Dirty Tube

Authentic tube overdrive from a tube

The GT Dirty Tube overdrive rack effect module compares to the slightly different versions of the Tube Driver pedal from Chandler, Butler, and Baja. Eric Johnson, David Gilmour, and others have used the Tube Driver to create their epic tones.

Description

If you want great tube tone, then why not use a tube effect? As you may know, that's exactly what Eric Johnson, David Gilmour, and numerous others have done. They use a popular tube pedal called the Tube Driver. If you know about the Tube Driver, you'll know there were different versions of it made by different companies. There's some debate about which is best, but technically they are all very similar. Differences between the individual tubes (every tube is slightly different) likely account for as much difference in the tone between different Tube Drivers than the design differences do, although it is a mixture of both. The GT Dirty Tube compares to the various versions of the Tube Driver (Chandler, Butler, Baja). But we've added some great features for players and tweakers. You can choose to stay closer to the original pedals, or select some extra features that add to usability and allow you to dial in tonal options that aren't available in the originals.

The original pedals, as well as the available derivative versions of the circuit, tend to have differences in three areas. First are some very small, but definitely audible, differences in tone shaping through the op amp portion of the circuit. The second is a difference in how the tone controls work. The third has to do with how the tube is powered. Our Dirty Tube allows you to select your preferences for the tone shaping portion. Our method of powering the tube is unique, due to how we power our modules, but allows us to run the tube at high voltages, instead of the starved voltages used in some pedals. The original Tube Drivers used wall power to get those voltages. We don't have to do that. We also have a single tone stack we offer. It is like the originals that have the Hi/Lo tone controls. It is based on a passive Baxandall tone stack. That tone stack is cool. At noon on both knobs, you get flat response. The Hi knob only affects the hi frequencies. And the Lo knob only affects the low frequencies. You can easily dial in mid-scoop or mid-hump tones, by either turning both controls up or both down.

The main feature we've added is the ability to have two footswitchable channels, each with their own Gain, Hi, Lo, and Volume controls. We all want two channels on our tube drive effects! The channels can be identical or tweaked to be slightly different. But with separate controls they can be adjusted independently so you can switch between two different tones on the fly. In addition, we have some options for switchable tone shaping for each channel. Of course, there is an optional Bias control for the tube. It lets you adjust from full and rich to starved and splatty. The results depend on your tube. You may have noticed in the picture above that there are also Cut controls for each channel. They are meant to help shape the gain after the op amp gain but before the tube gain. They don't really have the impact we were looking for, so we are dropping the Cut option. If you are interested, let us know as we may still have some boards that support that option. Or we may tweak it in the future to have more effect.

At low Gain levels, you can add some tube thickening to your clean and low drive settings. Or you can set Gain low and Volume high to get a nice tube boost. Cranking the Gain up will put you into tube overdrive and then distortion territory, with easy feedback on tap. Of course this will be stacked with your amp, so you'll be getting your final result depending on your amp and amp settings. You can also control the Gain a good amount with your guitar volume control. Touch sensitivity is pretty good, especially as the Gain is increased.

You can vary the tone considerably with your choice of tube. The usual tube is a 12AX7, but even among 12AX7's there is a considerable variety of gain, noise, and tonal differences. Or you can select 12AT7, 12AU7, 12AY7, or others to reduce gain for better clean and OD tones with less distortion. Any of those tube types, including 5751 and other 12AX7-compatibles can be tried. You can order your module with or without a tube, depending on the tubes we have in stock or what you may already have.

This next part may be a little confusing, but let's give it a try. Our two-channel design has considerable flexibility in how it can be configured. At the top level, you can think of our circuit as having three sequential sections. First is the "pre-tube" section. Then the tube. Then the Tone section. There are two pre-tube sections for the two channels. There is a single tube section that is shared between the channels. Then there are two Tone sections, again for the two channels. Two channels give you separate independent controls for each channel. But you can select which controls you want in each channel and which you want to share between the two channels. A little more detail is needed to explain.

The pre-tube section contains a switch for selecting the channel Voice, the small tone-shaping differences between the different versions of the circuit (Chandler, Butler, Baja, Bass). It also contains the Gain control. The tube section is shared between the two channels. The only control it has is the optional Bias control. So one Bias setting applies to both channels. The tone section has the Hi and Lo tone controls, as well as the Volume control. We can build the circuit with one or two pre-tube sections with the same or different choices for options in each of the two, or shared controls if there is only one pre-tube section. We can also build the circuit with one or two Tone sections, either sharing or duplicating the tone and volume controls. If we build it with only one pre-tube and only one tone section, then it is a single channel module. To have two channels, you must have either two pre-tube sections or two tone sections or two of both the pre-tube and tone sections. Each of those sections has its own controls and options, so if you want certain controls/options to be separate for each channel, you have to have two of those sections of the circuit that have those controls/options. Hopefully that is enough info for you to understand what independent and shared controls you can have in a two-channel configuration. It's actually not that difficult, just detailed.

Why would you want to share controls in a two-channel configuration if you can easily have them be independent? The answer is because if you choose all the controls/options and make them independent between the two channels, you'll end up with about a dozen front panel controls, making a much wider module, taking up more space in the enclosure, and costing more. That may in turn push you into needing another enclosure. And that, of course, means 3U more rack space and some additional cost. A 1.5" module can have 4 controls. A 2" module can have up to 8 controls. A 3" module can have up to 12 controls. Of course, it is your choice!

We offer two standard configs, but you can tweak them to your desired config with the available options. Since this can be a little confusing for this module, just drop us a line and we're happy to help you get the config you want and make any trade-offs you want in sound, size, and cost. If you have a preference for a particular version of the circuit, look carefully at our option descriptions below. There are several differences between the different versions that we think make little or no difference in the resulting tone. For those, there are no options - we build them that way, which should not matter. The other differences between them are captured in our options. Just select options to get what you want. Some of them come at no additional charge.

A few words about tubes…

First, they have to warm up and also break in. It takes a few seconds for them to warm up. Until then, you'll get no sound. So plug the module in and power it up a few seconds before you need it. It doesn't have to be switched to have the effect on, just powered up will do it. Tubes also take a while to break in and give consistent tone. That should happen after a few hours of being powered up. Some capacitors also take a little time to break in. The cap break in time probably won't be noticed, but the tube break in might well be audible. So give each tube a chance to impress you before you switch to another tube!

With tubes, you can pay a bit more to get better tone, or at least different tone. Yeah, there's the difference in price between, for example, a new JJ and a vintage Mullard, and that might be worth considering. But there are other things that may be worth considering first. It used to be that tubes were sort of luck of the draw - you got what you got. But now you can get very well-tested tubes and choose your specs much better. Here are some considerations.

Type of tube. There are several models of pin-compatible tubes, including 12AX7, 12AT7, 12AU7, 5751, etc. There are also diffferent European vs British/American names for them. Those models have some different properties, but a key one is usually their gain range. Even within a type, such as 12AX7, you can get tubes tested to have different gain ranges. If you want high gain, get a tube with a higher gain rating - duh. If you prefer cleaner boost and low drive ranges, get something with a lower gain. We generally get tubes rated for higher gain, since our controls let us manage the gain. Having higher gain available gives us more options. If we really want lower gain, then we get a tube model that has a lower overall gain range, but still get one nearer the top end of that lower gain range. No rules here - just preferences.

Noise. Higher gain levels will amplify noise levels - that's just how it goes. However, some dealers will check for actual noise levels, not just spec noise levels, in their tubes. Gain isn't the only factor. Individual tubes can have varying levels of microphonic noise caused by vibrations within them. Some can be so noisy as to be useless, others much more quiet. It's always a good idea to shop for a known quiet tube. The dealer may not be able to tell you how quiet they are, but they can separate tubes into more quiet and less quiet, and that's usually worth paying extra for.

Balanced. All the tubes that work in this module are really like two tubes in one. They have two gain circuits. We cascade those two gain circuits and share the result between the two channels in our module. Each of those circuits has its own gain value. So in the above section that discusses gain, you might see gain specified as two numbers, one for each channel, if they are actually measured. There is a wide spec for acceptable levels of gain, so the two circuits could be very different, the same, or somewhere in between. If you get a balanced tube, the two halves will have the same or very close gain values. Both values matter in this module, but they do not need to be balanced as long as you are getting the overall amount of gain you desire. However, we are suspicious of tubes that are not balanced or at least reasonably close to the same on both sides. Worn out tubes will often get low on one or both sides. But, as long as they are both in spec, they should be OK. We still prefer balanced because we swap tubes around and balanced tubes sometimes work best for other purposes, like in some positions in amps.

Brands and models. Yes, different brands and models do sound different, even if you get say, low noise, medium gain, and balanced tubes. Trying different tubes can get out of hand, particularly if you start trying expensive vintage tubes. That's up to you. We have brands and models we like for different effects, amps, etc, but those change with our preferences over time. We have a decent selection of new and vintage tubes we've tried for different purposes. But be careful, it is easy to get fakes - very easy. When it is all said and done, what works best for us is choosing a good dealer that does accurate testing on their tubes so that we can choose brands/models/specs reliably to match our needs. There are current production tubes that are very good. We don't have any relationship with any tube dealers, other than we buy tubes just like everyone else does. Eurotubes is a great place to get JJ tubes of all models and specs. Tube Depot and Tube Amp Doctor (TAD) have also been great for us. Our demo Dirty Tube has a TAD balanced 5751 in it which sounds great. If you do get a tube that doesn't suit you, hang on to it. Chances are good it'll work for something else.

Break in. Sometimes you can get tubes that have been left in the testing machines for long enough that they are already broken in. Normally, the break in time isn't a big deal to us. We just do it and go on with life. However, it is probably true that if you are looking for particular specs in your tube, you might find that spec readings taken before, during, and after break in could vary. I don't really know how much, but sometimes as you break in tubes, you can tell the difference. Some of the difference may be related to caps and other components also breaking in, so there is no really clear way of sizing this up. Just sayin'...

Configurations

Dirty Tube, 1 channel - You can tweak the configuration, but the standard 1-channel config comes with:
- "Thicker" tone shaping caps
- Gain, Hi, Lo, and Volume controls
- Internal Bias trimmer
- TL072 JFET op amp
- JJ 12AX7 tube
- 1.5" module width
Dirty Tube, 2 channel - You can tweak the configuration, but the standard 2-channel config comes with:
- Channel 1 - "Thicker" tone shaping caps
- Channel 2 - "Thick" tone shaping caps
- Channel 1 - reduced Gain control
- Channel 1 - Gain, Hi, Lo, and Volume controls
- Channel 2 - Gain, Hi, Lo, and Volume controls
- Internal Bias trimmer
- TL072 JFET op amps
- JJ 12AX7 tube
- 2" module width

Note that regardless of configuration, each channel can be further configured to compare to either the Butler, Chandler, and/or Baja versions of the Tube Driver, as well as a GT bass version. See the options below.

Options

Two Channels (Pre-Tube only) - Build a config with two separate pre-tube sections. Select One Channel configuration and add this option.
Two Channels (Tone only) - Build a config with two separate tone sections. Select One Channel configuration and add this option.
Two Channels (Full) - Build a config with 2 separate pre-tube sections and 2 separate tone sections. Select Two Channel configuration to get this option.
Toggle Channels - By default, you will get a second footswitch input to control the channel selection if you have two channels. If you prefer, we can make that a front panel toggle switch instead. It takes up extra front panel space but doesn't use the second footswitch on your footswitch unit and it is a bit cheaper.
Tone Shaping Caps - There are two of these Voice capacitors in each pre-tube section. One at the input of the circuit and one between the op amp and tube sections. You choose the values for both caps. For the input cap, choose "Thick" if you prefer the Butler or Baja value. Choose "Thicker" if you prefer the Chandler or GT value. Choose "Bass" if you want more bass. For the second cap, choose "Thick" for Chandler, Butler, or Baja. Choose "Thicker" for the GT value. Choose "Bass" if you want more bass. The "Bass" caps are more for bass than guitar but will work with either. However, the more bass or thickness you select for these caps, the more chance of losing some clarity as you turn up the Gain. If you are keeping the Gain low (maybe a clean/crunch channel), then Thicker may be a good choice since that'll give you a bit more bass with the reduced gain. If you want a distortion channel, you may prefer the reduced bass of the "Thick" channel. "Thick", "Thicker", and "Bass" are just our labels for least to most bass. The three brands of Tube Driver (Chandler, Butler, and Baja) use a mixture of the "Thick" and "Thicker" values in the two positions.
Switched Tone Shaping Caps - Can't decide which caps you want? This option may be for you. Get all three settings, "Thick", "Thicker", and "Bass" on a switch. These will give you three fairly distinctive settings to select from. But if you want to save some money and space, remember that the Tone stack also covers a lot of Tone settings. The main difference is that these caps come ahead of and in the gain stages, while the Hi/Lo tone controls come after all the gain. You can do this for one or both channels.
Custom Gain Controls - The originals all used the same Gain control, which covers from clean-ish to distortion, and makes a good default. But the wide range of the control may make it a little difficult to find your exact settings. If you want a clean channel, we can put in a control with a lower maximum level of gain. That makes it easier to dial in lower-gain sounds. We can also put in a higher gain control for those that need every little drop of gain. You won't notice much difference, and you may get some more noise or squealing. You can select this for each channel. You may choose to set it up like an amp, with Channel 1 being lower gain, maybe more bass, and Channel 2 being more gain, maybe less bass. Your choice!
Custom Op Amp - By default we use TL072 JFET op amps because they are readily available, don't color the tone too much, are quiet, and are tubey as they distort. The originals used either 4558, TL072, or TL082 op amps. You can choose any 4558-compatible op amp. You can do this for each channel.
Custom Tube - We provide a JJ 12AX7 by default. You can select something different, or we can ship it without a tube and you can use your own. We don't mind ordering tubes that are generally available, but we'll let you hunt down those rare, used, or vintage tubes.
External Bias Control - By default, you'll get an internal Bias trimmer. If you think you'll want to adjust Bias frequently, then opt for the external Bias control. Mostly you might want to adjust Bias if you are trying a different tube, then set and forget it. If you are trying a different tube, you'll obviously have your enclosure open to do that, so you can access a trimmer then and set it for the new tube. Or put the Bias control on the front panel. This is probably one of the best available options, but it may cause you to have a wider module. If you have space, it's a good one to consider.
Tone Control - Some of the early Tube Drivers had a simple high-cut Tone control instead of the Hi/Lo tone stack. Our boards don’t really support this option properly, but we may be able to build it this way. Check on availability as our boards sometimes change. The Hi/Lo stack is better, but it does take up more front panel space if you have separate tone controls for each channel.
Tweaked Tone Stack - You can probably ignore this option, but we'll put it out there for you Super Tweakers! We use a passive Baxandall tone stack, as do many of the original pedals. There are numerous ways to select component values to change the way the controls respond. If you're just dying to get that special configuration tone stack, here's your chance. We recommend that you Google "Duncan Tone Stack Calculator" and use the "James" model to design/verify what you want.
Other Tweaks - There are some other minor tweaks we can make in the circuit for those of you that know the circuit and have *very* particular needs. Pull-down resistors and noise caps are in this category. Let us know what you have in mind and we'll let you know if it'll work with our board.

Front Panel

On/Off indicator LED
Volume
Gain
Hi
Lo
(optional) Channel 2 On/Off indicator LED
(optional) Channel 2 Volume
(optional) Channel 2 Gain
(optional) Channel 2 Hi
(optional) Channel 2 Lo
(optional) Bias
(optional) Voice switch(es)
(optional) Tone(s)

Rear Panel

Audio In
Audio Out
On/Off Footswitch
On/Off Override
(optional) Channel Footswitch
(optional) Channel Override
DC Power
DC Power LED

Module Width

1.5" to 3" module width, depending on options
left or right wing module configuration may be available, depending on options

Power Consumption (aprox)

Note that this module draws considerably more power than most of our others. It will pull a few hundred mA. You can power it with our 18V Power module with no problem. Just balance the two power channels as best you can. You can also power it with a 18VDC regulated power supply. Look for one that can give you somewhere between 750 - 1000mA (more is better). Also, if you are poking around inside, be careful with the heat sink on the power board. It gets hot. If the voltage regulator and heat sink get too hot, the power will shut down. You'll hear the effect fade out and then stop working. If it does that, let it cool off and let us know. We haven't had any problem with this yet, but it may be possible and require a different power configuration if it becomes an issue. When you first power it up, it will draw 400-500mA very briefly as it charges capacitors, then it should settle down to around 275-325mA, well within specs for that voltage regulator that gets hot.

Base Configurations

Part #	Description	List Price
MOD-DTUBE1	Dirty Tube Module, One Channel	$265
MOD-DTUBE2	Dirty Tube Module, Two Channel	$335

Options

Part #	Description	List Price
MOPT-DTUBE-PT	Two Channels (Pre-Tube only)	$50
MOPT-DTUBE-T	Two Channels (Tone only)	$50
MOPT-DTUBE-TC	Toggle Channels	$-39
MOPT-DTUBE-TCAPS	Tone Shaping Caps	$0
MOPT-DTUBE-STCAPS	Switched Tone Shaping Caps (per channel)	$19
MOPT-DTUBE-CGC	Custom Gain Controls	$0
MOPT-DTUBE-OPAMP	Custom Op Amp	$0 - TBD
MOPT-DTUBE-TUBE	Custom Tube	TBD
MOPT-DTUBE-NOTUBE	No Tube	-$20
MOPT-DTUBE-BIAS	External Bias Control	$19
MOPT-DTUBE-TONE	Tone Control	TBD
MOPT-DTUBE-CUSTTONE	Tweaked Tone Stack	$0 - TBD
MOPT-DTUBE-TWEAK	Other Tweaks	TBD