and short lived in tube manufacture. The kinks refer to the verboden negative resistance region where the plate voltage swings below the screen.

3. The KT series was the European answer to the beam tube (6L6) and some say RCA worked with the Brits developing it but get RCA to admit that! News of it was released in the first volume of RCA news in 1936 which I have studied in great detail. There is a good article on Wikipedia which I find to be accurate and the first time I have read that the Brits licensed it to RCA. In RCA news they claim to have invented it. Of course with David Sarnoff at the helm what else could one expect from the man who financially ruined Major Armstrong, the inventor of most every early radio circuit including FM and Philo Farnsworth, the father of Television. Search 6L6 on Wikipedia for more.

4. The 6L6 internals, known as the "mount" were used, with a bit larger plate, in the 807, 1625 and the first horizontal sweep tube the 6BG6. These are all essentially 6L6s.

5. The smaller brother of the 6L6 is the 6V6 beam tube and the 7 pin 6AQ5.

6. The beam tube is the output tube in the All American Five, the most popular table radio made by everyone.

7. The KT-66 is close but not quite a 6L6 as it has higher dissipation and heater current.

8. The KT-88 is the Brits answer to the Tungsol 6550

9. The KT-77 is a distant cousin to the EL-34 and was created to be a drop in replacement by Genelex. However the EL-34 is a true pentode and the KT-77 is a beam tube. The curves are quite different. The American version of the KT-77 is the 6CA7. To my knowledge no American company every made EL-34s as a true pentode. This is a rare example of making a entirely different tube as a drop-in replacement. At this time I cannot think of another.

10. The KT-90 was the creation of David Manley and Electron Industria (EI) in Nis Yugoslavia. It was a bastardization of a EL-509 and has some rather bad characteristics. When I visited the factory I chastised them for that but they said "David made us do it" What David wanted was a tube he could run at 550 Volts in an Ultralinear amp. The early Genelex tubes could do that but the modern ones couldn't.

11. The KT-120 and KT-150 are not drop in replacement for KT-88s but close enough for most amps that can handle the heater current and somewhat different bias. They are a creation of New Sensor/Electroharmonix/Sovtek headed by Mike Matthews. I have no complaints about them and time will tell. I see here pro and con reports of its sound.

12. I fully believe that the sound of a tube cannot be expressly stated as it varies with how it interacts with the amplifier. We do however see more use of the EL-34 in ultralinear circuits and the KT-88/6550 in pentode circuits. I have found this to be true in my own designs also for purely technical reasons.

I saw this post when I was searching 6L6's which have become a favorite tube. It was a reply in a thread asking why there are few 6L6 audio amps. Last weekend I ran Sylvania 6L6GAY's with a Holland Pope 80 and CV-181T in the amp. NU 27's UK 6SN7 and Tungsram 80 in the pre and was pretty surprised. The little 6L6GA's sounded much better than the GE labeled Westinghouse L6GC's with a 274 rectifier. The 6L6 is usually considered a guitar tube.