Superlead preamp build

Discussion in 'Luthiery, Modifications & Customizations' started by jarnozz, Oct 11, 2017.

  1. jarnozz

    jarnozz Let´s Get Wasted!

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    Hi there!


    I haven't been active on this forum for a while now. As far as I remember, there were some people interested in building amplifiers and electronics in general. I am an electrical engineer and work with digital electronics, embedded design and digital signal processing. A hobby of mine is analog design and music related gear in general! During my absence here I have made a few very cool projects. One of them is the Superlead preamp.


    spoiler: I won't submit the schematic since it's an original design and I am designing an amplifier that uses this preamp design! Who knows what the future will bring.. I will explain why I made certain decisions and how I designed the preamp. When I talk about potentials it's about voltage. (boys will call it a voltage drop, men will call it a potential difference two points expresses in the unit of volts. My old professor said that phrase a lot..)


    Let’s start with a little lesson in tube amplification in general. I will be using Merlin’s (aka the valvewizard) images to help explaining the subject. Calculations and values taken from graphs are not 100% correct but will work 100% of the time. I could give accurate calculations and formulas that will make your head spin. These are too difficult to explain and understand without knowledge in electrical engineering.


    Let’s take a single stage 12AX7/ECC83 tube as an example.


    The 12AX7 consists of 4 parts. The heater, anode, grid and cathode. The heaters function is to heat up the cathode of the vacuum tube. A chemical paste, that releases electrons when heated, is applied to the cathode. The cathode of the tube is heated to around 777 degrees Celsius (1430.6 Fahrenheit for the American reading this). When the cathode is heated it will form a cloud of electrons called space charge (yes, space charge is really a thing in engineering). All these electrons are negatively charges. The anode of a vacuum tube is put at a higher potential as the cathode. This positively charged plate inside the tube will attract the electrons. This will cause a constant movement of electrons if the signal applied to the grid is a stable DC potential. The other term for electron flow is current flow. It appears that the flow of electrons moves from the lower potential to the higher potential, in other words from negative to positive (They got that wrong when inventing electricity. Everyone though electrons flowed from high to low like nature.). The electrons travel a very small distance but reach a velocity of 5927KM per second when they strike the anode. ( 13.258.321 miles per hour)


    Now let’s take a look into amplification.


    If we apply an AC signal to the grid of the vacuum, we will see the same AC signal, but 180 degrees shifted in phase, appear on the anode. This happens because applying a current to the grid will alter the flow of electrons. The flow of the electrons is will decrease if the grid is made negative. This will cause less negatively charge electrons to reach the anode. This in turn will raise the DC potential at the anode. The same happens when the grid is made positive. This is also how a vacuum tube used to be biased. A different power source would apply a constant negative potential to the grid of the vacuum tube. You will not find this guitar amplifiers because cathode bias is in 99.9% of the cases applicable and cheaper.




    The cathode bias takes advantage of the constant flow of electrons if no signal is applied to the grid. The grid can now be put at ground potential with a 1meg resistor. Let’s assume the constant current flow is 1mA. Now let’s place a 1K5 resistor in between the cathode and ground. Because current flows through this resistor, a potential will be created. When 1mA flows through a 1K5 resistor a potential of 1.5V will be created. The tube will see the cathode as a ground potential and the grid as a -1.5V potential.




    Knowing your amplification can be done with the use of load lines found in the datasheet of the said tube. These graphs will have the plate current (Ia) on the horizontal axis and the anode potential in the unit Volt (V) on the vertical axis. Let’s take a 100K anode resistor with a source potential of 300V. When these is no anode current, the plate of the tube will be the same as the source potential. If the current will be maximal (300/100K=3mA) the anode potential will be zero volts. Now draw a line between these points and work from there.




    The moment everyone has been waiting for… Distortion. This rather interesting phenomenal happens when the grid curves are not spaced evenly.




    To avoid headaches, even harmonics are derived from the fundamental harmonic and will make the signal sound fuller and warmer, while odd harmonics make the signal sound rather fuzzy but also more aggressive and tighter when added in the right amountJ.




    I won’t talk about intermodulation distortion because that is a quite difficult subject so here is a graph!




    Another loved topic is clipping. Clipping happens when the grid of the tube exceeds its operating limit. The designer’s choice of components will determine when clipping occurs. There are two types of clipping, namely warm and cold. Warm clipping is caused when the grid of the tube is made negative beyond the linear region and unequal spacing of the grid curves occurs. (In the HIFI world we stay within these boundaries to keep linearity). The anode of the tube cannot get higher than the source potential so the signal gets clipped. This does not happen abrupt like with transistors, but exponentially.




    Cold clipping is caused when the grid of the valve becomes positive. The anode potential will drop to the same potential as the cathode and cannot go lower. This also happens exponentially.




    The adding of a cathode bypass capacitor will have a huge impact on the actual tone of your amplifier. If an amplifier has a lot of treble all the time, it usually means there is a poorly chosen bypass capacitor somewhere. A valve will amplify the difference between cathode and grid potential. Because the flow of current is not a constant, the DC bias of the cathode will start to follow the signal applied to the grid. This in turn reduces amplification, distortion and is called cathode current feedback or cathode degeneration. Why would you ever do this you ask? It increases headroom. Eliminating this effect can be done by adding a cathode bypass capacitor. Any rapid rise in cathode current will now be diverted into charging the capacitor, and if cathode current falls the capacitor will supply the deficit from its own charge. Another way of looking at it is to say that the capacitor decouples the AC signal to ground, so signal current does not flow in the cathode resistor and the DC bias potential remains unchanged. The exact relationship between the gain of the stage with frequency and the size of the cathode bypass capacitor is not straightforward, but the shape of the frequency response is. It forms a shelved response shown in the image below. The frequency at which the gain starts to rise from its lower level can be calculated. 1 devided by 2*PI*cathode resistor value * cathode bypass capacitor.




    As to why tubes wear out:



    “If you look inside a lit tube, you will see a glowing red stem in the middle of it. This is called a cathode sleeve. The manufacturer has coated the outside of the sleeve with a proprietary white powder, which emits electrons, making tube action possible. The sleeve is heated to incandescence by a coil of insulated wire stuffed inside called a filament. Your amplifier powers it. Heat causes the powder to deteriorate over time and emit fewer electrons. Tube performance dies with the powder. Shorts are caused by excessive heating of the elements, which causes them to warp and touch. Power tubes and certain small-signal types such as the 7199 are prone to shorts because of internal heat build-up. "Gas" refers to air molecules, which have forced their way inside. This is typically caused by a faulty glass-to-metal seal, where a wire passes through the glass envelope to the tube element. Gas is bad news because it causes the tube to conduct more heavily (run hotter), reducing its life.” (thank you Doug Roccaforte for not making me type this myselfJ).s

    Took a little longer to make this post so I won’t go in depth on the superlead. That will have to wait until tomorrow. Here is a little sneak peek of the schematic design without values. Note that this is semi PCB so not all components and features are present of the PCB. the bright switch with its components is not here for instance.




    I hope some of you have found this interesting to read and stick along for the rest of the build!
     

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