|
Fustica bron voor condensatoren en Vintage audiocomponenten |
||
|
|
TUBE
RATINGS and their significance |
part 2 |
|
For part 1, see: Tube ratings 1 |
||
|
RECEIVING TUBES The ratings of all receiving tubes currently used in new equipment are
set up according to the design-center system. Older and obsolescent types of
receiving tubes still have absolute maximum ratings because these types are
used only for renewal purposes and, therefore, design-center values are of no
practical value. Receiving-tube types rated on the design-center system are Identified in
the Receiving-Tube Section either by a large star in the index corner or each
data page or by the statement "Maximum Ratings Are Design-Center Values"
preceding the ratings on each data page. TRANSMITTING TUBES The ratings of transmitting tubes grouped in the Transmitting-Tube
Section are on the basis of the absolute system. This system enables the
transmitter design engineer to choose his design values so as to obtain
maximum performance within the tube ratings. Such design procedure has been
considered practical for large transmitters where adequate controls are
usually incorporated in the design, and ordinarily an experenced operator is
present to make any necessary adjustments. The maximum ratings given for each transmitting type on its data pages
apply only when the type is operated at frequencies lower than some specified
value which depends on the design of the type. As the frequency is raised
above the specified value, the radio-frequency currents, dielectric losses,
and heating effects increase rapidly. Most types can be operated above their
specified maximum frequency provided the plate voltage and plate input are
reduced in accordance with the information given in the table "Transmitting-Tube
Ratings vs. Operating Frequency" in the front part of the
Transmitting-Tube Section. For certain air-cooled transmitting tubes two sets of absolute maximum
values are shown to meet diversified design requirements. One set is
designated as CCS (Continuous Commercial Service) ratings, while the other is
called ICAS (Intermittent Commercial and Amateur Service) ratings. Continuous Commercial Service
is defined as that type of service in which long tube life and reliability of
performance under continuous operating conditions are the prime consideration.
To meet these requirements, the CCS ratings have been established. Intermittent Commercial and Amateur Service
is defined to include the many applications where the transmitter design
factors of minimum size, light weight, and maximum power output are more
important than long tube life. These various factors have been taken into
account in establishing the ICAS ratings. Under the ICAS classification are such applications as the use of tubes
in amateur transmitters, and the use of tubes in equipment where transmissions
are of an intermittent nature. The term "intermittent" is used to
identify operating conditions in all applications other than amateur in which
no operating or "on" period exceeds 5 minutes and every "on"
period is followed by an "off" or standby period of at least the
same or greater duration. ICAS ratings are considerably higher than CCS ratings. They permit the
handling of greater power, but tube life under ICAS conditions, of course, is
reduced. However, the transmitter designer may very properly decide that a
small tube operated with ICAS ratings better meets his requirements than a
larger tube operated with CCS ratings. Although such use involves some
sacrifice in tube life, the period over which tubes will continue to give
satisfactory performance in intermittent service can be extremely long
depending on the exact nature of the service.The choice of tube operating
conditions best fitted for any particular application should be based on a
careful consideration of all pertinent factors. RECTIFIER TUBES Rectifier tubes used principally in receiving equipment are rated
according to the design-center system, while those used primarily in
transmitting and laboratory equipment are rated according to the absolute
system. The method of identifying which rating system is used for any
rectifier tube in this Handbook is the same as that for other tubes in the
particular section of the Handbook in which data for the rectifier tube are
given. The ratings of rectifier tubes are based on fundamental limitations in
the operation of the tubes themselves, and in general include the following:
maximum peak inverse plate voltage, maximum peak plate current, and maximum
d-c output current. Maximum peak inverse plate voltage
is the highest instantaneous plate voltage, which the tube can withstand
recurrently in the direction opposite to that in which it is designed to pass
current. For mercury vapor tubes and gas-filled tubes, it is the safe top
value to prevent arc-back in the tube operating with in the specified
temperature range. In determining peak inverse plate voltage on a rectifier tube in a
particular circuit, the equipment designer should remember that the relations
between peak value of inverse plate voltage, rms value of input voltage, and
average value of output voltage, depend largely on the characteristics of the
particular rectifier circuit and the power supply. Furthermore, the presence
of transients, such as line surges and keying surges, or waveform distortion,
may raise the actual inverse plate voltage to a peak higher than that
calculated for sine-wave voltages. Therefore, the actual inverse plate
voltage on a rectifier tube should never exceed the maximum peak inverse plate
voltage rating for that tube. The peak inverse plate voltage may be determined
with an electronic peak voltmeter of the self-contained battery type. In single-phase, full-wave rectifier circuits with sinewave input and
pure resistance load, the peak inverse plate voltage is approximately 1.4
times the rms value of the plate-to-plate voltage supply. In single-phase,
half-wave circuits with sine-wave input and pure resistance load, the peak
inverse plate voltage is approximately 1.4 times the rms value of the plate
voltage supply, but with condenser input to filter, the peak inverse plate
voltage may be as high as 2.8 times the rms value of the plate voltage supply. Maximum peak plate current is the highest instantaneous
plate current that a tube can safely carry recurrently in the direction of
normal current flow. The safe value of this peak current in hot-cathode types
of rectifier tubes is a function of the electron emission available and the
duration of the pulsating current flow from the rectifier tube in each
half-cycle. The value of peak plate current in a given rectifier circuit is largely
determined by filter constants. If a large choke is used at the filter input,
the peak plate current is not much greater than the load current; but if a
large condenser is used at the filter input, the peak current may be many
times the load current. In order to determine accurately the peak plate
current in any rectifier circuit, the designer should measure it with a
peak-indicating meter or use an oscillograph. Maximum d-c output current is the highest average plate
current which can be handled continuously by a rectifier tube. Its value for
any rectifier tube type is based on the permissible plate dissipation of that
type. Under operating conditions involving a rapidly repeating duty cycle (steady
load), the average plate current may be measured with a d-c meter. In the case
of certain mercury-vapor tubes where the load is fluctuating, it is necessary
to determine the average current over the time interval specified on the data
pages for these types. In addition to the above ratings for rectifier tubes, other ratings may
be set up for a rectifier tube when the service in which the tube is to be
used makes such ratings essential for satisfactory performance. Such ratings
are: maximum surge plate current, and maximum heater-cathode potential. Maximum surge plate current is the highest value of
abnormal peak currents of short duration that should pass through the
rectifier tube under the most adverse conditions of service. This value is
intended to assist the equipment designer in a choice of circuit components
such that the tube will not be subjected to disastrous currents under abnormal
service conditions approximating a short circuit. This surge-current rating is
not intended for use under normal operating conditions because subjecting the
tube to the maximum surge current even only once may impair tube life. If the
tube is subjected to repeated surge currents, its life will be seriously
reduced or even terminated. Maximum heater-cathode potential
is the highest instantaneous value of voltage that a rectifier tube can safely
stand between its heater and cathode. This rating is applied to certain
rectifier tubes having a separate cathode terminal and used in applications
where excessive potential may be introduced between heater and cathode. For
convenience, this rating is usually given as a d-c value. ------------------- * Types rated according to the absolute system have no
identification on their data pages issued prior to April 1. 1942. Sheets
issued after that date carry the statement "Maximum Ratings Are Absolute
Values" preceding the ratings. ** Types rated according to the design-center system are
identified on their data pages either by a large star in the Index corner or
by the statement "Maximum Ratings Are Design-Center Values"
preceding the ratings. This statement is used on sheets issued since . |
||
|
"Tubes are for Music, transistors are for TV and electronics in general" |
||