4-Knob Tube Driver Build.

[DrNomis_44]

Hi Everyone,


Been seriously thinking of building myself a 4-Knob Tube Driver pedal, I've built one a few years ago but found that it was a bit of a pain to cram all the circuitry into the small Diecast Aluminium box that I normally use to build my DIY pedals, this time round I'm going to build it into a bigger box like the one I used for my Doctor Overdrive pedal.

So, what is a 4-Knob Tube Driver anyway?, it's a distortion pedal that uses a real 12AX7 high-gain Twin Triode Tube/Valve to produce that distinctive overdriven Tube/Valve sound that lots of guitarists seem to like a lot, the circuitry is very similar to the circuitry in my Baja Real Tube Overdrive pedal, except that it has two tone controls, one for the Lows and one for the Highs, in addition to a Drive control and a Level control, the sound is similar to the Baja Real Tube Overdrive, but it has less mids in it.

Next fortnight I'm going to be working on getting all the parts for the 4-Knob Tube Driver together, once that's all sorted I will be documenting the build process in this thread with a write-up and series of build process pics so you can see how I go about building it, I will also include a hand-drawn circuit diagram together with an explanation of how it works.

Stay tuned for updates.

[Andy40]

Sounds great Doc. I am going to have a go at the Greanbean tube screamer soon. Going to get the parts and breadboard it out. However, I'll have to draw the PCB for etching (EEEK!) more learning stuff.

[DrNomis_44]

Sounds great Doc. I am going to have a go at the Greanbean tube screamer soon. Going to get the parts and breadboard it out. However, I'll have to draw the PCB for etching (EEEK!) more learning stuff.

Tube Screamers are always good DIY projects since the circuit is relatively simple, yet still sounds good, I think I remember seeing a Vero/Stripboard Tube Screamer layout on the internet somewhere, if my memory serves me right most Madbean pedal projects usually feature PCB artwork that you can use with blue press and peel film, you use a standard laser printer to print the PCB artwork onto the press and peel film and then you transfer it onto the copper foil of the un-etched PCB material, using a standard electric clothes iron, the idea is to use the black toner powder from the laser printer as the etchant resist to shield the wanted copper foil from the etchant, Jaycar Electronics normally stock both the blue press and peel film and the etchant, I actually used that method to etch a PCB for my Univibe pedal, and got pretty good results, I used an etchant called Ammonium Persulphate which Jaycar do happen to stock, the laser printer I used was an old HP type (I think, will have to have a look for it later on today).

Actually, since I've had some experience with it, I should do a thread about using blue press and peel film to etch PCBs for DIY pedals, the process is pretty simple.

[DrNomis_44]

I might consider having a go at designing a PCB layout using a bit of software called DIY Layout Creator this time around rather than just do a Veroboard layout, for this pedal build, I'll have to download the software and refresh my memory since it's been a while since I last designed a circuit board with it though, if I manage to design a good layout I'll definitely make it available for use by forum members.

[DrNomis_44]

Just doing some googling about the Tube Driver, there's lots of websites about it so I thought I'd post a link here for you guys:

https://www.google.com.au/?gfe_rd=cr...er+Tube+Driver

[DrNomis_44]

Okay, so let's have a look at the Tube Driver circuit, here's one I found on the internet, but it's for the 5-knob version:




There's only a very minor difference between the 5-knob Tube Driver circuit and the 4-Knob Tube Driver circuit, in the 4-Knob circuit, the two cathodes of the 12AX7 Valve are connected directly to the V-- supply, other than that the two circuits are functionally the same.

[Andy40]

looks complicated doc I cant zoom in on schematic, not that i would understand it. Im keen to see how it works out.

[DrNomis_44]

looks complicated doc I cant zoom in on schematic, not that i would understand it. Im keen to see how it works out.

It's actually quite a simple circuit, and not really as complicated as it looks, I think that's probably because of the power supply circuit, I'll do a write-up about how the circuit works soon.


I tried to find a circuit diagram that would let the viewer zoom in but the one I posted was the best I could do at the time, perhaps if I do a hand-drawn version and scan it into my computer, that may sort out the zooming in, I'll get that sorted out soon.

[DrNomis_44]

Okay, here we go with a discussion of how the Tube Driver circuit works, the Tube Driver circuit itself is made up of five circuit building blocks, and I will go through and explain what they do and how they work, I'll try and keep it free from any complex maths so it's easy to understand:


Input:

This consists of a 10k resistor in series with a 33nF (.033uF, 333) cap,with a 1M resistor going from the 33nF cap to circuit ground, there's also a 47pF cap parallel with the 1M resistor, which is there to filter out any radio-frequencies from the input signal, there's also a couple of diodes connected to the 33nF cap, the 1M resistor, and 47pF cap, and these two diodes connect to the +V and -V supply, what are they for?, these are input protection diodes for the next stage, all the parts in this circuit block form the input of the Tube Driver circuit.

Input Buffer:

The input circuit block is connected to the + input of an Op Amp (the small triangle symbol) which forms a Buffer, this is to convert the high-impedance of the input to a low output impedance so that it is not loaded-down.

Op Amp Amplifier Circuit:

The signal from the Buffer, and passes through a 22uF cap and then onto a resistive divider-circuit formed by a 680 Ohm resistor in series with a 1M resistor, this slightly reduces the signal from the Buffer, next the signal passes through another 22uF cap which is in series with another 680 Ohm resistor, and then the signal goes to the - input of the second Op Amp, you'll notice that this Op Amp has it's + input going to circuit ground so that it has a ground-reference, there's also a 500k Log pot connected from the Op Amps output to it's - input, and there's a 120pF cap in parallel with it, the cap is there to prevent the Op Amp from self-oscillating at high gain-settings, the 500k Log pot in conjunction with the 680 Ohm resistor on the Op Amp's - input ultimately determines how much gain the Op Amp produces, basically, the Op Amp is being used as a clean-ish booster to boost the signal to the point where it starts to overdrive the next circuit block, which is....


Two-Stage Valve Amplifier:

This circuit block is formed by two sections of a 12AX7 High-Gain Twin Triode valve, the signal from the Op Amp Amplifier Circuit is applied to another resistive-divider (formed by two 10k resistors) via a 10nf cap, and then is applied to the grid connection of the first Triode section, this signal appears on the anode of the first Triode section, but it is much larger and inverted compared to the signal on the grid, next, the signal passes through the 10k resistor in series with the 47nF cap before being applied to the grid of the second Triode stage, when you increase the gain of the Op Amp Amplifier, the signal will cause the first Triode stage to start clipping on one half of the signal-waveform, and then the second Triode stage will start to clip the other half of the waveform, the two 47k anode resistors both supply current to the anodes and also set the gain of the Triode stages too, the signal from the second Triode stage is then applied to the next circuit block.

Tone Stack:


All the parts in this circuit block form what's called a tone-stack, in this case we have a tone-stack with two controls, one controls the Lows, and the other controls the Highs, the signal from the two Triode stages passes through the Tone Stack and then end up at the 100k Log Level control pot, this gives us a way to control the output signal level.

The two Triode stages in the Tube Driver circuit are what generate most of the overdrive/distortion in the circuit, the Op Amp Amplifier does contribute some distortion at high gain settings too.

[DrNomis_44]

Update:


I've decided to build two versions of the Tube Driver, there's the 4-Knob version and there's also a 3-Knob version which uses a different circuit that runs off a single power supply, the original 3-Knob was actually designed to run off a 9V DC supply, I'm going to see if I can optimize the circuit to run at a higher voltage, like +18 V DC since my pedal power supply can supply a regulated output at +18V DC, but the two Tube Driver pedal build projects will have to wait till I've finished paying off the new Fender USA Strat I've got on layby.