Dean Markley Overlord to B K Butler Tube Driver

So, I've got one of these and I've got to admit - I agree with the advert - it's ugly as sin but it does sure cook an amp nicely, but it could be better. It's the Dean Markley Overlord Tube Overdrive pedal. Using a REAL 12AX7 vacuum tube it creates realistic tube overdrive and distortion tones! It has some close relatives:

Now check out the schematic for the Overlord:

Now, as I stated earlier it has some close relatives, not only looks wise but circuit wise too. Here's the B K Butler Tube Driver:

Now there are plenty of variations on this design from B K Butler himself, Chandler and Tube Works among others. I took a look at various schematics with a view to modifying the Overlord to full-fill it's true potential:

Tube Works - Real Tube

Tube Works - Blue Tube

Tube Works - 303 Smooth

Chandler - Tube Driver (4 Knob)

And the newly updated Tube Driver itself (with the "Bias" mod):

New version B K Butler Tube Driver

And another version by Bajaman - The Real Tube Overdrive with a regulated power supply and simplified tone control:

Baja - Real Tube Overdrive

Now what I want to do is combine the best parts of each of these variations into one and modify my Overlord into the ultimate tube overdrive pedal... So, what am I planning to do:

1 - Make this thing True Bypass; the bypass switching currently employed by Overlord is terrible, a switch to true bypass will greatly improve the bypassed tone.

2 - Incorporate the "Bias" mod, here's a description of what the bias mod actually does from the B K Butler website:

"The Bias control is very helpful as it adjusts the amount of current applied to the cathode of the tube. Eric Johnson used to spend a lot of time looking for perfect tubes to use in his standard Tube Drivers, but now he can just set the bias easily with the stock tube. Just go up or down a few clicks and he nails what he wants. The Bias control also adds dynamics and allows you to custom match your Tube Driver for any particular pickup/amp combination.

Technically it varies the electrical 'strength' of the tube and that affects gain, texture and dynamics. It does not change the EQ directly but because the way the tube responds can be varied; you will hear EQ differences at various settings.

Most players find a 'sweet spot' about 2:30 - 3 o'clock on the control.

All in all, it is a 'pro tool' type of control that each player can set to their own subjective taste."

3 - Remove the in chassis power transformer and utilise a 16VAC adaptor with a modified regulated power supply a'la the Baja Real Tube Overdrive to reduce hum and circuit noise.

4 - Modify the stock component values to match the re-issue B K Butler Tube Driver. The "Mids" control will be sacrificed and replaced by the Bias control.

5 - Upgrade poor quality components to higher quality equivalents and use shielded wire for all signal carrying off-board connections to ensure this thing sounds as good as it can!

So after all that what am I going to end up with? Well, hopefully a super clean and cool version of the well known (and widely used) "Tube Driver" circuit. Check back here to see my build/mod report shortly :-)

Geofex.com - How to Measure Germanium Transistors Correctly

Germanium transistors - be them the work of God or the evil creations of Satan - people still love them for their Fuzzy goodness 30+ years after they went out of production and general use. So, lets say you've managed to get hold of some germanium transistors, they look all right, nice and shiny, but then you pop them in modern multimeter to measure their gain and numbers such as 300-500Hfe pop out. A quick check on their datasheet suggests they should only have a gain of 50-150Hfe, so why the huge reading on the multimeter? Leakage - germanium devices "leak" current from their collectors through their base junction and on to their emitters, this gives the illusion of the transistor being "on" even without any voltage/current at it's base to actually turn it on. Modern silicon devices don't suffer from this phenomenon nearly as much (at all) so the modern multimeters just disregard and presume there is no transistor leakage, this is the reason for that false gain reading - the meter just adds the leakage to the actual gain of the transistor.

So what's the problem with leakage? Well, a transistor is a "Semi-conductor" - in some conditions it is "off" and doesn't conduct and in others (when the base current reaches a sufficient level) it is "on" and does conduct. If there is too much leakage in a certain transistor it can't turn "off" and is essentially useless as a semi-conductor as it is always on!

So how do we determine the real gain and the leakage present in a germanium transistor? Well the great R.G Keen of Geofex has written a brilliant article (many years ago now) explaining just how to do this:

"Picking transistors for FF Clones


If you have a batch of germanium transistors, how do you tell which are going to sound good and which will not?  To a first order, you can just gain select them in a DMM that has a transistor checking range. However, all modern DMM's assume that the transistor being testing has no leakage at all. They just put a metered amount of base current in and look for how much collector current comes out. With germanium's inherently higher leakage, this just makes a leaky device look like a higher gain device.

This is one way to sift the leakage from true gain. You hook up a couple of resistors and a DMM to the device, and the resistors set up conditions you can control to see what is what. If you really want to do this, get a 2.2M resistor and a 2.4K ; better, get one each 2.2M and 2.49K metal film 1% resistors. This will set you back about US$0.30 if you get them from Mouser, and slightly more or less than that from other sources. If you're going to do much of this, get a transistor socket to, so you can easily test a large number of devices.

If you are satisfied with an indication of gain but are willing to settle for lower accuracy, you can carbon film at 5%, but recognize that the accuracy will be less. If you can, get several 2.4K resistors and measure them. You may find one that's closer to 2.472 ohms, which would be ideal. I'm being picky about the ohms because if you get exactly 2.2M and 2472 ohms, and use a 9.0V battery,  you'll find that the voltage across the resistor will be numerically equal to the indicated gain! That's why the somewhat odd resistor values, and the discussion on the values. It makes the final numbers on your DMM come out about right - multiply the voltage by 100, and that's the gain.

To do the test, stick the transistor in the socket, and read the DC voltage across the 2.4K resistor. The resistor will convert any leakage current from the transistor into a voltage that you can then read on your meter. A 2472 ohm resistor is 2.472 volts per milliamp, so a milliamp of leakage will cause 2.472 volts to display. That is incredibly too much leakage, so any transistor that does that is not going to be useful for a FF. In fact, although it will differ a bit, any transistor that shows more than a few micro amps of leakage is suspect. Because of the resistor scaling, the indicated value on your meter is "false leakage gain" and will have to be subtracted from the total reading that you do next.

To test the total gain, press the switch that connects the 2.2M resistor to the base. This causes a touch more than 4 microamps of base current to flow in the base. The transistor multiplies this by its internal gain, and the sum of the leakage (which doesn't change with base current) and the amplified base current. If the transistor has a gain of 100 and no leakage, the voltage across the 2.4K resistor is then (4uA)*(100)*(2472) =  0.9888V - which is almost exactly 1/ 100 of the actual gain. Pretty neat, huh?

But we know that germanium really does have leakage - that's why were doing this little dance in the first place. So, let's say that the device leaks 100uA to start with. We stick the device into the socket, and read the voltage before we press the switch. It reads (100E-6)*(2472) = 247mV. So the leakage is making the meter believe that there's a "gain" of almost 25 with no current into the base at all.

How much leakage is too much? 100uA is common, 200 happens pretty often. More than 300uA means the device is suspicious, and more than 500uA I would say is bad.

Let's say the device really leaks 93uA, and has a gain of 110 - a prime specimen. What happens when we test? We chuck the thing in the socket, and read (93uA)*(2472) = .229V. Then we press the switch, and read 1.330V. To get the real gain, we subtract 0.229V from 1.330V and get 1.101V. The true gain is just 100 times the reading.

Hey! How come it's 110.1, and not 110? Well, that's from this being an imperfect world, and from this tester being built with some approximations. The exact base current is 4.046...uA, assuming that the transistor's base conducts that much with a forward voltage of 0.1V (reasonable with germanium at these currents) and that the battery is *exactly* 9.0000V, and that the resistors are 2.20000M, and...   well, you get the picture. 0.5% accuracy is doggone fine for work with such blunt tools, and much better than you actually need to make a fine sounding FF. Besides - if you're clever, you'll flip the switch and watch the voltage while you  put your finger on the transistor. Simple finger heat will make the gain rise rapidly. What's the real gain? All of them are - at the temperature and conditions of the moment."

This instructional taken from the Geofex article "Technology of the Fuzz Face": http://www.geofex.com/article_folders/fuzzface/fffram.htm

Fred Briggs - Tourbox #1

Here's a couple of images of the pedals that will soon be whizzing their way around North America as part of the first tourbox I've ever done! On the left is the "Psychadelia Fuzz", a sweet overdrive/fuzz which can produce an awesome nasty octave/ringmod effect. On the right is the "Rangemaster MKII", my take on the classic Rangemaster Treble Booster. With the addition of darlington paired germanium and silicon transistors, and "Filter" and "Aggression" controls it has a huge tonal range. Centrally is the "Unknown Fuzz", a more traditional fuzz box that uses piggy-backed silicon transistors to emulate the warmth of germanium transistors without all the draw backs!

Fred Briggs - Rangemaster MKII Build

The second steel enclosure has been used - and here's the outcome. I built a version of the Rangemaster I call the "MKII". It uses a germanium and silicon transistor in a darlington pair to open up a huge amount of options. This has been done before as Pete Moore showed with his "RangePig" treble booster, I just chose to expand on the idea and add a few further modifications and refinements;

- a "Filter" contol to slide between a full range and treble boost, 

- an "Aggression" control to allow you to blend in the mellow germanium tone with the more aggressive silicon sound 

- and a Bias trim pot to allow you to dial in just the right voltages on those transistors to ensure you get the very best tone from them). 

Anything from standard Rangemaster treble boost right on to almost a fuzz faceish boost is available with this circuit. With the inclusion of all the modifications and refinements it becomes a very versatile booster pedal and a worthy addition to your pedal board.

And here's the schematic:

You Tube Tutorials - Tone Bender MKI & Echoplex

Here's some interesting youtube videos for you. First up - a couple of videos on how to build your own Tone Bender MKI replica:

Echoplex restoration: