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B.K. BUTLER TUBE DRIVER - Page 2

©Kit Rae. Article written in 2007. Updated in 2010, 2011, 2013, 2015, 2016, 2017

TUBE DRIVER 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.

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