Vacuum Tube Amplifier Design Program



Vacuum Tube Amplifier Design Program

Instructor: Chris Wen, Ph.D.

Program Objectives:

This course is focused on the design of vacuum tube preamplifiers and power amplifiers. Electronic components such as resistors, capacitors, transformers will be discussed to help students (or designers) select the most appropriate components. After this course, students will be able to design and DIY their own vacuum tube amplifiers.

Total Hours: 12 hours

Schedule: 10:00-12:00, 12:30-14:30, June 9, June 16, June 23

(The schedule can be changed after the first meeting if all students agree)

Major topics:

1. Basic electronics

Ohm’s law, Kirchhoff's Voltage and Current laws, Thevenin’s Theorem

2. Understanding electronic components

1) Resistors: type, characters, standard resistance, color band

2) Capacitors: type, specifications, selection

3) Transformer: power transformer, audio output transformer, transformer materials

3. Understanding vacuum tubes

1) vacuum tube theory

2) direct heated vacuum tube and indirect heated vacuum tube

3) vacuum tube types

4. power supply design

1) using solid state diodes

2) using vacuum tubes

5. basic building blocks

1) common cathode triode amplifier

2) miller capacitance

3) decoupling capacitor

4) cathode follower

5) shunt regulated push-pull (SRPP) amplifier

6. distortion

7. preamplifier

8. Power amplifier design

1) single ended amplifier

2) push-pull amplifier

9. demonstration

1) preamplifier

2) single-ended amplifier

Vancouver Business and Technology

#6615, 8181 Cambie Road, Richmond, BC V6X 3X9

Tel: 604-2782176, 778-8898132

High Voltage, Danger!

1. Vacuum Tube = High Voltage

(12AX7: 300V-350V, 300B: 400V-450V, EL34: 450V, KT88: 450V-500V)

(211, 805, 845 >1000V)

2. It is the current that flow through your body kills.

3. Body resistance

The resistance of our skin varies from person to person and fluctuates between different times of day. In general, dry skin isn't a very good conductor having a resistance of around 10,000 Ω, while skin dampened by tap water has a resistance of around 1,000 Ω.

4. Physiological effect

Using Ohm's law, we may derive the voltages lethal to the human body. This is given in the following table:

|Electric Current |Voltage for |Voltage for |Physiological Effect |

|Amperes |10,000 Ohms |1,000 Ohms | |

|1 mA |10 V |1 V |Threshold of feeling an electric shock |

|5 mA |50 V |5 V |Maximum current which would be harmless |

|10-20 mA |100 - 200 V |10-20 V |Sustained muscular contraction. "Cannot let go" |

| | | |current. |

|50 mA |500 V |50 V |Ventricular interference, pain, respiratory difficulty |

|100-300 A |1000- 3000 V |100-300 V |Ventricular fibrillation. Can be fatal. |

Basic Electronics

1. Ohm’s law

V = I x R

Voltage (Volt) = I (Ampere) x R (ohm)

I = V/R, R=V/I

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2. Kirchhoff's Current Law (KCL)

This law is also called Kirchhoff's first law, Kirchhoff's point rule, Kirchhoff's junction rule, and Kirchhoff's first rule.

The principle of conservation of electric charge implies that:

At any point in an electrical circuit where charge density is not changing in time, the sum of currents flowing towards that point is equal to the sum of currents flowing away from that point.

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3. Kirchhoff's Voltage Law (KVL)

This law is also called Kirchhoff's second law, Kirchhoff's loop rule, and Kirchhoff's second rule. It is a consequence of the principle of conservation of energy.

The principle of conservation of energy implies that:

The directed sum of the electrical potential differences around a circuit must be zero.

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4. Thévenin's theorem

The Thevenin theorem states that any real source may be represented as an ideal potential source in series with a resistor. In many cases, one may use the Thevenin circuit to solve electronics problems that might otherwise be tedious at best. The ideal potential source is called the Thevenin Voltage, VTH; The resistance is called the Thevenin Resistance, RTH.

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Thevenin Voltage is the voltage appears across the load terminals when you disconnect the load resistor.

Thevenin Resistance is the resistance looking back into the load terminals when all sources have been reduced to zero.

Example: Find the current flow across R5. R1 = 5 kΩ, R2 = 6 kΩ, R3 = 5 kΩ, R4 = 3 kΩ

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Solution:

(1) Find VTH

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(2) Find RTH

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Example:

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5. Capacitor circuit

(1) Parallel

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(2) Series

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3) reactance

The current through the capacitor is proportion to the rate of voltage change across the capacitor.

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Since capacitors conduct current is proportional to the rate of voltage change, they will pass more current for faster-changing voltages, and less current for slower-changing voltages. What this means is that reactance in ohms for any capacitor is inversely proportional to the frequency of the alternating current:

[pic] [pic]

The unit of the reactance is ohm.

6. inductor circuit

The relationship between the voltage dropped across the inductor and rate of current change through the inductor is as such:

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Since inductors drop voltage is in proportion to the rate of current change, they will drop more voltage for faster-changing currents, and less voltage for slower-changing currents. What this means is that reactance in ohms for any inductor is directly proportional to the frequency of the alternating current. The exact formula for determining reactance is as follows:

[pic] [pic]

The unit of the inductive reactance is ohm.

Understanding Electronics Components

Resistors

Type of resistors:

1. Carbon Film Resistor

Low price, most popular, large temperature coefficient, tolerance 5%

Power: 1/8 W - 3W

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2. Metal Oxide Film Resistor

Can withstand temperature up to 200℃, higher power, temperature coefficient is about 350PPM/℃, tolerance 2%, 5%

Power: 1/4 W – 5W

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3. Metal Film Resistor

High precision, low noise, temperature coefficient is less than 100PPM/℃, tolerance less than 1%.

Power: 1/8W - 3W

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4. Carbon Composition Resistor

The resistive element is made from a mixture of finely ground (powdered) carbon and an insulating material (usually ceramic). The mixture is held together by a resin. The resistance is determined by the ratio of the fill material (the powdered ceramic) and the carbon. Tolerances with 10% and 5% are the most common.

The main advantage of Carbon Comps is their pulse handling capability. This is due to the fact that the entire rod conducts. Typically the 1W CCR1 can handle 35J. This is in contrast to 4J for a wirewound resistor of similar dimensions.

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()

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()

5. Wire Wound Resistor

(1) Wirewound resistor

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(2) Cement resistor

Cement resistors are made by winding resistance wires around non-alkaline ceramic core, which is added with a layer of heat and humidity-resistant and non-corrosive protective material.

Power: 2W-50W

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(3) Aluminum Housed Wirewound Resistor

Power: 5W-120W

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Resistor Color Code (or Color Band)

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Standard Resistor Value

The Electronic Industries Association (EIA), and other authorities, specify standard values for resistors, sometimes referred to as the "preferred value" system. 

E12   10% tolerance

E24     5% tolerance (and usually 2% tolerance)

E48     2% tolerance (also for inventory cost control in place of E96)

E96     1% tolerance

common ratio [pic]:

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Where N is the E series number.

Standard EIA Decade Values Table (10-1000)

|E12 |10 |

|vacuum |1.00000 |

|air(sea level) |1.00059 |

|Aluminum Oxide |7.0-12.0 |

|Ceramics |5.0-6.0 |

|Mica |3.0-6.0 |

|Polyester(PET, Mylar) |2.8-4.5 |

|Polycarbonate (PC) |2.9-3.0 |

|Polyethylene (PE) |2.25 |

|Polypropylene (PP) |1.5-2.2 |

|Polystyrene (PS) |2.4-2.6 |

|Teflon (PTFE) |2.0 |

Capacitor Equivalent Circuit

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ESL ( Equivalent Series Inductance): mostly results from wounding plates.

ESR (Equivalent Series Resistance): intermetallic resistance between leads and plates, and resistance from plates.

Dielectric Absorption: Dielectric absorption is the inability of a capacitor to discharge completely to zero. This is sometimes called battery action or capacitor memory and occurs because the dielectric of the capacitor retains a charge.

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There appears to be a strong correlation between the subjective sound quality of capacitors and their dielectric absorption.

Polar plastics:

Some plastics are polar. At a molecular level within the dielectric, there are permanently charged electric dipoles. Under the influence of an external electric field, these dipoles attempt to align themselves to that electric field. When we apply an AC field, energy is absorbed as we successively align these dipoles first one way, and then the other, so that we incur a loss that rises with frequency. This is sometimes known as electrostatic hysteresis.

|material |Dielectric absorption |polar |

|PTFE, Teflon |0.0002 |N |

|PS |0.0002-0.0005 |N |

|PP |0.0005 |N |

|PC |0.001-0.01 |Y |

|PET, Mylar |0.002-0.015 |Y |

Dissipation Factor (tangent of loss angle δ)

DF = (ESR/Xc) × 100% = ESR/(1/ωC) × 100%

Where ωis the frequency angular, ω=2πf.

|Type of capacitor |Dissipation factor |

|Electrolytic Capacitor |0.1-0.4 |

|Plastic film capacitor |0.001-0.01 |

Direct Current Leakage, DCL

DCL is the current leakage inside a capacitor. Usually it is listed as a specification for a electrolytic capacitor. This is due to that the electrolytic capacitor uses very thin aluminum oxide film as the dielectric material and uses high-purity aluminum foils which are etched with billions of microscopic tunnels to increase the surface area in contact with the electrolyte.

Direct Current Leakage is proportional to the working voltage and the capacitance of the capacitor.

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Where k is a constant, between 0.01~0.03. The smaller the better, but will be more expansive. The capacitance is inμF and DCL is in μA.

Type of Capacitor

1. Film capacitor

(1) Plastic film capacitor

a. plastic film and foil capacitor

b. metallized plastic film capacitor

(2) Paper capacitor

2. Multi-Layer Capacitor

(1) Ceramic Capacitor

(2) Mica Capacitor

3. Electrolytic Capacitor

(1) Aluminum Electrolytic Capacitor

(2) Tantalum Electrolytic Capacitor

Plastic film capacitor:

a. plastic film and foil capacitor

b. metallized plastic film capacitor

plastic film and foil capacitor

This is the most important class of capacitors for use in valve amplifier, as we will use these for coupling stages and also for precise filters.

Film/foil capacitors consist of two metal foil electrodes made of aluminum foil separated by a piece of plastic film. The plastic film can be polyester, polypropylene or polycarbonate. The thickness of the plastic film typically ranges from 2 μm to 20 μm, while the aluminum foil thicknesses range from 5 μm to 9 μm.

A film/foil capacitor is made by alternating two pieces of aluminum foil with two layers of plastic film. These interleaved layers are wound around a spindle in a manner that prevents the metal layers from touching.

Metallized plastic film capacitor

Metallized film capacitors differ from film/foil capacitors in the sense that the aluminum foils are replaced by a layer of metal vacuum deposited onto the film itself. The metal layer is typically aluminum or zinc that is extremely thin in the range of .02 μm to .05μm.

The advantage of these capacitors is their reduced physical size and their self healing property.

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Type of plastic film:

Polypropylene (PP)

Polyester (Mylar)

Polycarbonate (PC)

Polystyrene (PS)

Teflon (PTFE)

Polypropylene and polyester film capacitors are most common. Mylar capacitor is good for general non-critical application. It is cheaper than other plastic film capacitors. Teflon capacitor is very good, but it is big and expansive.

Self-Healing

The self-healing property is exclusive to capacitors with metallized films and is their single biggest advantage over film/foil capacitors.

Self-healing is a phenomenon where in the event the electrodes are exposed to each other instead of the capacitor shorting, the capacitor repairs itself. This repairing of the capacitor is due to the thinness of the foils used.

In a film/foil capacitor when the foils are exposed to each other, the foils would touch and short together rendering the capacitor useless.

Paper capacitor

A paper capacitor is made of flat thin strips of metal foil conductors that are separated by waxed paper (the dielectric material). Paper capacitors usually range in value from about 300 picofarads to about 4 microfarads. The working voltage of a paper capacitor rarely exceeds 600 volts. Paper capacitors are sealed with wax to prevent the harmful effects of moisture and to prevent corrosion and leakage.

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OIL CAPACITORS are often used in high-power electronic equipment. An oil-filled capacitor is nothing more than a paper capacitor that is immersed in oil. Since oil impregnated paper has a high dielectric constant, it can be used in the production of capacitors having a high capacitance value.

Ceramic Capacitor

A Ceramic Capacitor is so named because it contains a ceramic dielectric.

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MICA CAPACITOR

A Mica Capacitor is made of metal foil plates that are separated by sheets of mica (the dielectric).

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Aluminum Electrolytic Capacitor

An aluminum electrolytic capacitor consists of a wound capacitor element, impregnated with liquid electrolyte, connected to terminals and sealed in a can. The element is comprised of an anode foil, paper separators saturated with electrolyte and a cathode foil. The foils are high-purity aluminum and are etched with billions of microscopic tunnels to increase the surface area in contact with the electrolyte.

While it may appear that the capacitance is between the two foils, actually the capacitance is between the anode foil and the electrolyte. The positive plate is the anode foil; the dielectric is the insulating aluminum oxide on the anode foil; the true negative plate is the conductive, liquid electrolyte, and the cathode foil merely connects to the electrolyte.

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Tantalum Electrolytic Capacitor

What is Tantalum?

Tantalum is a chemical element in the periodic table that has the symbol Ta and atomic number 73. A rare, hard, blue-gray, lustrous, transition metal, tantalum is highly corrosion-resistant.

The cathode electrode is formed of sintered tantalum grains, with the dielectric electrochemically formed as a thin layer of oxide. The thin layer of oxide and high surface area of the porous sintered material gives this type a very high capacitance per unit volume. The anode electrode is formed of a chemically deposited semi-conductive layer of manganese dioxide, which is then connected to an external wire lead. A development of this type replaces the manganese dioxide with a conductive plastic polymer (polypyrrole) that reduces internal resistance and eliminates a self-ignition failure

Compared to aluminum electrolytics, tantalum capacitors have very stable capacitance and little DC leakage, and very low impedance at low frequencies. However, unlike aluminum electrolytics, they are intolerant of voltage spikes and are destroyed (often exploding violently) if connected backwards or exposed to spikes above their voltage rating. Tantalum capacitors are also polarized because of their dissimilar electrodes.

Working voltage is limited up to 50 V.

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Transformer Materials

Cold Rolled Grain Oriented Steel

Conventional CRGO (Cold Rolled Grain Oriented Steel) materials (M4, M5, M6) are used regularly for cores in Transformers.

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Cold Rolled Grain Oriented Steel

|Material |Density |Core Loss W/KG |Magnetic Flux |Stacking |

| |Kg/dm² | |Density |Factor % |

| | | |T | |

New number |Old number |Thickness,

mm | |W 17/50 |W 17/60 |B 8 | | |30Z120

30Z130

30Z140 |Z8

Z9

Z10 |0.30 |7.65 | ................
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