Translated from the German musiker szene magazine July 1999
Note: Please don't email me saying this or that statement is wrong! I only translated this article!Click on a tube picture to see an enlarged version.
Power tubes of the 6L6 type are the most widely used glass bulbs in the history of tube amplifiers. Be they Fender, Boogie, Soldano: 6L6 tubes are glowing inside nearly every U.S. made amplifier. While the classic tube types made by General Electric and Philips/Sylvania are history today, manufacturers from China and Eastern Europe strive to achieve the high quality of their predecessors. We ordered examples from all available types of 6L6 tubes and have tested them extensively.
6L6 tubes stand for the "American sound". While the Europeans from Vox and Marshall put their trust into continental types such as EL84 or EL34, the classic American tube amps are usually equipped with 6L6 tubes. These can put out 25 watts of power (hence 50 watts as a pair) and are famous for their warm, round character. Nearly all Fender amps are running these tubes. Because the American manufacturers had to close down their production lines years ago (the small numbers made the production unprofitable) companies from Russia, China and Czechia specialized in manufacturing copies of the classic tube types. We tested some originals and many replicas for sound and character.
On the whole 11 tube types were ordered as pair from the German Tube Amp Doctor Supply. These specialists have every type in store.
As mentioned above, the original General Electric and Philips/Sylvania tubes set the standard for all the replica types. Hence so-called NOS type have been additionally tested. These are specimens from old age production but originally packed and unused as of yet. But of course the supply is very limited, and it is just a matter of time until the last ones will be put into the trash bin as "burned out", after having been used in some amplifier. It is all the more important to carefully inspect the many replica types.
For our tests we chose a "Blackface" Fender Super Reverb from 1966. This amp with four 10' speakers stands for the typical 6L6 sound and clearly demonstrates the different sound characteristics of the individual tube types.
How does on begin such a task? How best to spot the differences? Of course in the beginning all tubes have to be adjusted for a given zero-signal current (or bias value). For this purpose the Super Reverb has a bias pot to allow for adjustment. We chose 30 milliamperes as a standard. Next the tubes have to be matched, which means the current values should be exactly the same for both tubes. The Tube Amp Doctor Supply delivered perfectly matched tubes. We measured a maximum deviation of 2 milliamperes. A dismatching of more than 5 milliamperes can lead to crossover distortion, weak and disharmonic sound. Of course the tubes have to warm up properly. After testing they should cool down somewhat before they can be removed from their sockets. Hence up to 45 minutes passed between each test run, so a direct comparison was nearly impossible. Our auditive memory is very limited. The solution was recording the sound results using a DAT recording machine. First came the tests, later on the results were compared. It was a perfect success. By the way, sound purists can order a CD with our recordings from the musiker szene publishing house for a nominal charge of 40 DM (about 20$, tr.)
The first tubes to be tested, of course, were the 6L6GC manufactured by General Electric. "GC" stands for "Glass container", by the way. Nearly every Fender amp ran these tubes throughout the 70's. Their sound is a classic and an ideal starting point. Of course, these tubes are no longer available, except one finds a few unopened packs at the local radio store or a garage sale. In that case one should buy them. The GE tubes sound excellently. They perform a warm and round sound with pearly highs. Forget about the Bright-Switch! When the amp is turned up higher, a soft distortion shows, which is somewhat scratchy in the high end. For my taste this is a little bit too much. At full power they tend to become rather unstable and muddy. Well, but that is the good old GE sound.
The 6L6GC-STR/China from Ruby Tubes is manufactured after the American Philips/Sylvania type in China. Incidentally Ruby Tubes recently acquired the chinese tube factory. "STR" stands for "Special Tube Request." This tube cropped up as a serious competitor. It sounded extremely forceful with a hefty punch in the midrange. The silky highs are pleasantly restrained. Hence the STR delivered a fantastic distortion sound. Whoever relishes a warm and thrustful punch should test these tubes. They make mincemeat of the old GE types.
In the case of the 6L6GC-C/China we have an inexpensive 6L6 variant from China. It is available both in "Coke-Bottle" design or with a straight glass bulb. These are used presently in most current production amps such as Mesa/Boogie, sound very tightly and clear yet deliver a little less power than the STR type. The sound is comparatively neutral and without noteworthy overemphasize. It is easy to overdrive and delivers an appealing distortion sound which is a little too muddy. To play loud economically and yet replace tubes on a regular basis, this 6L6GC-C is a good choice.
The Czech 6L6GC by Tesla is remarkable for its glowing red socket. These types have a comparativly thin basic sound with a remarkable peak in the higher midrange (1000-1500 Hz). Hence their sound is harsh and nasal. This can be unpleasant for the first row of an audience in a club. But the attack comes very sharp and accentuated. A real tuning possibility for those who want more attack from their amplifier. But this model has few things in common with the classic GE sound.
The 6L6GC from Sovtek is manufactured by Reflektor in Russia and is a more economical version. It shows basically a round sound, but overdriven tends to become unstable. The distortion sounds come along rather dull and flabby, which is not much fun. The low price seems to be the only advantage of this model.
The 6L6GC by Svetlana is a wholly different animal. These tubes sound incredibly clear and open, have a fantastic attack and give a wholly new face to the old Fender amp in respect of highs. The distortion sounds are very convincing as well. Vintage distortion at its finest! Stevie Ray Vaughan fans will find an ideal choice in this tube. An insider tip for friends of Texas Blues.
The good fame of their predecessors is recreated by the 6L6GC/WXT+ by Sovtek. These sound much more freshly and balanced than their more economical sister types. Here it is again, the classic 6L6 sound with fat midrange. These are not exactly as open sounding as the Svetlana types, but all the more powerful. A very good Blues tube for aggressive play without effect ado. To be sure, these don't surpass the high standard of the STR tubes, but for reason of their airy response these tubes will gain friends among clean players.
With the 7581A by Philpis/Sylvania we are back in the NOS region. These tubes are legend and are no longer available. Only clever collectors will be able to get specimens of these rarities. What we have here is a military grade version of the famous glass bulbs from the U.S.A. These used to be manufactured for military electronics and hence they are extra rugged, reliable, and long lasting. This stableness makes for a glassy clear sound with gorgeous attack. The 7581 types are very fast-responding, but somewhat harsh in details. In other words, they do not sound as sweet as the General Electric types or the Chinese STR ones. Yet when overdriven they always remain clearly accentuated. Only at very high volume levels sharp distortions appear. In the case of our test amp they work perfectly for adherents of straight clean sounds with explosive attack.
Maybe the most popular version of the 6L6 is the Sovtek 5881 WXT regarding modern amp models. It is used in nearly every recent Fender amplifier and was used by Marshall and Mike Soldano. In this case as well we have a Military grade of Russian (Reflektor) manufacture. Usually delivered with a narrow socket, it has become available with a taller socket for some time. It is famous for extreme reliability and is said to achieve best results in the distortion region. Our test could only confirm this. Clean the 5881 sounds like the classic 6L6 with clear response and openness. In the distortion region this was by far the absolute favorite of this test, since this variety sounded so homogenously and powerful like no other tube tested. No wonder specialists like Mike Soldano tuned their High-Gain Channels for this tube. The high production numbers of these tubes indicate their high reliability. After all, who wants to deal with warranty complaints about broken tubes? Those who find themselves unable to decide or simply do not have the opportunity to test all tubes find an ideal all-round version that should satisfy every demand. One can blindly order a Russian 5881 and become happy with it.
A 6L6WGB/USA from Philips/Sylvania is the ancient ancestor of the 5881 tubes that were used in Fender Bassman amps from the 50's. It can easily be distinguished by virtue of its smaller glass bulb. These tubes sound incredibly compact and closed. They do not have much headroom in the treble region, but still show a very good response and have a very tight bass sound. They obviously excel in the mid region, because in this case the guitar's strings come out perfectly thru the speakers. Often the sound of these tubes is criticized as too midrangy or too small. In an amplifier like our test amp however this was very convincing. In case the tubes deliver too much bass the results get muddy in the lower region at high volume levels. Not so with the WGB. Even at full force the sound stays clear and punchy. Great! This tube may also do good to amplifiers normally equipped with the smaller 6V6 types. For example in a Fender Deluxe Reverb they deliver excellent sound results with high stability. Also, friends of midrangy Solo Sounds à la Robben Ford or Larry Carlton should test these tubes.
In the end we allowed ourselves a test with the new KT66-CRT STR/China from Ruby Tubes. This model has a different designation as well as deviations in design, but it can nevertheless be plugged into any amplifier originally equipped with 6L6 tubes. But they need much higher heater currents than their relatives. Apart from a possibly required bias readjustment one should measure the heater voltage. Should the meter show much less than 6.3 volts then beware! The power transformer may die per overheating. In our test amp this, however, was no problem. KT66 basically are HiFi tubes and came into use in the 200 watts tops from Marshall. Stevie Ray Vaughan used to run a quartet of KT66 in his famed Dumble Steel String singer amp. Whoever has heard his record "The sky is crying" (eg Little Wing) has had a glimpse of how these tubes sound. They cost about double the price for 6L6 tubes, but show sound properties that simply threw us off base! Regarding linearity and clearness these tubes surpass all others. They are exrtemely loud and thrustful and have excellent dynamics. These tubes let the guitar's strings twang so metallically and fresh that we almost could not believe it. As well in the distortion region - and considering their headroom this means extreme volume levels - the sound of the KT66 always remains clear and stable. The sound quality of a Stevie Ray Vaughan is really newly discovered with this tube. We also recommend this tube to Rockabilly players. A "thick" Gretsch unfolds its true capabilities thru these "thick" tubes in an uncomparable way.
The Bottom Line
It is not easy to do justice to all these different tube types. Yet we could ascertain that they all have their individual character. Replica types such as the Ruby STR or the Sovtek 5881 set a new standard and can even surpass the American originals in certain sound parameters. The Svetlana is a real revelation for Texas Blues players and the KT66 the no-compromise-choice for High-Enders. There is no reason to mourn the old times. We still have good enough choices concerning quality and quantity to make our tube amps sound great. Content probably © 1999 musiker szene Euskirchen/Germany.