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Seymour Duncan SFX-11 Twin Tube Blue Review

The Twin Tube Blue is a useful pedal for blues players with an interest in effects

Dunlop Eddie Van Halen Signature Wah
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SIGNAL CHAIN: Tom Anderson Cobra Special-S with 3 mini humbuckers cable to the unit by a Mogami Platinum 12' and from the unit with an Alleva-Copollo 20'.
The Twin Tube Blue is the third in a series of Twin Tube stompboxes manufactured by Seymour Duncan and Co., best known for their full line of excellent pickups. Each in the series uses a different subminiature triode tube; the “Blue” uses the 6111 military spec tube, hence the designation SFX-11. I’m sitting at my computer pondering the 3 lb. “Blue” and wondering what might the other two units do that this one doesn’t, and vice versa. Maybe it’s the old “collect all three and trade ‘em with your friends” thing… or not. All I can say is this unit does a lot and does it well.

On the outside, the unit is painted a very pretty metallic blue with well-done graphics and labels in contrasting colors. The bottom cover is black with a large corrugated rubber “slip strip” and slotted vent openings for cooling on each side. Four screws hold the beefy steel top and bottom together. The left hand side has a status light at the top to show that the unit is engaged. When the light is out, the unit is in true-bypass mode. Below the light is the master Tone section with separate Bass and Treble controls, affecting both Rhythm and Lead channels equally (but of course not affecting the bypass signal). Below that is the logo, and at the lower edge of the chassis is the On/Bypass switch, brilliantly located so that it may be actuated easily with a toe (preferably in a shoe).

The right side of the top has the Rhythm channel, with separate Volume and Gain knobs. Below that is the Lead channel, with separate Volume and Gain controls as well. Below that, similarly placed to the bypass switch, is the Channel Select switch, which allows either the rhythm channel or the lead channel to enter the signal path, but not both. The rear of the blue cover has a 1/4" input for the instrument and a like output into the amp (or other device). There is also an input for the 16V 560mA AC power supply from a wall wart—the only way of powering this unit.

Removing the bottom cover reveals a very neat layout and tidy soldering. The mounting of some of the heavier components is reinforced by neatly placed dabs of flexible glue. There are some beefy capacitors in there that could definitely knock your socks off, so don’t be trying this at home unless you are sure you know what you’re doing. The 6111s have wires extending from their bases (instead of pins) and the wires are nicely insulated from each other and neatly soldered to tabs. A skilled tech should be able to change tubes in about 15 minutes, should that ever become necessary. The tubes are held to the board horizontally with a removable metal clamp and cushioned somewhat by silicon or Teflon tabs on the clamp and the board. All components both inside and out, except the footswitches, are mounted to a fiberglass printed circuit board. Components appear to be of good quality. The pots and switches are labeled “Alpha,” a reliable brand.

Time to light this baby up!
First, I tested the bypass by A/B’ing the unit in bypass mode with a direct connection and heard no change. I then read the very well-written instruction manual and set the unit up using my Clark Beaufort 2x10" and Mogami Platinum and Alleva-Coppolo cables (both very neutral sounding).

At middle, but appropriate, gig volume levels, the rhythm channel sounded very transparent and easily duplicated the bypass sound when dialing in less gain and more volume on the channel, with the master Tones set for Bass at 11 o’clock and Treble at 2. I could do the same with the lead channel, but had to add bass and back off the treble. This indicated that the rhythm channel used a different voicing than the lead channel. Indeed, further tweaking and listening showed the Lead channel with more top and a bit less bottom than the Rhythm channel. This makes sense if you plan to use the unit in the standard way, since you’d want the lead to come forward in the mix without adding mud to the overall sound of the band.

I set up other scenarios with the bypass, providing a solid rhythm sound and adding more gain to the Rhythm channel and Lead channel, but being careful not to overdrive the input of the amp at the same time, as this changed the character of the sound of the guitar and added some unpleasant (to me) forms of distortion. The unit outputs 2 Volts at max before clipping, so there is overdrive potential there. Everything I tried without going over the top retained a sweet tube sound. I then thought of using the unit as a two-step overdrive, which might actually work well with some amps.

I switched to my ’71 Princeton Reverb and set up the basic sound at a volume where it was slightly overdriven. I then set up the unit with low gain and high volume on both channels to produce an intrinsically clean sound but with incremental increases in volume, and thus was able to overdrive the Princeton in steps, each step having a useful tone and volume in a blues gig setting—all the while producing the sweet sounds of a Princeton Reverb (with a 12" Weber Alnico) in torture… er, I mean “enhanced interrogation” mode.

Another cool thing about the unit is that its Volume control doesn’t start at the volume of the input guitar but can be used to lower the volume instead. In other words, if you really love the sound of your amp going straight out in bypass mode for solos, you can use the unit to instantly lower the volume and at the same time change the tone and character of the sound at two levels.

To round out the testing, I used a number of other amps, including a Fender ’69 Champ and ‘90s Hot Rod Deluxe, an Ampeg ’65 Reverberocket, an Allen “Old Flame” and a Clark Beaufort, as well as multiple guitars with both humbucking and single-coil pickups. In all cases, the amps and guitars sounded like their glorious selves, but with the addition of smooth, clean tube tone changes produced by the effect.

A few concerns surfaced during use. The unit became somewhat microphonic at high-gain settings, causing particularly the bypass switch to “clank” loudly when actuated. The unit ran quite warm, which is understandable since the tubes are run at normal (high) plate voltages. Just don’t block the vents. Of more concern is the wall wart power supply. It is a real oddball voltage and runs very warm. I personally would not gig with this pedal without a backup power supply. I checked the internet and found no aftermarket replacements. Digikey, the supplier cited in the instructions by Seymour Duncan, listed the part as obsolete and unavailable. I did not find its exclusion from the warranty, so along with all else, it’s covered for one year (except tubes, which are covered for 90 days). Tubes are tubes, and prone to failure. These particular tubes are heavy duty and designed for mechanical stimulation as might be found in handheld devices, such as microphones. I suspect they will last a long time if the unit is not abused.

There are no value markings on any of the controls, but the use of “chicken head” knobs makes clock-hour values calculable—I’ve used small color-coded stick on dots found at office supply stores to indicate approximate setup for particular amps and guitars with final tuning by ear.

The Final Mojo
This is by far the best blues-oriented pedal I have ever used. I have used many over many years, and I still own the original BK Butler Tube Driver, a TS-9,etc. The Twin Tube Blue is so good it doesn’t sound like a pedal. I need one.
Buy if...
you're a player of amplified blues with no hang-ups about pedals.
Skip if...
you're an acoustic player, or an amplified player who doesn't use effects.

MSRP $325 Seymour Duncan -