It's tough to find the perfect test-bed amplifier. Many amps have effect loops, reverb, tremolo, master volume and any number of other features that could affect your perceptions of tone. We wanted to eliminate as many of these factors as possible. Our final selection was a reissue ‘59 Fender Bassman. This particular amplifier had been modified to classic 5F6-A specs, using a solid pine repro cabinet, Jensen speakers, and NOS (New Old Stock) pre-amp tubes. A Mullard GZ34 rectifier tube was used instead of the stock solid state module. Additionally, the power supply components were upgraded and a variable bias adjust added to facilitate the use of different tubes. Tone controls were set at half way and left there.
There is a widely held belief that NOS are the only really good tubes available. The reality is that there are a number of manufacturers making excellent quality power tubes today. They are not the same as the oldies but they can hold their own with the classics.
The Tung-Sol 5881 may be the best option available for guitar toting tone freaks. The magic is definitely not in the name. The new T.S. 5881 has it all going on. The pair I evaluated has been seeing steady gigging use for about 8 weeks. In this case, about 20 hours a week in a 1964 Fender Bassman running flat-out. One tube has drifted about 1ma, but that's after 150+ hours of full power usage.
The tone is everything you would expect from NOS for about 1/3 of the price. These are full spectrum bottles that deliver everything from the classic, deep, "piano-like" lows to crystal clean highs. They are very tolerant with regards to biasing; allowing anything from 50% - 80% of rated plate dissipation to be dialed in at idle while still producing great tone. The construction differs slightly from the NOS in my possession but they are very close. Microphonics has not been an issue at all; no ringing, pops, ticks or squeals - unless you create them with a guitar. The overdriven tone is thick but articulate and just gets fatter the harder they are driven. Dime your amp and then use your guitar's volume and tone controls to dial in just about anything you want.
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The construction seems to work well at holding things in place. Both electrical and mechanical noise were not an issue. This is a tube rated for 23 watts and 500 volts so I wouldn’t run it real hot in daily use. The tone is very balanced with no notable weighting in the response. I did find that it was softer than a 6L6, there’s no big bass thump and no sparkling highs, just a pleasant flat response. This may be just the thing if you have an amp you find too bright or if you like to use pedals. To my ears it has that sound I usually associate with an EL34, that broad midrange response.
30 Watt Tubes
Sylvania 6L6GC (used for reference) - Another classic brought in for reference purposes. Unfortunately, microphonics were a problem, causing it to rank below comparable Svetlana and Sovtek tubes. It had a sweet sound but it shows that buying rare NOS tubes can be a bit of a crapshoot.
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JJ 6L6GC - Big, open, articulate; these are the words that came into my head when listening to the JJ tubes. The build quality is very rugged with thick glass and a well supported plate assembly. Perhaps this is a factor in the extremely low noise and lack of microphonics that this tube exhibits. The tubes have great tone and nicely capture the “6L6 sound”, characterized by sparkling highs, warm mids, and controlled bass response. Biasing was easy and well within the range of the standard bias adjustment circuit in the test amp. A great choice for any of the Fender amps made in the 60’s and 70’s.
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Preferred Series 6L6GC - The Tube Store has another terrific entry into their Preferred Series of vacuum tubes. The Preferred Series 6L6GC.
This tube is an homage to the black plate 6L6GC made during the first golden age of tubes. It has the looks and the tone to match. The plate design is classic American 6L6. It can be found in tube designs from Sylvania, RCA and General Electric. A tall bottle with a unique dome, the shiny black plates are supported by heavy mica spacers at the top and bottom of the plate structure, and topped off with dual top mounted getters. The glass looks very smooth and is a medium thickness. The base of the tube is not real vintage looking, as it is blue instead of black. An attractive change for those that run open chassis designs in their audio equipment.
This tube has a sound that I would characterize as smooth and warm. The high end is articulate but not sharp or brittle. Bass response is bigger than most but not boomy, so you still get good note definition. The midrange is very nice and thick without any tendency to sound nasal. In short, this sound is very much like an NOS tube made in the 1960’s. What it does not have is any perceptible hiss or microphonics. It is a very quiet tube and should be great in combo guitar amps.
Like all Preferred Series tubes, the PS-6L6GC has double the standard tube warranty and is burned in, tested and matched on precision equipment.
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Mullard 6L6GC - First off, I should state that Mullard never produced a 6L6GC. This is not a reissue, but a brand new tube. The closest Mullard made in England would be the KT66 and KT77.
Physically, this tube is a kissing cousin the Tung-Sol 6L6GC-STR. Base, internals, micas and spring clips are virtually identical. You would assume they would sound identical, but they don’t. The clever folks at New Sensor must have been changing the coatings and materials while sticking with the established stamping and assembly.
Bass response is firm and the sound remains detailed. I put some of these in my '59 Harmon Kardon and they were very pleasant to listen to. No ear fatigue and plenty of detail. They also performed well in my '53 Fender Bandmaster. Lots of punch and some really nice bluesy sound when wound up.
Because of the short bottle configuration, these should be ideal in any amp, including high gain combos.
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Sovtek 5881WXT - Solid construction and good performance. While not outstanding, the 5881WXT is probably the most commonly used tube of its type. Many amp manufacturers have been using these reliable workhorses as original equipment components. A good choice for general purpose use but not as rich in harmonic content as others in this class. If you like to run your guitar straight into the amp, there are better choices. If you prefer to run through a bunch of rack equipment or like major crunch and overdrive sounds then this tube will work well.
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Sovtek 6L6WXT+ - Guitar-cable-amplifier. If this describes the rig you use, then this is one of two modern tubes you should consider. This is not a repackaged 5881. Extremely low noise and a lack of microphonics combine with excellent ratings for detail and harmonic content to provide a tube of exceptional quality. These tubes can be used in any tube amp. Plenty of power and great tone. We have tested these tubes with plate voltages ranging from 425 to 500 volts without problem. Biasing can be set from conservative to hot; again, no real problem and great tone will be delivered from most settings.
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Winged "C"/SED 6L6GC - Tremendous harmonic content and an overall depth in sound make these another excellent choice for the purist. They can handle solid plate voltages and are very well constructed. Consider these tubes as an excellent choice for any type of music, delivering crystal cleans and smooth breakup with plenty of punch. Unequaled clarity and complex harmonics, plenty of sustain and depth.
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Svetlana SV-6L6GC - The tube has a nice balanced sound that is not as bottom heavy as say the TAD and not as bright as the SED. The midrange response is very nice. No honk and no really pronounced EQ shelving as you manipulate the tone controls of the amp. Very smooth and balanced frequency response that sounds great driving Celestion speakers which tend to be a bit honky in the mids and extreme in the Bass and Treble. Clean sounds are pretty chimey and overdrive is thick with lots of sustain. I tried a Fuzz Face, Vox Fuzz, Linear Power Booster, reverb and delay. Effects sound great and don’t make the tube mush out. Microphonics and hum simply aren’t factors with the SV6L6GC tube since they use a very good system for mounting the internals. They give off a good bit of heat and this is facilitated by oversized wings on the plate structure. One of the best applications for these tubes would be in Marshall JTM 45 and Bluesbreaker reissues. These amps usually ship with the Sovtek 5881 WXT+ and sound pretty dead. I have a meticulously built JTM45 head and these tubes sound better than Siemens EL34’s or Shuguang KT66’s. I wouldn’t believe it if I hadn’t done it to my own amp. I’m running the tubes at 40 mA and 466 plate volts. I ran them up to 48 mA without problem.
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TAD 6L6GC-STR - Very close tonally to the JJ 6L6GC tube with a bit more going on and easily justifying the extra cost. These are labeled as an STR tube. STR standing for Special Tube Request. When a buyer goes to the tube maker and delivers a specific set of specifications the tube goes into production as a S.T.R. One of the best know is the old Sylvania 6L6 tall bottles with the designation STR387. The TAD 6L6GC-STR is matched for plate current and transconductance and has the look of an old Sylvania. Using the old school U.S. style 6L6 plate design and a shiny black plate material they look like American manufactured stock. The sound is all there. The TAD is very percussive or punchy if you prefer. This makes them great for palm muting or “chunky” down-stroke rhythm playing. They break up a bit earlier than a JJ or Svetlana so you can get a bit nasty by digging in with your pick or clean things up by rolling back your volume control. They had the best tone biased at about 70% of rated plate dissipation. That’s about 21 watts at idle for a tube with a design max output of 30 watts. Both this tube and the TAD 6L6WGC-STR had some initial drift in plate current and required about 20 minutes to settle. I would recommend installing the tubes with an initial bias setting close to your target and check them again after ten minutes in play mode with a final check and adjustment after twenty minutes.
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TAD 6L6WGC-STR - A completely different animal than the TAD 6L6GC-STR. That’s not a bad thing and shows that the company is really working on developing unique products. A completely different plate structure than their other 6L6. These are short bottles just like the American 6L6WGC but they chose to use a proprietary plate structure. Sonically, I found these to be brighter than the GC version and tonally similar to a G.E. black plate 6L6. I have a customer with a 1962 Fender Concert amp with 4 10” Oxford speakers. I have tried just about everything in stock to make this amp sound good. Previously the only thing that made us both happy was NOS G.E. Alas, one of the tubes died and so did my stash. I tried a lot of tubes and had left the TAD until the end because it didn’t have the look this customer was used to. To my surprise I tried the 6L6WGC-STR and it was magic. I got the same tone as the old G.E. tubes but the biasing was colder than what I used for NOS tubes in the past, closer to 55% or about 17 watts at idle as opposed to 20 – 25 watts (70-80%) that most people seem to like for rock or blues.
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Tung-sol 6L6GC-STR - A nice solid short bottle 6L6 that looks more like a 6L6WGC but has 6L6GC performance ratings. The plate structure is very similar to a Svetlana with the extra cooling wings and the bottles are very similar to the Tung-Sol 5881. One could be led to believe that the parent company used bits and pieces from current production and came up with the Tung Sol 6L6GC-STR. This is partly true but inaccurate. These are a very rugged tubes with a unique, “dusty-black” plate coating. The sound is closer to a 5881 than a 6L6GC but they have a higher output rating. Biasing in my sample tubes was very particular. If you bias them hot they lose some definition and take on that tweed amp tone. With these tube hot could be anything above the 70% mark. During the test I found that idle settings between 15 watts and 20 watts (50-60%) provided a clean, warm sound that would overdrive nicely. A lot of techs go for the 70% mark, or about 21 watts at idle. The Tung Sol 6L6GC-STR starts to change character at this point. The result is not unpleasing and they are rated for more output than a 5881 or 6L6WGC so by all means experiment. This is one of the few tubes I’ve tested where I would start at 18 watts idle (60%) and then slightly adjust up and down to find the spot your ears like best. Check again with your test equipment to make sure you’re in spec and you should be good to go.
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Tung-sol 6L6G - If tubes were sold by the pound these would be very valuable tubes. I tend to classify the Tung Sol 6L6G as a specialty tube. This is not a normal 6L6 tube in today’s market. This tube is aimed squarely at the vintage market or amplifiers where looks are as important as sound. The Tung Sol 6L6G is enormous. A quick glance may lead you to believe you are looking at something exotic, perhaps a large power triode. It is taller than any 6L6 you can readily find. It has the slope shoulder bottle that many refer to as a “coke bottle” and it is extremely heavy. I seldom see anything old or new that has glass as thick and well formed as this Tung Sol. In fact it is very similar to a Tungsol 6L6 from the early 50’s I keep in my vintage stash. The new tube is still a bit bigger and has much thicker glass.
The vintage look is very cool but there are some issues that need to be addressed. Being so tall, this tube will not fit into all amplifiers so you have to be sure about your size requirements. The base of the tube has a tapered bottom like a KT66. This means that the Fender style “bear trap” retainers won’t grab the base. These guys will fall out if mounted inverted without proper retainers because they are very heavy. If you have an amp where the tubes sit upright this isn’t a big issue. If you have an amp where the tubes hang inverted, the basket and spring type retainers must be used. This is the price of looking cool.
Let’s not forget to talk about the sound. Although labeled a 6L6G this reissue Tungsol meets all the requirements of a 6L6GC. In fact, the internal assembly looks almost identical to the standard Tung Sol 6L6GC-STR. It will work fine in fixed bias and cathode bias designs and sounds just like the standard Tung Sol 6L6GC. Nothing really old or vintage about the sound. If you like the standard 6L6 this company makes you’ll like the sound of this flashy cousin.
The Tung Sol 6L6G sounds great and looks amazing. It won’t be for everyone, but if you have a cool piece of vintage gear or something on display there’s really nothing on the market that can compete with it.
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Tung-sol 7581 (6L6GC) - This is a new tube from Tung Sol and so far the field reports are quite good. Like most things out of Russia, the Tungsol 7581 tube bears an uncanny resemblance to another Russian tube. In this case the Svetlana 6L6GC. It's not identical, but shares the same tall, straight, bottles and general plate structure. The support spacers are similar, as is the plate structure. Again, there are differences, and we can't see the part of the tube where the magic happens because it's all encased in the plate structure.
What we do know about the Tungsol 7581 tube, is that the tube sounds great. In fixed and cathode biased tube amps, there was no issue with noise or microphonics. The construction over all and the pin finishing looks good. Testing didn't reveal any hot spots in the frequency response, at least none that could be observed by listening. The 7581 tube has a smooth and balanced sound like we've come to expect from Tung Sol. It reacted well to tone changes generated by the amps and the guitars that were used to test it.
It's not an Uber tube like the old 7581-A (keep reading below for the new Tung-Sol 7581A), but the majority of 6L6GC tubes made today, including this one, should be good up to 500 plate volts. Not many amps will go there but there are a few exceptions. Music Man and Ampeg both run higher voltages but this tube should fare very well. I installed a set in an Ampeg V4 in lieu of the stock 7027 tubes and the sound was great. Big and snappy, while remaining warm and having good note definition.
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35 Watt Tubes
Tung-Sol 7581A - 7581A gets an A+. The 7581A was a tube that stood head and shoulders above a standard 6L6GC. There have been a few attempts to recreate this tube but it was never really accomplished. The 7581 that Tung-Sol has been selling is really a high grade 6L6GC and the tube store advertises it as such. That may be changing very soon. Today I got a sample of a new 7581A from Tung-Sol and it looks to be the real deal. If not, it’s the closest thing I’ve seen, with its roots in another personal favourite the 7591A that has been shipping from Russia for many years.
The 7581A does not use the familiar plate stamping that you see in the Tungsol 6L6GC, 7581 and Kt66. This bottle has a plate very similar to the 7591 but is enhanced with large cooling fins, a larger 6L6 size bottle and thick mica’s top and bottom. The bottle is just big enough to hold the plate structure, so you get a lot of power from a pretty compact tube. I think this tube could replace the KT66 that is so hard to fit into a reissue Marshall Blues breaker and probably the 7027 used in Ampeg’s.
Balanced, is the first word that comes to mind when I think about the sound. Other descriptive words would include detailed, rich and responsive. In my single ended amp I was running it class A with about 44 ma of current and it was just great. The bass response, midrange and treble were very balanced and the EQ could essentially be left flat. Very rich sounding with enough shimmer and harmonics to put a smile on my face. Construction quality is high, with nice bottles, a single halo getter, 8 support points for the plate structure and nicely finished pins. The next re-tube of my beloved vintage Harmon Kardon integrated amp will feature these tubes. Based on this small sample I have to say that this will be a best seller.
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