Resistors

Overview

Resistors are electronic components which restrict the flow of electrons through a circuit. The electrical resistance of a resistor is measured in ohms. The symbol for an ohm is the greek capital-omega: Ω.

There are two main circuit symbols used for resistors. One consists of a jagged line representing the wire used in a resistor and the other is a small rectangle.

Types

There are two distinct categories of resistors:

• Fixed resistors
• Variable resistors

Fixed resistors are by far the most widely used type of resistor. They are used in electronics circuits to set the right conditions in a circuit and have a fixed resistance.

Variable resistors consist of a fixed resistor element and a slider which taps onto the main resistor element. This gives three connections to the component: two connected to the fixed element, and the third is the slider.

Examples of fixed resistor types include:

• Carbon Composition: Oldest type of resistor and was very common. Has higher tolerance and higher noise levels.
• Carbon Film: introduced in early days of transistor technology. Lower tolerance and lower noise levels than carbon composition.
• Metal Film: Metal film. Lower tolerance and lower noise levels than carbon film.
• Metal Oxide: Metal Oxide on a ceramic rod. Has low tolerance, high accuracy, low noise.
• Wire Wound: This type is used for high power applications. They are wire wound and have very low noise levels.
• Surface Mount: This type of resistor is used in automated manufacturing and provide high level of performance.

Examples of variable resistor types include:

• Potentiometer: resistance changes with rotation of slider.
• Variable: resistance changes with rotation of slider.

Examples of other types of resistor include:

• Thermistor: resistance changes with temperature
• Photoresistor: resistance changes with level of light

Resistors in Amplifiers

There are a number of resistor types which can typically be used in amplifiers. For amplifiers which are mass produced SMT (surface mount) are often used. The following list covers the majority of types which are used in modern amplifiers

• Potentiometer:Volume, Tone, Presence, Gain, Bias Controls.
• Variable:Bias Controls.
• Wire Wound:Voltage Dropping
• Metal Oxide:Voltage Dropping, Cathode, Plate etc
• Metal Film:Plate, Grid Leak, Grid Stopper, Cathode, Tone
• Carbon Film:Plate, Grid Leak, Grid Stopper, Cathode, Tone
• Carbon Composition:Plate, Grid Leak, Grid Stopper, Cathode, Tone

There is a massive amount of discussion on the internet about the influence of resistor type on voicing and tone of an amplifier. The following list provides some summarised views:

• Mojo from Carbon Composition:Carbon Composition resistors can influence tone due to the higher levels of distortion and harmonics they produce where there are fluctuating voltages. This occurs due to the resistor composition, low accuracy and tolerance. This can be said, to a lesser extent, when comparing carbon film with carbon composition and comparing metal film with carbon film.
  
  To achieve the best levels of compression and harmonics carbon composition resistors should be used where there are high voltage swings i.e. power section and not preamp sections. Typical application includes 1) plate resistors in the power section 2) Placement of one carbon composition in one PI plate and a metal film in another PI plate will thicken the tone
  
  Using carbon composition resistors can lead to higher noise levels and less reliability however on the flipside can provide better tone (all subjective).



• Noise:There are three types of noise which contribute to the noise floor of an amplifier: shot, thermal and contact. These types are influenced by the material, value, physical size of the resistor and current which passes through them. To keep noise to a minimum it is best to work with metal film and oxide types where there is a greater level of current e.g. plate, and these same types at the initial stages of amplification i.e. 1) Input 2) Grid to Ground 3) Reverb 4) Effects

Purpose

• Remove very high frequencies such as RF from entering the signal path. This is achieved by the resistor working in conjunction with the tube capacitance (about 100pF) to form a high frequency low-pass filter. Please note that only frequencies above the breakpoint are attenuated
• Reduce treble response
• Prevent oscillation (motorboating / parasitic oscillation / squealing)
• Reduce blocking distortion.

Note

• Grid Stoppers can vary from 1.5K to 470K in value. The lower value being more applicable to output tubes and the higher value for pre-amp tubes.
• When installing grid stopper resistors they should be placed as close to the tube pins as possible to form the high frequency low-pass filter.
• Do not place the grid stopper before the grid leak or this will result in signal attenuation.

Grid Leak Resistor

Purpose

• Provide the grid with a reference to ground. The grid will therefore maintain a stable charge of around 0V. There is no voltage drop because no current will flow through the grid.
• Provides a path to ground for the current which go to the grid.
• Provides the signal path impedance to ground. The higher the resistance the less signal to ground and therefore higher gain.

Note

• Grid Leak resistors are generally high in value. Can be from 500K to 3M in value

Cathode Resistor

Purpose

• To regulate the current flow through the tube when no signal is present.
• To produce a voltage drop thus making the cathode have a positive voltage. Because the cathode is positive in relation to the grid, which is 0V, the grid is negative relative to the cathode. This is required for bias.

Note

• In fixed bias amps there is no cathode resistor. A separate circuit provides the negative DC voltage to the grid.
• Cathode resistors generally span from 250 to 10K in value.
• Cathode resistors for output tubes are generally at a higher power rating than preamp tubes.

Plate Resistor

Purpose

• DC Function: Used to reduce the B+ voltage to the level required by the tube plate.
• AC Function: Converts the plate current output into voltage to be applied to the next grid or output transformer. Please note that the current flow increases and decreases in unison with the grid charge changes created by the input signal. When the current flow increases the voltage across the plate resistor increases and when the current flow decreases then the voltage decreases.

Note

• Plate resistors generally around 100K in value.
• The higher the plate resistor the more signal will pass through to the next stage rather than through B+ path. This means that there will be higher gain. Conversely, the lower the value of plate resistor will result in more signal passing through B+ path and less gain.