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.
The reissue Gold Lion KT66 was tested later. For this tube's test I wanted a cathode biased amp and used a 1958 Stromberg Carlson P.A. head driving a Marshall 4x12 cab loaded with Celestion G12T-75 speakers. The cathode biasing is basically running the tubes flat out all the time. The pre amp is a 6SJ7 pentode followed by a 6SL7 for the tone controls and another 6SL7 for the phase splitter. An Epiphone Casino was used for guitar since the P-90 pickups are right between standard single coils and humbuckers in terms of output.
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.
What is a KT type of tube?
The KT series of tubes was developed by MO-Valve aka Genalex in response to Mullard’s development of the pentode. In a pentode a fifth element, the suppressor grid was added to reduce secondary emissions and ensure maximum electron transfer from the cathode to the plate. The kinkless tetrode (KT) tube was developed with beam forming plates to channel that electron flow and avoid infringing on Mullard’s patent. That is a very simplified answer but the difference is important. Pentodes tie the suppressor grid to a pin on the socket. In an EL34 this is pin 1. In most guitar amp applications this pin is externally jumpered to the cathode, in this case pin 8. The KT series of tubes ties the beam forming plates internally to the cathode and pin 1 is unused. The ratings between KT tubes and pentodes are similar. In America this type of tube was referred to as a beam tetrode. This includes the 6L6 and 6V6 as common examples, although they are often referred to incorrectly as pentodes. Do a search on the web and you will find weeks worth of reading as to what people think is the better design or what sounds better. The KT66 and 6L6GC are kissin’ cousins but the similarity ends there. The same holds true for the EL34 and the KT77.
Gold Lion KT66 - Switching from the stock 6L6GC to the KT66 produced an immediate and very noticeable improvement in sound. The only way to describe it is "BIG". It was like hitting the loudness switch on your stereo receiver, or fat switch on a guitar amp. At low to moderate volumes you get a big open sound across the guitar spectrum. Very articulate with little compression, excellent touch response and dynamics. Heater current was in the 1.3 amps per tube range as opposed to 900mA commonly found in a 6L6. The test amp had good iron and there were no issues with heating or reduced heater voltage. Do not assume your amp will be as stable. If you do not know if your amp can handle the additional heater current draw, contact the manufacturer before installing KT66 tubes.
We tested a set of Shuguang KT66 tubes for comparison. They produced similar results at low to moderate volume ranges. The next step was to crank things up and listen to the nature of the tube's overdriven tone. At higher volumes the difference between the Shuguang and the Gold Lion becomes more apparent. The compression in the Gold Lion tubes really seemed focused on the highs and upper mids, producing a smooth, singing sustain as each note bloomed. Chords compressed very nicely without loss of individual note definition or mushiness. By comparison, the Shuguang tubes were more compressed in a slightly lower frequency band, leaving the highs a little more raw sounding. While both sounds are very useful, I feel the Gold Lion tubes are better for those that want that smooth Santana sustain and the Shuguang tubes are the way to go if you want the raw sound of reverend Billy Gibbons.
The Gold Lion tubes are suitable tarted-up with impressive gold screening on the bottles. The bottle height is shorter than the Saratov or Shuguang and that will make it easier to get them into tight fitting amps like the Marshall Bluesbreaker reissue. No funny metal bases to deal with but a basket type retainer will be required as the molded base is tapered just like the originals and bear trap style retainers won't grab them. The overall construction is impressive. Nice thick glass with no signs of sagging or bubbles, nice molded bases, beautiful silk screening and clean pins. Hopefully the manufacturer will maintain this level of quality over time. The plate structure looks a lot like the Saratov KT66 but has better support rods and spacers. Take the cages off your power amps because these tubes want to be seen.
JJ KT66 - The JJ has got to be the most rugged tube I’ve seen in years. The construction reminds me of a GE 6550, including the metal base ring. JJ chose not to follow other KT66 makers by ignoring any physical resemblance to the original British tubes. This is actually a good thing in many respects. The bottle on this tube is 6550 diameter and height. The glass bottle on my sample was very thick and well formed. The Chinese and Russian variants use two sets of supports for the plate structure while the JJ has a third support element in the very top of the bottle. It’s hard to see but very effective when combined with the thick glass and heavy base, leaving very little room for microphonics. The tone is very balanced and centered around some thick warm midrange tones. It handles high and lower voltages on the plates equally with no problems biasing. Tonally these tubes rock, literally. A pair of these tubes can be biased to deliver crystal clean that will thicken into juicy crunch when pushed into overdrive. If you like 6550’s in your Marshall (many do) this tube would be an excellent choice for an alternate. Of all the KT series tubes on the market this is my choice for classic rock and they are definitely built for the road.
Saratov KT66 - The Saratov like its Chinese counterpart tries to capture the look of the old KT66 and the sound. The Saratov is well made and looks very much like the products from Genelex and MoValve of old. The sound is good and solid, very firm lows and thick mids. This contributes to a tube that is a darker sounding than the Chinese Shuguang. This is a good choice for people that want to use it in a Marshall plexi or 800 series. Care must be taken when putting these tubes into some amps. The bottles are very tall and wide. You have to measure the tube socket spacing and available headroom. Like the Chinese KT66 this tube won’t fit into some heads. Testing was done at 505 VDC on the plates and 500 VDC on the screen grids without any problems. I treat this tube like a 25 watt so it biases more like a 5881. 16 to 18 ma of idle current sound better than 20 to 21 in the test rig. A small difference but it seems that you get nicer sounding breakup when biased this way. Remember, even though these will bias with a range of voltage used for a 6L6 tubes, each KT66 draws about 1.3 amps of heater current. Your power supply’s heater tap will use 50% more current compared to a pair of 6L6 tubes. Check the heater voltage to see if it sags down to 6 volts or less. If it drops like this you are likely pulling too much current from the heater winding.
Shuguang KT66 - The Shuguang is functional art. A great job has been done in nailing the look of the original KT66. This is a huge tube when compared to others used in guitar amps. You are advised to have a careful look at space availability before installing these guys. The specs on this tube are very similar to the 25-watt originals. They draw more heater current than a standard 6L6: 1300mA instead of the usual 900mA. You should be sure that your amp's power transformer has the capacity to handle the extra current draw. Bias these tubes hot for best results. While most tubes we tested were biased at around 60% of their rated plate dissipation, the Shuguangs sound very bright and crystalline in this range. Some may find it too bright. At 75% of rated dissipation, these tubes really provide that feeling of rolling thunder. If you want Country clean, bias them for 60-70%. If you need earlier breakup and a beefier sound then turn up the heat, but keep an eye on your power tranny.
Sovtek KT66 (discontinued) - Everything you read in the review of the Sovtek 6L6WXT+ holds true for this tube. A larger than average curved glass envelope and metal base ring make this a different package than the familiar 6L6. Biasing and current draw are not issues, just the same as any 6L6GC. The sound of the Sovtek KT66 is great and usable for any application. If your amp has “bear trap” tube retainers they will have to go because of the size of the tube's base but also the bear trap clamp will ground the metal base to the chassis and cause a malfunction.
Tungsol KT66 - Not your average KT66. In fact it’s more like an extremely rugged 6L6GC in a KT88 bottle. Heater current is the same as a 6L6 and the biasing is more like a 6L6. You might want them just for the look but the tone is all there. Big and open with very solid lows and smooth treble response. The mids are well balanced and don’t go to the extremes of sounding scooped or nasal. Like all the Tung Sol branded tubes, the build quality is top notch. The over sized base has the metal band you usually find on a KT88 tube, and the large bottle helps dissipate heat if you like to run your tubes hot or if you have cathode biasing in your amp. If your amp has “bear trap” tube retainers they will have to go because of the size of the tube's base but also the bear trap clamp will ground the metal base to the chassis and cause a malfunction. If your amp requires the tubes to pass through a hole in the chassis for socket insertion you’re also out of luck. Home audio is where I would use these bottles since they are really functional art objects. Bottles like this should be on display. If you have a Marshal head I think you might find them to be an alternative worth exploring. They will give you great tone but since they draw similar current as a 6L6 you will draw 1 amp less heater current in a 50 watt head and 2 amps less in a 100 watt head. Your power transformer will run cooler and you will get closer to the 6550 tone that many Marshall enthusiasts love.