Cathode bias

From Wikipedia, the free encyclopedia

CATHODE BIAS

Perhaps the best discussion of cathode bias, in terms of clarity and simplicity, is to be found in one of the many editions of "The RCA Receiving Tube Manual", in the applications section. This manual was published in many forms and editions for over 40 years, but has long since been out of print. Edition RC-17, released around 1971 is likely to be the most available. Information in this article is based on that edition.

In order for a vacuum tube to operate in a fairly linear region of its characteristic curve, the grid element must be maintained at a bias voltage more negative than the cathode. Early experimenters and manufacturers used a battery to provide this bias. This battery, called the "C" or bias battery provided voltage but almost never was called upon to deliver current. Thus, such batteries lasted nearly as long in service as they would have on a shelf. In 1985, The Department of Engineering and Technology at Cuesta College in San Luis Obispo, Califronia was presented with a "C" battery date stamped 1927. Department Chairman W.E. English and Instructor W.T. Hanley conducted experiments which demonstrated that the battery still performed satisfactorilly in its originally intended role more than 50 years after its manufacture.

Battery bias, however, is fixed, and doesn't accommodate differences between a new tube and one that has aged, differences between various tubes of the same type, or substitutions that may be made in tube type by repair technicians. Cathode bias automatically accounts for all these possibilities. It is inherent in the technique that the bias level is set by the operation of each individual tube.

To establish cathode bias, a resistor is placed between the emitting element, or Cathode and the negative return of the "B" or HT supply. Current drawn through this resistor by tube conduction places the cathode slightly more positive than the negative return. The grid input is returned directly to the negative supply, causing it to be negative with respect to the cathode. Thus, changes in tube conduction are automatically compensated by changes in bias.

This scheme inherently introduces dynamic odd harmonic distortion. To overcome this problem, the bias resistor is typically shunted by a capacitor. In general, the capacitor value is selected such that the time constant of the capacitor and bias resistor is an order of magnitude grater than the period of the lowest frequency to be amplified. The capacitor thus acts as a dynamic battery, and maintains the bias constant through input signal swings.

Another problem is a slight reduction in gain. The cathode, or bias resistor appears in series with the plate, or load resistor. The bias voltage must be subtracted from the total "B" or HT voltage in gain calculations. In most circuits, this problem is easily overcome by selecting a load impedance at least two orders of magnitude greater than the bias resistance. For example, a 1K bias resistor will have virtually no effect if the load impedance is at least 100K. These values were, in fact, used by Leo Fender in many of his guitar amplifier designs. Refer to "The Tube Amp Book", schematic section.

Excellent discussions of cathode bias may also be found in "Fundamental Electronics", by Bernard Grob, published in no fewer than eight editions over the years by McGraw Hill. Air Force Manuals 52-8 and 101-8 also cover this subject, although they have long been out of print. The problem is that those of us who still understand the technology are getting old fast, and nobody writes much about it any more.

This category requires continual maintenance to avoid becoming too large. It should list very few—if any—article pages directly, and contain primarily sub-categories.

Please diffuse articles into relevant subcategories as needed.

Electronics is the study and use of electrical devices that operate at relatively low voltages by controlling the flow of electrons or other electrically-charged particles in devices such as thermionic valves and semiconductors. The pure study of such devices is considered as a branch of physics, while the design and construction of electronic circuits to solve practical problems is part of the fields of electrical engineering, electronic engineering and computer engineering.

The main article for this category is Cathode bias.