Vacuum tube technology always means bulky, heavy cases, dangerously high voltages, and complex multi-channel power supplies. Or does it?
This little amplifier is powered by a 12V wall-wart AC/AC transformer adapter. Its anode voltage is only 56 volts. It's high time to learn how this voltage is created from 12V and how a simple tube amp actually works.
We will also see on the oscilloscope screen that that unique tube sound signature is loved by audiophiles and musicians, especially guitarists. And then, we'll talk about the difference between long and short vacuum tubes.
The amplifier uses two 6BA6 pentodes. One can also use 6BD6, European EF93, and Chinese 6K4. In today's experiment, I specifically used 6K4s because they are inexpensive and easy to get.
Instead of pentodes with long characteristics and long bodies, one can use short ones like 6AK5, EF95, 6J1, and 6J2. They will also sound great but in a different way.
To understand the difference between a remote (long) cutoff and a sharp (short) cutoff of a vacuum tube, let’s check the principles of their operation.
The simplest vacuum tube is called a diode because it has two electrodes: a negative cathode and a positive anode. They are both contained in a glass or metal container with air pumped out of it so that its molecules do not interfere with the movement of electrons.
The silverish substance at the top of the flask is a gas absorber. It removes residual gas particles from the lamp that haven't been pumped out during the manufacturing process or those released due to heating.
The cathode is usually a nickel straw coated with barium or strontium oxide. The oxide coating is necessary to enhance electron emission.
Inside the tube, there is a heater—a tungsten spiral, like in an incandescent light bulb. The spiral is electrically isolated from the tube, which allows several tube heaters to be powered by a single source of current. The cathodes of those tubes can have different voltages and different signals at the same time.
The most common tubes are designed for heater voltages of 6.3 and 12.6 volts. The model numbers of American and Chinese tubes, unlike European ones, begin with one or two digits of the filament voltage. For example, 12AX7, 6BA6, and today's 6K4.
There are also directly heated cathodes, which are just tungsten spirals. They are typically used in battery-powered tubes. For example, 1N34, 2L34, and their Chinese analogs 1A2 and 2P2. This cathode is easier to heat and keep heated, which saves battery life.
The anode is usually a metal box covering the cathode. The anodes of low-power tubes are made of nickel or iron, and those of high-power ones are made of molybdenum, tantalum, or graphite, as they can withstand higher temperatures.
The surface of the anode is often made black and ribbed to radiate heat efficiently. The tube's electrode cannot transfer heat through convection because no air is around it.
The thermal conductivity of the electrode standoffs that complete the electrical loop to the output terminals is tiny. So, the only way to transfer heat from the inner parts to the outside environment is through thermal radiation.
The heated cathode emits electrons, and a cloud of electrons forming around it is called space charge. Like charges repel, space charge prevents further emission of electrons from the cathode.
Suppose the anode has negative or zero potential relative to the cathode. In that case, nothing happens, and the current won't flow through the tube from the cathode to the anode. But, the positive potential of the anode attracts electrons. They will fly towards the anode, and the current will flow.
Thus, a vacuum diode allows current to flow in only one direction—from the anode to the cathode. After all, the direction of the current is always indicated by an arrow from plus to minus; this direction is for the movement of imaginary positive charges.
In P-type semiconductors, charge carriers are indeed positive; in electrolytes, both positive and negative; and in metal wires and vacuum tubes, negative electrons.
So, a vacuum diode can be used for rectifying AC current (then the tube is called a kenotron), detecting an amplitude-modulated radio signal, and for many other purposes where current needs to be passed in only one direction.
Things get much more enjoyable when, between the cathode and anode, the mesh is added. Such a tube with three different electrodes is called a triode.
When the mesh is negatively charged relative to the cathode, it repels electrons. It makes it harder for them to reach the anode until the flow is completely blocked.
When the mesh's potential is positive relative to the cathode, it attracts electrons from the cathode. Some electrons are captured by the mesh wires and never reach the anode. They create a grid current, a very small one because the wires are thin and the gaps between them are huge.
Therefore, most of the electrons attracted from the cathode and accelerated by the positive potential of the mesh fly right through it and reach the anode. The anode current is much greater than the mesh current.
However, any tube circuitry must have a so-called grid leak, an electrical loop, usually just a resistor between the mesh and the cathode, ensuring the return of electrons from the grid to the cathode. Otherwise, electrons will accumulate on the insulated mesh, and a significant negative potential will occur, which will block the lamp.
So, a vacuum triode is a device that controls the anode current by changing the grid voltage. Suppose a resistance is included in the anode or cathode circuit. In that case, the voltage drop across it will correspond to the anode (or cathode) current according to Ohm’s law.
Thus, we get an amplification of voltage, current, and power. A small change in grid voltage with a tiny grid current controls significant changes in output voltage and current.
This way, one can amplify the signal from a microphone, the pickup of a vinyl record player or guitar, radio waves from an antenna, and so on. And if a transformer with a loudspeaker is connected to the anode circuit, we get a loud-speaking sound amplifier.
The usual mesh of a regular "short" tube has an equal distance between the wires. Or should we say coils because the wires are actually coils winded around the mesh posts.
The image shows how the negative potential of the grid shields the positive electric field of the anode, preventing it from attracting electrons from the cathode.
The mesh of the “long” tube is wound unevenly, with different distances between the turns. And this is not a sign of carelessness; it was done intentionally.
In those places where the distance between the wires is wider, the electric field of the anode does not immediately switch to the grid. Still, it continues to reach the cathode, attracting electrons. This allows for the extension of the characteristic of the tube, which is the main difference between a remote (long) cutoff and a sharp (short) cutoff.
What is the nonlinear transfer characteristic of the tube needed for? Doesn't it distort the signal? And the answer is yes, it distorts, and when applied to guitar amplification and music reproduction, this helps to get interesting results.
However, remote cutoff tubes were originally intended for circuits with automatic gain control. The dependence of the amplification factor, or µ (Greek letter "mu"), on the grid bias voltage, allows one to make a voltage-controlled amplifier (VCA).
And suppose this offset is set by an amplitude envelope detector (like the one in VU-meters). In that case, we get greater amplification of the strong signal and less amplification of the weak one, that is, compression and stabilization of the signal level. This can be very useful and necessary in many cases.
So, let's get started with our amplifier. I took a popular tube buffer board for a home audio system as a base for my project; only a couple of modifications were needed. Here is the diagram for this board.
The amplifier consists of two identical stages. Pentodes here are used as triodes; the second mesh is connected to the anode. The tubes are connected according to a common-cathode circuit.
R13, R18—grid leak resistance. R14, R19—cathode bias resistances, providing a negative grid potential relative to the cathode. R15 and R20 are anode load resistors outputting the amplified signal.
The cathode heaters are connected in series since the supply voltage of the circuit is 12 volts alternating current, and the nominal filament voltage of each lamp is 6.3 volts, so the total is 12.6 volts.
In this scheme, the heaters are powered by a smoothed direct current. This is an ideal way to power them because it eliminates the possibility of AC mains hum remaining in the output signal.
The voltage for powering the heaters is provided by a half-wave rectifier D1 and a pulsation-smoothing RC filter R1 C13 after it.
The tubes are beautifully illuminated by blue LEDs (LED1 and LED2). Since LEDs cannot withstand reverse voltage, they are powered by direct current from the filament rectifier D1. R10 and R11 determine the LED current.
An anode supply of 56 volts is obtained using voltage quadruples consisting of two doublers: positive voltage +28V D2, D3, C1, C2, and negative voltage -28V D4, D5, C5, C6.
Two capacitance multipliers are assembled on capacitors C3 and C7 and transistors TR1 and TR3. Another name for such a circuit is an electronic choke.
The capacitance multiplier is an emitter follower with a capacitor at its input. At the output, the repeater will maintain a constant voltage smoothed by a capacitor.
Thus, the capacitance of the capacitor is multiplied by the transistor gain plus one, β + 1. The minimum gain of the 2SD667A-C and 2SB647A-C transistors is 100.
Thanks to the capacitance multiplier, we have the equivalent of filter capacitors with a capacity of over 4700 microfarads. This ensures almost perfect filtration of the anode power.
Current limiters are assembled on transistors TR2 and TR4. Suppose the voltage across the resistor R4, through which the anode current flows, reaches a value sufficient to open TR2. In that case, it opens and bypasses the base junction of TR1, closing it.
Current protection TR4 R8 works in precisely the same way. And resistors R5 and R9 form additional RC filters with capacitors C4 and C8.
To get a two-stage monophonic guitar amplifier from a single-stage stereo amplifier, connect the left channel output to the right channel volume control input.
Apply a sinusoidal signal with a frequency of 500–3000 Hz to the input of the amplifier, and you'll see a pure sine wave on the oscilloscope screen.
Increasing the signal level, we widen the upper half-wave and narrow the lower half-wave.
This waveform is characteristic of the sum of a sine signal with its second harmonic, the same sinusoid, but with double the frequency and half the amplitude.
These even harmonics are the secret to the beauty of the tube sound that we adore so much. They can be obtained without a tube by digital or analog simulation. Still, in tube stages, they arise naturally with no trickery.
The amplifier board was originally intended to act as a buffer between components in a home stereo system. That is, to match the high output impedance of a signal source (for example, a player) with the low input impedance of a receiver (for example, an amplifier).
It also eliminates the influence of connecting cables. It slightly enriches the signal with tube harmonics, which add crispness and beauty to the sound of individual instruments in a composition. High gain is not required from such a tube buffer; the signal level at both the input and output is linear.
I will use the amplifier as a guitar one, so I'll solder a couple of 100 uF 16V capacitors in parallel with the cathode resistors R14 and R19, positive terminals to the cathode of the lamp.
This will ground the cathodes for AC without changing the DC bias. The gain of the amplifier will increase.
Now, when the volume control knob comes close to the right-most position, the upper half-wave acquires a flat top with no sharp bends, indicating a soft tube limitation.
The tube preamp is ready for testing. As a power amplifier, we will use a board based on the TDA2030 chip in standard configuration as an electronic load with Cabsim, Torpedo Captor X.
Connecting the Squier Bullet Mustang HH guitar directly to the amplifier, as well as through a homemade Landtone Phoenix Song Overdrive pedal (we have a separate post on it),
The first sound test of this small amplifier made me smile. It has an unexpectedly pleasant, clean sound and a nice light overdrive. Higain, of course, will require additional equalization and power amp simulation.
I will get my hands on sharp cutoff pentodes in the upcoming future and compare their sound as well as the oscillograms of sinusoidal signal processing.
Thank you for your time!
Hi! I'm Kevin! I am a very curious engineer :))
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