Originally Posted by FloridaBoy (Post 843870)

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.

Originally Posted by Analog Addict (Post 843945)

Of the 6L6GC versions, I like the GEs the best.

Originally Posted by Analog Addict (Post 843945)

The 6L6 is a wonderful sounding tube in it's many configurations. My particular favorites are the 6L6G, 6L6GA, and the 6L6GAY. Of the 6L6GC versions, I like the GEs the best. The guitar amp guys tend to gravitate towards the RCA 6L6GC black plates, which command premium prices. So much so, I couldn't resist liquidating a quad last week. But to my surprise, they went to an audio guy. I have a lot of 6L6WGBs, 5881s, etc, but the gem of my 6L6 stash is a NOS quad of WWII vintage Sylvania JAN 6L6GAs with the U.S. anchor printed on the base. I'm saving them for when I build a PSE beam tetrode amp....:thumbsup:

Originally Posted by Musica Amantem (Post 843938)

Bombadil: But if one is looking for single-ended 300B / 2A3-type sound, this isn't it.

It's a matter of choices. I believe the Triode-strapped Inspire amp is best within a continuum of output tubes, OTBE, ranging from the 6V6-6L6-KT66-KT77-6550 and KT88 (at the other extreme). Not that other tubes are not interesting, but the tradeoffs involved in said range of output tube specs are more attuned to the mid-range magic of the Triode configuration, with the KT77 at the climax of that optimal tradeoff. Pentode configurations are different, I'm sure.

Dialing-in the LP-27a and Triode-strapped Inspire amp with those considerations, through synergy-triggering input and rectifier tubes, makes a big difference. In my case, mid-range magic at affordable output tube prices and about 5 watts (relative to the 2 watts of the best true-Triode tubes) is an objective. As FloridaBoy stated, there's an added synergy potential through the dialing-in of the preamp, which I frankly had been missing, so there's an opportunity there to improve things further at reasonable costs. In the end, what makes sense to me is maximizing the mid-range performance (transparency, tri-dimensionality, instruments' timbre and texture), where most of the audible music spectrum lies, while still preserving acceptable levels of bass and dynamic qualities.

Originally Posted by Bombadil (Post 843943)

The two types of tubes I've liked the least in my SE KT88 HO have been 6V6 and EL34. I've tried 4 different brands of 6V6 and while the sound has a lot of good qualities, I find them too forward in the upper midrange and too light & ill-defined in the lower bass. All four brands have had a similar sound. I've only tried two brands of EL34 but in my amp & system, they are lacking in high frequency detail and imaging, and are a little loose and fuzzy on lower bass. Great on vocals and blues sound very good with them. But overall I find them to come up short on many types of music.

Also my treasured Sylvania fat boy 6CA7s, which sound so good in my triode push-pull amp, are only a little better than the EL34s in my Inspire. However I like my EI 6CA7s much better in the Inspire.

I'm sure a significant factor in this are my Von Schweikert speakers. They excel in creating a big, wide, deep soundstage, which I love. And they go deep (sub-30Hz) while being very clean and neutral. No mid-bass emphasis, as found in so many speakers. I've grown addicted to that ultra-clean, well-defined bass. Tubes which come up lacking on the bass end are exposed, as are tubes which compress detail & imaging.

Originally Posted by FloridaBoy (Post 843974)

I'm using CV-181 T in the amp UK in the pre and don't think the BX/BL tubes will be back. I think you need a couple more of them in different flavors. A pair of 6L6GA's too. They are superb with the copper V-cap. :D