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THE KEEP ....Mistress Mystery Page. ..


©Kit Rae. Article written in 2007. Updated in 2010, 2011, 2013, 2015, 2016

B.K. BUTLER REAL TUBE OVERDRIVE - The Tube Driver is a booster/overdrive pedal with an IC and vacuum tube driven preamp circuit inside, used by guitarists such as David Gilmour of Pink Floyd, Eric Johnson, Billy Gibbons, Joe Bonamassa, Joe Satriani, Kenny Wayne Shepherd, J Mascis, David Holt, Keith Urban, and others. It was designed and built by audio electronics engineer B.K. Butler (Brent Butler). Around 1978 Butler designed and began making an early version of the Tube Driver, one of the first tube overdrive pedals. It has evolved over the years and there are many versions (a list can be found HERE). It generates anything from a smooth, light overdrive tone, to a classic rock-style heavy distortion. At high gain, it is very reminiscent of the lead crunch tone of a Marshall JCM800 amplifier. At low drive it can deliver a Fenderish smooth gain boost or a bluesy overdrive. It is not very versatile sounding through some amps, but exceptionally good with others. It can be difficult dial in a tone setting that hits the sweet spot, and some people find it finicky and hard to get along with due to some design issues, but in the right setup, with the right amp, nothing sounds quite like it. The TD has a very unique voice that I have never heard another overdrive replicate.

The Tube Driver looks like something from the bridge of the Enterprise from the 1960s Star Trek television show. The standard 4 knob version features a master volume, HI (treble), LO (bass), and drive control. The 5 knob version adds a MID knob. Inside is a hybrid fuzz - pre amp circuit. The hard clipping part of the distortion comes from the integrated circuit chip, or op-amp. That strong signal is fed into a vaccuum tube, which does not work to amplify the signal as it would in the pre amp stage of a typical amplifier. It filters and colors the sound with the tube characteristics. The tube is actually only running at 12v, a fraction of what these tubes run in a typical amplifier. This is called a "starved plate" design, and you won't see much tube glow, if any, if you look inside. It would seem that the tube itself would make very little difference to the sound, but it actually is an integral part of the sound, and different tubes or tube types will alter the sound in different ways. Butler has always made these stock with 12AX7 tubes, but owners typically experiment with different tubes to tailor their tone.

I think the TD sounds best with clean amps, like the Hiwatt Custom 50 or Custom 100 (DR103), or similar amps like the Reeves Custom 50/100. It also sounds excellent with certain Fender amps, particularly the Fender Bassman, or it's high powered step brother, the Marshall JTM-45. The four knob Tube Driver sounds best with amps that have a moderate mid range, like a Hiwatt. For mid-scooped amps, like the Fender Twin Reverb, the Real Tube version with the mids knob is better. Some amps that do not have a lot of head room and break into distortion at high volume do not play well with it. It also does not sound good when certain pedals are before or after it in the signal chain dues to the high impedance output. Unlike typical effect pedals, the Tube Driver includes a built in power cord, as it uses an internal transformer, which can be noisy for certain users.

TUBE DRIVER VERSIONS - There have been numerous version of the Tube Driver made since the 1980s, but these are all of the 4 knob versions. A mostly complete list of all the other versions can be forund HERE.

•1985 - B.K. Butler/Chandler Tube Driver. Marked TUBE DRIVER™ and CHANDLER INDUSTRIES-PATENT PEND. on the front.

•1986 - B.K. Butler/Chandler Tube Driver. Marked TUBE DRIVER®, Concept & Design: B.K. Butler and CHANDLER INDUSTRIES, INC on the front.

•1988 - Chandler Tube Driver. Knockoff made by Chandler after Butler parted ways with them. Only marked with script Chandler logo on the front, no BK Butler.

•1993-94 - 911 Tube Driver - Made by Butler's Tube Works company. A four knob version of the Tube Works 901 Real Tube pedal, which was an improved Tube Driver.

•2005 - 911 Tube Driver. A reissue of the 1990s Tube Works Tube Driver.

Chandler TD

Shown above (left to right) - The extremely rare original 1985 BK Butler Tube Driver, the revised 1986-87 era BK Butler Tube Driver, the unauthorized 1988 knockoff made by Chandler after Butler parted ways with them, the 1990s Tube Works 911 Tube Driver, and a B.K. Butler reissue of the 911 Tube Driver from 2006.

I have owned several versions of Butler's Tube Driver, including a few original Chandler/BKB versions from the 1980s, a 1989 Real Tube with 5 knobs, a 911 model, a few 911 reissues with the bias knob, a 1996 Ibanez Tube King, and a Tube Works Smooth pick pedal, among others. I have played most of the other variants as well and I suggest staying away from any but the original 4-knob Tube Drivers branded with both BKB and Chandler, Tube Works branded 911 Tube Drivers made in the USA, the USA made Ibanez Tube King, or the BKB Branded 911 Tube Drivers made since 2006. The three knob version and other Taiwan made versions may be found for much less money, but they do not sound the same. I do not recommend the knockoff Chandler version with the attached wall wart either (the one without BK Butler's name on it), as those have a different tone circuit that sounds like a poor imitation of the BKB circuit.

Various versions of the B.K. Butler Tube Driver Circuit.
Top row (left to right) - the crappy Chandler knockoff from 1988, three BKB/Chandler Tube Drivers from 1986-87, and an Ibanez Tube King (TK999US) from 1995.
Bottom row (left to right) - a BK Butler 911 Tube Driver reissue from 2007, a 911 reissue with bias knob on back from 2008, an original 911 Tube Driver from 1994, a 901 Real Tube from 1987, and a 903 Smooth pick pedal from 1997

There are basically two different circuit boards used in the original BKB/Chandler 1986-87 Tube Drivers. The first one had a light green-tan colored pcb with the trace and resistors on one side, everything else on the other. The trace side is marked 9 3151•Z. I have seen two slightly different sets of component values on that board, and in my experience and based on what other owners have said, no two sound the same. I currently own two and have played four side-by-side. Two of them sounded identical to each other (and identical to the second version mentioned below) and the other two sounded very different. Those two required the drive pot to be above notch 3 to get any sound.

Butler had a firm in California manufacture a second circuit board for him sometime in 1987. The trace side was dark green on this version. Most components were soldered on the front side, but a few caps and resistors were on the trace side. It had no identifying numbers or letters, and I have seen two slightly different sets of component values on this pcb. I have owned several of this version and each sounds nearly identical. David Gilmour used two 1986-87 era Tube Drivers in his massive Pink Floyd rig in 1994.

In 1987 Butler made made an improved version called the Real Tube through his Tube Works company. It added a mids knob, allowing a strong mid boost to be dialed in, getting into the Tube Screamer territory. It had a much wider tone range than the earlier 4 knob versions. With the mids knob dialed almost off, the tone range is basically the same as the 911 version Butler made in the 1990s, and the 911 reissue he began making in 2005. The 5 knob Blue Tube Enhancer (model 903) and Smooth pick pedal (model 303) are lower gain versions of the same circuit, suitable for mid gain overdrive and rhythm playing, or boosting an amp or high gain pedal.

Around 1993 Butler made a new version of the older 4 knob Tube Driver through his Tube Works company, called model 911. It was basically the Real Tube circuit without the mid range knob, not the older BKB/Chandler circuit from the 1980s. The circuit layout and pathways were very similar to the 1980s circuits, but there were several component value changes across the circuit that affect the way it sounds. Butler sold the Tube Works company in the 1990s and they were later shut down. In 2006 Butler reissued the 911 Tube Driver by custom order through his Butler Audio company. This version was used by many popular guitarists, including David Gilmour, who used in his 2006 On an Island pedal board and 2015/16 Rattle That Lock pedal board.

In 2008 Butler added a bias knob as an option. That was a feature I found made little difference to the sound, but some people find it useful. The TD comes stock with a 12AX7 tube. The bias knob allows you to reduce the gain, similar to what would be accomplished by changing to a lower gain tube like the 12AU7. The full + setting is the same as the non bias version, but I found that rather than softening the distortion with the bias reduced, it tended to make the sound die faster. I found it more effective to simply change the tube, but others really like this feature.

B. K. Butler

B.K. Butler still makes 911 Tube Drivers by hand (at the time this article was written) through his Butler Audio company, HERE. Used models are also plentiful and easy to find online. If you want to read more about B.K. Butler and the Tube Driver, there is a good interview over on the fantastic Tone From Heaven website. A list of most of the Tube Driver versions can be found HERE.

SOUND CLIPS - These are almost all Pink Floyd/David Gilmour inspired tone clips. With just a light drive setting the TD works great with a neck pickup for solos and blues playing. It is perfect for songs like Shine on You Crazy Diamond and Coming Back to Life. The sound holds its own against a Big Muff when cranked into full overdrive. Here are clips of different Tube Drivers played with a Stratocaster fitted with EMG-SA pickups through a Fender Twin Reverb and a Reeves Custom 50 (Hiwatt Custom 50 replica), and a P-90 Les Paul into a Reeves or Fender Bassman.

BKB/Chandler Tube Driver - Demoing the different pickup position tones on a Strat. Position 5 (neck), position 4 (neck+middle), then position 2 (bridge+middle), 12AU7 tube.

Castellorizon - BKB 911 Tube Driver with 12AX7 tube. EMG Strat, TC Nova delay into a Reeves Custom 50.

5 A.M. - Les Paul with P-90 pickups and Bigsby tremolo into a Fender Bassman. Boss CS-2 compressor > 1980s BKB/Chandler Tube Driver > Boss GE-7 > TC Nova delay into a Fender bassman.

Louder Than Words solo (no backing track) - This is just the BKB 911 Tube Driver with a Demeter Compulator into a Reeves Custom 50

Louder Than Words solo (with backing track) - Demeter Compulator compressor >BKB 911 Tube Driver > Electric Mistress > Echorec delay > Reeves Custom 50

Coming Back to Life solos - Boss CS-2 compressor >1980s BKB/Chandler Tube Driver > Boss CE-2 chorus > TC Nova delay. First solo with EMG-SA neck+middle pickup. Second with neck pickup. Reeves Custom 50.

BKB/Chandler Tube Driver - Light drive/boost demo. TC Nova delay. 12AU7 tube.

Clips below are played with a 2008 American Standard Strat, Seymour Duncan SSL-5 bridge pickup, Fender CS69 neck pickup, or Seymour Duncan SSL-5 bridge pickup, into a '65 Fender Twin Reverb RI or a Reeves Custom 50.

LIGHT DRIVE / BOOST SETTING - Example of the Tube Driver light drive setting with a Fender Strat into a Reeves custom 50. Settings shown above.

MP3 Sound Clip - Tube Driver light boost setting, Boss CE-5, and Nova delay. 12AX7 tube.

MP3 Sound Clip - Noodling with Tube Driver light boost setting, light Boss CE-5 chorus, and TC Nova delay. 12AX7 tube.

OVERDRIVE SETTING - Example of the Tube Driver overdrive setting with a Fender Strat into a Fender Twin Reverb. Settings shown above.

MP3 Sound Clip - This is just the TD with 12AX7 tube and some delay from a TC Electonic Nova delay.

CAN A TUBE DRIVER HANDLE LEADS LIKE A BIG MUFF? - It can, and Gilmour has been using the Tube Driver as his main distortion since the 2006 On an Island tour finished (according to him and his backline tech), and his TD overdrive settings from the same tour produced a very muffish distortion. It does a great lead tone, although it does not have quite the same huge bottom end and scooped mid range you find in a Ram's Head era Big Muff. I was going for the Pulse era live tone here. A Civil War Big Muff was used for the song Sorrow, so I'm using the Tube Driver here to compare to a Muff. Strat into a Fender Twin Reverb.

MP3 Sound Clip - Sorrow solo with Boss CS-2 compressor >Tube Driver > Boss CE-2 chorus, TC Nova delay

BOOSTING A BIG MUFF WITH A TUBE DRIVER - A Tube Driver can be used as a light boost before or after a Big Muff in the signal chain. Muffs like to be driven hard and like a loud, clean tube amp to sound good, but sometimes that is not enough for lead or solos, especially with modern Muffs. By 'boost' I don't mean a volume boost, but more of a blend of the two pedals. Driving a Muff with a driver can make the Muff tone come alive, smooth out the tone while adding some gain and mids, and help it cut through in a band mix better. Most modern Muffs and some vintage Muffs can really benefit from this type of boost. The Big Muff sound is slightly warmer when placing before a Tube Driver, and slightly sharper when placing it after. The tone of a Tube Driver is fairly transparent when using as a boost, but you can also lightly color the Muff tone when boosting.

BIG MUFF BOOST (settings shown above)

Tube Driver set for boost AFTER a Civil War Big Muff - smooth tone
MP3 Sound Clip - Boss CS-2 Compressor > Tube Driver > Big Muff

Tube Driver set for boost BEFORE a Civil War Big Muff - grittier tone
MP3 Sound Clip - Boss CS-2 Compressor > Big Muff >Tube Driver

OLD 1986-87 ERA B.K. BUTLER/CHANDLER TUBE DRIVER VS 2000s ERA B.K. BUTLER TUBE DRIVER - The core circuit design on the 2000s reissue 911 Tube Driver is similar to the original 1985-87 versions, but there were several component value changes across the circuit that affect the way it sounds, as well as different pots. Do they sound different from each other? Tube Drivers of different eras can sound slightly different from each other anyway, primarily because of differences in the way different tubes filter the sound, but also because of how the circuits were slightly revised over the years, and how the components have aged. There was a 1986-87 circuit (with two variations), a 1987 circuit (with two variations), a model 911 circuit board from the 1990s, and a reissue of the 911 that Butler started making around 2005. Each requires different settings to sound the same. However, when using the exact same tube, each can be dialed to sound nearly identical, with the exception of some of the 1986-87 models. If we look at the more common circuits from 1987, the LEVEL, HI, LO, and DRIVE knobs need to be set very differently to get the same tone on a 911 or 911 reissue Tube Driver. The photos below illustrate the differences using a 1987 Tube Driver and a 2007 reissue model 911 Tube Driver.

Light drive settings for the common 1987 BKB/Chandler Tube Driver and settings to match the same tone on a 2007 model 911 Tube Driver

Overdrive settings for the common 1987 BKB/Chandler Tube Driver and settings to match the same tone on a 2007 model 911 Tube Driver

The primary difference is much more low end available in the 2007 version than the 1987 version, so it can be dialed into much heavier, fatter tones. There is slightly more drive/gain available on the 2007 TD when set to 10 than most of the 1980s TDs. The 2007 TD offers drive/gain all the way from 1-10, whereas on most of the the older TDs there is no volume at all until the drive gets up to around 2. The 2007 TD also has "notched" pots, like the Tube Works versions, meaning you feel tiny notches click by as you turn the knobs. Notches 1-2 do little on the 2007 TD drive knob, then at notch three there is a huge jump in volume and drive.

Below are a few comparison clips. These clips are not the correct overdrive settings for Shine On You Crazy Diamond, just what I had set at the time I banged them out. The drive is set around 85%. Both TDs use the same GE brand 12AX7 tube. Strat into a fender Twin Reverb.

MP3 Sound Clip - BK Butler/Chandler Tube Driver from 1980s

MP3 Sound Clip - BKB Butler Tube Driver made in 2007

Shown Above - The original 1986-87 BKB/Chandler Tube Driver circuit with components on both sides. These can sound wildy different from unit to unit.

Shown Above - The second version of the BKB/Chandler Tube Driver circuit from 1987 with most components on the front side

Chandler TD

Shown Above - The simplified, cheap, and poor sounding Chandler Tube Driver circuit

Shown Above - A 1994 Tube Works model 911 Tube Driver


Shown Above - A 2007 BK Butler Tube Driver circuit with bias knob, which was a reissue of the 911 Tube Works Tube Driver, not the 1980's Tube Driver

REAL TUBE by TUBE WORKS - Other than one being in super cool enclosure, and the other in a butt-ugly enclosure, the 4 knob 911 Tube Drivers and 5 knob Real Tube versions are basically the same patented B.K. Butler designed Tube Driver circuit with some component differences. The Real Tube includes a mid range knob, allowing for a wider range of tones to be dialed in. In that way, it is actually a superior pedal to the 911 Tube Driver, and housed in a smaller enclosure. It is powered by an internal transformer and includes a power cord like the Tube Driver. There was a USA made version by Tube Works beginning in 1987, and a Taiwan version made when Tube Works was sold to Genz Benz in the 1990s. The only real difference between the two was less expensive, cheaper parts in the Taiwan version. Some of those Taiwan pedals were made with a 9v or 12v DC power jack on the back, rather than a built in power transformer and AC cord on the earlier models.

IBANEZ TUBE KING TK999US - This is another B.K Butler design, made by his Tube Works company for Ibanez. It is basically the 5 knob Real Tube circuit with a few changes and improvements, like an active tone circuit rather than the passive circuit in the Real Tube. Unlike the 911 Tube Driver and Real Tube, the Tube King includes an output buffer so it is immune to most capacitance and loading issues that can plague those other pedals. It also runs off a standard 9v DC power jack, not an internal transformer, so noise issues associated with the internal transformer in the Tube Driver and Real Tube have been eliminated. When using the same tube, it can sound 90% the same as the Real Tube or 911 Tube Driver. The pcb includes the Butler patent number 5022305. Note: there were two versions, one from Tube Works that was made in the USA, and one made in Japan by Maxon for Ibanez and Maxon branded versions that were sold internationally. The Japanese version does not follow Butler's circuit.

4 KNOB 911 TUBE DRIVER vs 5 KNOB REAL TUBE - Other than the mid range knob, do the Real Tube and 4 knob 911 Tube Drivers from the 1990s and 2000s sound any different? Tube Drivers can sound slightly different from each other anyway, primarily because of differences in the way different tubes filter the sound and circuit variations, but when using the exact same tube, the 911 and 5 knob Real Tube versions can both dial in nearly identical tones. The US made version of the 5 knob Ibanez Tube King is also based on the Real Tube circuit and sounds very similar.

The photos above show what the Real Tube and Tube King settings need to be to match the tone and drive of the 2007 reissue 911 Tube Driver settings on the right

Even when using the exact same tube, the same knob settings on the Real Tube and Tube King will NOT result in a matching tone and drive on the 4 knob 911 Tube Drivers built after 2005. To get the Real Tube and Tube King mid range to match the pre set mid EQ in the 911 Tube Driver you have to dial the MID knob down to around 9:00. The highs and lows on the Real Tube and Tube King are also notched a bit differently than the 911. The photos above show what the Real Tube and Tube King settings need to be to match the tone and drive of the 911 Tube Driver settings on the right.

DAVID GILMOUR TONES - David Gilmour has been an avid Tube Driver user since the 1990s. He first used them for the recording sessions for Pink Floy's Division Bell album in 1993. Those same sessions also resulted in Pink Floyd's The Endless River album, which was not completed until 2015. He used 911 Tube Drivers extensively for his 2006 album On an Island and the subsequent tour, heard on the Remember Than Night and Live in Gdansk concert releases, as well as his 2015 Rattle that Lock album and tour. I have a page about Gilmour's various Tube Drivers and settings HERE.

David Gilmour's 1994 Pink Floyd pedal board with two 1986-87 BKB/Chandler Tube Drivers labeled #1 and #2. A page about Gilmour's Tube Drivers can be found here HERE

ERIC JOHNSON TONES - Eric was the first well know user of the Tube Driver back in the 1980s. Listen to Johnson's Cliffs of Dover from his 1990 album Ah Via Musicom for an example of his 1980s Tube Driver into a 100w Marshall amplifier. Four different Tube Drivers have been seen in his recording studio. He owns one of the first Tube Drivers made by Butler, which is marked Chander Electronics - Patent Pend. on the front, no BK Butler. The words Tube Driver are marked with a TM, as the tredemark registration had not gone through yet.

Shown above (left to right) - Eric Johnson's 1985 Chandler branded Tube Driver, his 1986-87 period BKB/Chandler branded Tube Driver (with Chandler blacked out), and his 2005/2006 period BKB branded 911 Tube Driver

Shown above - two versions of Eric Johnson's pedal board with Tube Driverss. He mounts the TDs on a piece of wood so he does not have to bend down far to reach the knobs.

Eric primarily uses the TD as a booster for his 1969 Marshall 100w plexi amps and to add gain to his lead tone through a Marshall JTM-45 or JTM-100 into a Marshall 4 x 12 cabinet. He sets the tone controls off and the drive at around 11:00. This is the setup, as he described it in Guitar Player magazine: "The lead chain goes from a stock, late-’60s Italian-made Vox CryBaby to a stock ’80s BK Butler Tube Driver loaded with a Yugoslavian 12AX7. I set the Tube Driver up on a block because, for some reason, it sounds better set apart from the rest of the pedalboard (Eric later said this was becasue it was easier to reach the knobs). The signal then flows to channels 1 and 2 of a ’69 100-watt Marshall Super Lead. [Settings: Polarity Switch Up, Presence 0, Bass 5, Middle 3, Treble 0, Volume I 9, Volume II 10]. A Monster Cable connects the head to the straight-bottom Marshall 4x12, which is loaded with 25-watt Celestion Greenbacks."

JOE SATRIANI TONE - Joe was an early user of the Tube Driver. Listen to Joe Satriani's track Surfin' with the Alien from his 1987 album of the same name. Joe described his setup for that song in Vintage Guitar magazine: "I used a Kramer Pacer made from spare parts… The tone on the title track is so big, throaty, and dry. It was the Kramer into a Vox wah and a Chandler Tube Driver into a Marshall half-stack. We used an Eventide 949 Harmonizer for the pitch-shift effect."


Shown above - J Mascis 5 knob Tube Works Real Tube Overdrives with J's settings, and his four knob 911 Tube Driver.

J MASCIS DIRTY "CLEAN" TONE - J Mascis of Dinosaur jr is another Tube Driver user. He uses the TD for a slightly dirty clean tone in his live rig. It is his always-on sound. He has used the older 5 knob Tube Works versions, and the modern four knob BKB version. Both are shown above with J's settings. J was spotted using the modern four knob version on his 2011 board, into a Hiwatt Custom 100 and two vintage Plexi Marshall 100w amp heads, all with Marshall speaker cabinets.

Shown above - Joe Bonamassa's preferred Tube Driver is the Genz Benz 911 version from the 1990s, shown above

JOE BONAMASSA TONE - Joe Bonamassa is also a Tube Driver user, and had this to say on his website: "I have three Tube Drivers and they are all different. Try the Genz Benz one.. that is actually the best in my opinion. Also using a 12AT7 in them helps.." The Genz Benz version was made in Taiwan after Butler sold his Tube Works company to Benz. It can be identified by the 911 model number in the pedal graphics.

Shown above - Joe Bonamassa's pedal board with a 2006 era 911 Tube Driver

RECOMMENDED SETTINGS - Below are recommended settings from the Chandler Tube Driver Owners manual included with the pedal way back in 1988!



REPLACEMENT TUBE DRIVER KNOBS - The yellow capped knobs used on the reissue 911 Tube Drivers made since 2005 are unique to B.K. Butler, as he owns the tooling. You can't find those exact knobs unless you buy them direct from him. The various versions of the yellow capped knobs used on the older Tube Drivers were cheaply made and broke easily, and the the yellow caps were prone to falling off and becoming lost. It is very difficult to find these same knobs now. Below is a comparison of the different knobs used, and a modern RS Knobs replacement.

RS Components makes a very similar grub screw knob with separate caps, shown below, but it is slightly taller and wider than the TD knobs. also sells the same RS knob and caps. The caps come in several colors, including red, for those of you that want the cool look of David Gilmour's #1 Pulse Tube Driver. They don't exactly fit the older Tube Driver knobs or the smaller knobs used on the 2000s Tube Drivers, but can be modified to fit.

RS Stock No. 465-9397

Yellow Caps
RS Stock No. 465-9426

Red Caps
RS Stock No. 465-9432

Rapid Electronocs sells more accurate knob and caps, but slightly larger than the TD knobs. They are made by Sifam in the UK. They are D shaft knobs, so they would need to be drilled out to make round. Sifam discontinued the round shaft hole version, the TPN150, which I believe was nearly identical to the original Tube Driver knobs.

Knobs: Sifam DCN150

Caps: Sifam C151

MODIFICATIONS - Changing Tubes - This is the simplest, most effective mod for a Tube Driver. Some people find the distortion too rough sounding, even and low drive settings, and want a smoother, cleaner sound. Since the distortion is generated by the op-amp, not the tube, it would seem changing the tube would have little effect on the level of distortion. A lower gain tube will not necessarily reduce the amount of distortion, but it can smooth and tame it at medium to low drive levels. What it does is color the distortion. Here is what the original Tube Driver Users Guide said about the tubes:

You can change the tone of your 'Tube Driver' pedal with a very simple modification, which can be completed with only a screwdriver. To add gain, you can substitute a 12AT7 tube, which will give more distortion and beefs up the mid range frequencies. To have a cleaner, more vintage sound, you can substitute a 12AU7 tube, which will clean up the tone and thins out the midrange. This tube works very well for players seeking a cleaner tone in the lower settings of the drive control.

In typical power amplifiers the preamp tubes can easily last 15 years or more with moderate use and 10 years or more with very heavy use. Since the Tube Driver is a starved plate design, the tube will last much longer (even though the original user guide said they only lasted 2-4 years!). Chances are you will never have to replace it, but you may want to experiment with different tubes to see which sounds best in the TD to you. The tube is usuallu glued to the socket with a rubber silicon, but you can peel or cut that away. The stock tube shipped with Tube Drivers was alwasy a 12AX7, but different brands of tubes or tube types may alter the sound in different ways. Some people don't really hear any difference when swapping tubes, and others hear drastic differences. In my experience, the differences are more noticeable at high volume, and at high drive settings on the TD.

Shown above (left to right) - An original long-plate Ei Yugoslavian 12AX7 tube re-branded "TUBE DRIVER" that shipped with the original BKB/Chandler Tube Driver in 1986-87, the same tube branded "CHANDLER ELECTRONICS" that shipped with some 1987 Tube Drivers and the cheap Chandler branded Tube Drivers in 1988-89, and a short plate, unbranded Goldern Dragon 12AX7 that ships with the reissue Tube Drivers made since 2006.

Original BKB/Chandler branded Tube Drivers from 1986-87, and the cheap Chandler version from 1988-89, shipped with a long plate Yugoslavian Ei 12AX7 tube with custom branding. There was also a version branded Real Tube that shipped with the Tube Works 911 Tube Drivers in the 1990s and some early 911 reissues in 2006. Butler called the 1970 'NOS' YUGO tubes, meaning "new old stock" tubes made in Yugoslavia in the 1970s. He ran out of them in 2006.

The stock tube that ships with the 911 Tube Driver made since 2006 is a Golden Dragon 12AX7 - a low noise, hi-fi preamp tube. It is unmarked, other than BK written on it with a Sharpie. It created a bit too much distortion for the low drive settings of the TD for my taste, so I swapped tubes to a lower gain 12AU7. Lower gain should give a smoother sounding overdrive, but that's not always the case with different tube brands. The 12AU7 did not really reduce the gain from the drive pot, but made the distortion less fuzzy sounding so the individual strings in a chord had more clarity.

Very few factories make vacuum tubes the way they used to be made these days, so it is worth noting that certain NOS (new old stock) tubes can sound much better than most modern tubes. I suggest staying away from cheap, noisy tubes, such as the Groove Tube brand. The best modern tubes seem to be the ones made by JJ Electonics. I have tried various tubes, including an old GE 12AU7, Ei Yugoslavian 12AX7, and JJ Electronics ECC83/12AX7 in my current TD's. The 12AU7, being a slightly lower gain tube than the 12AX7, was slightly smoother and cleaner sounding at low drive settings in my BKB/Chandler TD, but there was a less noticable difference in my 911 Tube Drivers. I tried 12AT7s in the 911 and did not hear a big difference vs the 12AX7. I have a cheap Electro-Harmonix 12AU7 in my 5 knob Real Tube that sounds as nice as the NOS tubes in my 911 TDs, so even a cheap tube can sound good.

Below are the tube gain factors if you want to try for a heavier or cleaner sound than the stock 12AX7.

Approximate Gain Factors
100 = 12AX7, ECC83, 7025, ECC803, E83CC, 6681
70 = 5751
60 = 12AT7, ECC81, 6201, 6679, 4024
45 = 12AY7, 6072
41 = 12AV7, 5965
19 = 12AU7, ECC82, 5963, 5814, 6189

MODIFICATIONS - Changing op-Amps - As stated, most of the hard clipping distortion comes from the op-amp, not the tube. You may notice that the op-amp on the circuit board is mounted in a socket in the modern BKB Tube Driver, so you can swap out other higher quality op-amps to see if you like the sound better. The op-amp is the small black boxed IC chip with 8 pins on the left hand side of the circuit in the photos below. You can pull it out by hand and replace it with any other 8 pin dual op-amp chip. I tried a JRC4558D (used in the older TD), and another that I think was an OP275. Those did not improve or change the sound to my ears at all, but I did have good results using Burr Brown OPA2134 op-amps. It was noticably smoother at low gain and I liked it better than the stock TL072 chip that shipped with my 911 TD in 2007. It only made a minor difference however. In a blind test, most people would find it difficult to hear much difference.

MODIFICATIONS - Remove the Internal Power Supply to Reduce Hum - To avoid the noise issues caused by the internal transformer being so close to the audio circuit (described in the KNOWN ISSUES section below), the transformer can be moved outside of the TD enclosure. This can be done by removing it, enclosing it in a protective plastic enclosure, and running longer lead wires back to the Tube Driver. This is exaclty what Chandler did with their copy of the Tube Driver in 1988. This is what their Owners Manual said: By locating the transformer outside of the box we have been able to accomplish three objectives: Noise and hum is significantly reduced. Shock hzard has is practically eliminated. The Tube Driver now conforms to international UL standards.

You can also remove the transformer and add an AC power supply jack on the back of the enclosure, so it can be powered by 12vAC (not DC) external power supply. The power supply needs to run at least 12.6vAC at 200mA-500mA. First disconnect the existing internal transformer that is riveted or screwed to the front end of the enclosure. Note which leads from the transformer run to the circuit board. Drill a hole and install a 1/8" power jack on the back of the enclosure. The polarity of the jack does not matter like it does on a standard pedal running DC, since the Tube Driver runs off AC. Solder the leads from the 1/8" jack back to the circuit board. Plug the external AC power supply in and you are ready to go.

It is also possible to convert the Tube Driver to DC power so it can run from a standard pedal power supply, as is the case with the Tube King and some Taiwan made Real Tube pedals. It is more complicated however, and requires running the leads from the DC power jack before the filtering section and after the rectifier section of the circuit. It is best to have an electronics tech do that unless you have some experience with this sort of thing.

KNOWN ISSUES - Below are some of the well known quirks with this pedal that may cause problems for people with certain setups, and some ideas for how to correct them or deal with them in your signal chain.

Hum Noise - The primary issue I have heard about is hum noise. Some people have hum noise so bad that it makes the pedal unusable. I have heard this from owners of old BKB/Chandler Tube Drivers, Tube Works Real Tube overdrives, and the later 911 Tube Drivers. It sounds like 50-60Hz noise or a ground loop hum. There are no shielded leads in the circuit and the input jack ground wire runs to the pcb, but the output jack is grounded directly to the enclosure, with no separate ground wire to the circuit. It's not a grounding or shielding problem however. The hum seems to be caused primarily by the proximity of the internal transformer to the Tube Driver circuit, which is a big no-no for effect pedal circuits. Butler has acknowledged that noise issues in the older units were due to noisy transformers. The hum also seems to be more prevalent with people running 240v than 110v, but not always. I had one Real Tube pedal with a hum problem, but that actually turned out to be caused by a bad ground, so I have no experience with the larger hum issue myself. All the various version of the TD circuit I have owned or tried have been hum free, with the exception of a low buzz on one of the 1987 Tube drivers when using single coil pickups.

The TD runs off AC power, not DC like most effect pedals. It uses a built in transformer (12.6VAC at 200mA) and has its own AC power cable, unlike most other effect pedals which use an external power transformer, plugged into a jack on the pedal. For the 911 reissie TD's Butler has changed to lower noise toroidal transformers like Pete Cornish uses in his custom pedal boards, but I have read comments from owners who replaced the transformer with the exact toroidal that BKB uses, and still have noise issues. All of my TD's have the older type transformers and I have never had this type of noise issue myself.

The hum sound could also be caused by the AC current used for the tube heaters. Some owners have eliminated the hum by hooking up tube heater pins 4 and 5 and the step down resistor directly to the power transformer output, so it is off the main circuit board. Several websites have details on how to do this.

Output Impedance - There is also a signal impedance issue with Tube Drivers and Real Tube pedals (not so with the USA made Tube King). As a general rule for a pedal board signal chain, you want a low impedance output connected to a high impedance input on each pedal to allow the widest spectrum of audio bandwidth to pass through the chain in order to keep your signal from degrading. The Tube Driver has a buffered, high impedance input, but no output buffer. The passive tone circuit hangs a high impedance signal on the output, so changes in loading and capacitance from long cables or other pedals in the signal chain can result in a loss of bandwidth, altering the tone in a negative way. High volume settings can place a load on the Hi and Lo controls, so different volume levels can affect the output sound differently. With improper impedance, some users find that turning the treble up can result in a loss of bass. Not everyone encounters this, and it will vary depending on the Tube Driver settings, tube used, and the load from the other pedals your signal chain.

You can check if there is a tone difference by comparing the sound when playing the TD in your pedal board (with all other effects switched off) to playing with just the guitar into the TD, directly to the amp. You can also hear this by taking two identical Tube Drivers, plugged in back-to-back, directly to the amp. Turn one on and play it at high drive and volume, then swap places keeping the same one switched on, and you can hear how the high impedance output from one changes the sound when going into the other.

So how does one balance the Tube Driver impedance in the signal chain? Placing a buffered pedal or buffer/line driver with an input impedance around 1MΩ immediately after the TD, or non buffered pedal with a similar high impedance input, can help the issue, as can simply placing the Tube Driver in a buffered bypass loop that includes an output buffer. Using a lower gain tube like a 12AU7 can also help reduce the output impedance of the Tube Driver.

How Impedance Works - There is a good article on impedance over on the AMZ website. Here is a simplified explanation. Guitar pickups output a small alternating current (AC) signal that varies from around 10KΩ to 50KΩ, depending on pickups, volume, playing, et cetera. The guitar's output impedance goes higher at high frequencies and lower at low frequencies. A circuit's input impedance is the resistance to that AC signal. It sounds backwards, but we actually want a high resistance, or high impedance, on the input receiving that weak signal. A low resistance will lose power in the pedal circuit, making that signal weaker, so less of it gets to the amplifier. That is what is meant by "loading" the signal. Signal loss means some of the audio frequencies of your tone are lost.

On the other end of the pedal is the output impedance, or the resistance to the AC current leaving the pedal. High output means LESS signal current gets to the output jack, so some audio frequencies are lost. Lower output impedance is better, because MORE of the signal voltage gets pushed through to the amp or next pedal circuit, so more of your audio frequencies are retained.

A good rule of thumb for a well balanced pedal board chain is to have the input impedance of each pedal be about 10x larger than the output impedance of the pedal before it. A typical well designed pedal circuit will have an input impedance of 500KΩ - 1MΩ and an output impedance of 1KΩ - 10KΩ. Ibanez/Maxon pedals and some older Boss pedals have an output impedance of 10KΩ. A typical modern Boss pedal, for example, is 1MΩ in, 1KΩ out. The Tube Driver also has 1MΩ input buffer, but no output buffer. Specs for the Tube Driver show a "typical" output impedance of 10KΩ with a moderate drive setting, which is high, but it changes depending on the knob settings and can be much higher - as high high as 90kΩ with high drive and volume settings. That means the input of the pedal or buffered circuit following the TD should be around 900kΩ or higher. A typical input buffer on a Boss has a 1MΩ input impedance, and 1MΩ=1000KΩ, so that type of buffer is a good match to follow the TD. However, some pedals, including older Boss, Electro-Harmonix, and Ibanez pedals, have a low input impedance of 500KΩ or less, which means a loss of some of your TD signal and alteration of the tone if one of those directly follows your Tube Driver in the signal chain.

Faulty Footswitch - Another issue is that the foot switches BKB used in the 2007/2008, and some earlier Tube Drivers, were unreliable. The switches in my 2007 and 2008 TDs broke within a year of use and had to be replaced. I don't know if Butler has since changed to a better switch, but he was aware of the problem and was offering replacements at the time - unfortunately the replacements were the same brand of unreliable switch.