RADIAL BEAM POWER TETRODE
4CX250B/M
7203A
indicated by grid-current values below approx. 15 mA. The
maximum permissible grid-circuit resistance per tube is 100K
Ohms.
ELECTRICAL
HEATER OPERATION - The rated heater voltage for the
4CX250B/M, 6.0 volts as measured at the socket, should be
maintained at this value to obtain optimum performance and
maximum tube life. In no case should the voltage be allowed to
deviate from 6.0 volts by more than plus or minus five percent
(5%). Short-time changes of +/- 10% will not damage the tube,
but variations in performance must be expected. The heater
voltage must be maintained within +/- 5% to minimize these
variations and to obtain maximum tube life.
SCREEN OPERATION - The maximum rated dissipation for
the screen grid is 12 Watts and the screen input power must
be kept below that level. The product of peak screen current
and peak screen voltage approximates the screen input power
when amplitude modulation is used. In some cases screen cur-
rent may tend to be negative. The 4CX250B/M shows reduced
screen current, compared to the 4CX250B, and is therefore
preferred for use in amplifiers where screen supply voltage
regulation is poor or where an increase in screen current above
the normal value will cause a significant reduction in voltage.
At UHF increased output (cavity) loading is recommended to
reduce screen current even if the overall tube efficiency is re-
duced somewhat.
At frequencies above approx. 300 MHz transit-time effects be-
gin to influence the cathode temperature. The amount of driving
power diverted to heating the cathode by back-bombardment
will depend upon frequency, anode current, and driving power.
When the tube is driven to maximum input as a class-C ampli-
fier, the heater voltage should be reduced according to the table
below:
FAULT PROTECTION - All power tubes operate at voltages
which can cause severe damage in the event of an internal arc,
especially in those cases where large amounts of stored en-
ergy or follow-on current are involved. Some means of protec-
tion is advised in all cases, and it is recommended that a series
resistor be used in the anode circuit (20 to 50 ohms) to limit
peak current and provide a means of dissipating the energy in
the event of a tube or circuit arc. For an oxide-cathode tube
such as the 4CX250B/M a maximum of 4 joules total energy
may be permitted to be dumped into an internal arc. Amounts
in excess of this may permanently damage the cathode or the
tube grids. Additional information is found Eimac’s Application
Bulletin #17 titled “FAULT PROTECTION,” available on request.
Frequency Mhz
300 and lower
301 to 400
heater Voltage
6.00 Volts
5.76 Volts
401 to 500
5.50 Volts
CATHODE OPERATION - The oxide coated unipotential cathode
must be protected against excessively high emission currents.
The maximum rated dc input current is 200 mA for plate modu-
lated operation and 250 mA for all other types of operation with
the exception of pulsed applications.
RF RADIATION - Avoid exposure to strong rf fields even at rela-
tively low frequency. Absorption of rf energy by human tissue
is dependent on frequency. Under 300 MHz most of the en-
ergy will pass completely through the human body with little
attenuation or heating affect. Public health agencies are con-
cerned with hazard even at these frequencies. OSHA (Occu-
pational Safety and Health Administration) recommends that
prolonged exposure to rf radiation should be limited to 10 mil-
liwatts per square centimeter.
The cathode is connected internally to four pins and all four
should be used to make connection to external circuits to low-
er inductance. At radio frequencies it is important to keep the
cathode leads short and direct and to use conductors with large
areas to minimize the inductive reactances in series with the
cathode leads.
It is recommended that the rated nominal heater voltage be ap-
plied for a minimum of 30 seconds before other operating volt-
ages are applied. Where the circuit design requires the cathode
and heater to be operated at different potentials, the rated
maximum heater-to-cathode voltage of 150 voltage should be
observed, regardless of polarity.
INTERELECTRODE CAPACITANCE - The actual internal inter-
electrode capacitance of a tube is influenced by many variables
in most applications, such as stray capacitance to the chassis,
capacitance added by the socket used, stray capacitance be-
tween tube terminals, and wiring effects. To control the actual
capacitance values within the tube, as the key component
involved, the industry and the Military Services use a standard
test procedure as described in Electronic Industries Associa-
tion Standard RS-191. This requires the use of specially con-
structed test fixtures which effectively shield all external tube
leads from each other and eliminates any capacitance reading
to “ground”. The test is performed on a cold tube. Other fac-
tors being equal, controlling internal tube capacitance in this
way normally assures good interchangeability of tubes over a
GRID OPERATION - The maximum rated dc grid bias voltage is
-250 volts and the maximum grid dissipation rating is 2.0 Watts.
In ordinary audio and rf amplifiers the grid dissipation usually
will not approach the maximum rating. At operating frequen-
cies above the 100 MHz region, driving power requirements for
amplifiers increase noticeably. At 500 MHz as much as 20 Watts
of driving power may have to be supplied. However, most of the
driving power is absorbed in circuit losses other than grid dis-
sipation, so that grid dissipation is only increased slightly. Sat-
isfactory 500 MHz operation of the tube in a stable amplifier is
3
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