TECHNICAL DATA

WATER COOLED POWER 8974

The EIMAC 8974 is a ceramic/metal, water-cooled power tetrode designed for very high power rf amplification. This tube is used for medium-wave and high frequency broadcast service at output power levels in the megawatt range.

The 8974 has a two-section thoriated-tungsten mesh filament mounted on water-cooled supports. The two sections may be fed from an ac or dc power source. The maximum anode dissipation rating is 1500 kilowatts steady state.

Large-diameter terminals are used for the control grid and the rf filament terminals. Filament power and filament support cooling-water connections are made through three special connectors. Anode cooling water connections are made with available hand-tightened fittings with O-ring seals.

GENERAL CHARACTERISTICS1

ELECTRICAL MECHANICAL Filament: Mesh, Thoriated-tungsten, two section Net Weight...... 175 lbs; 80 kg .Voltage...... 16.3 V Gross Weight….………..…….……380 lbs; 173kg .Current @ 16.3 V (nominal per section) … 640 A Operating Position...... Vertical, Base Down Frequency of Maximum Ratings (CW)...... 30 MHz Cooling...... Water and Forced Air Amplification Factor, Average, Grid to Screen.... 4.5 Maximum Overall Dimensions: Direct Interelectrode Capacitances (grounded cathode): .Length...... 25.50 in; 64.78 cm .Cin...... 1600 pF .Diameter...... 17.03 in; 43.26 cm .Cout...... 260 pF Maximum Operating Temperature: .Cgp...... 7.5 pF Envelope and Ceramic/Metal Seals...... 200*C Direct Interelectrode Capacitances (grounded grid) Available Filament Power Connector (not supplied Cin...... 690 pF with tube): .Cout...... 265 pF .Filament Power/Water Connector (2 req’d.):..EIMAC SK-2310 .Cpk...... 1.5 pF Filament rf Connector (1 req’d):...... SK-2315 Available Anode Cooling Water Connectors (not supplied with tube): Note: 2 connectors are required per tube: EIMAC SK-2320, SK-2321, SK-2322, or SK-2323

1 Characteristics and operating values are based on performance tests. These figures may change without notice as a result of additional data or product refinement. CPI/Eimac should be consulted before using this information for final equipment design. 2 Capacitance values shown are nominal, measured with no special shielding, in accordance with Electronic Industries Association Standard RS-191. 8974

RADIO FREQUENCY LINEAR TYPICAL OPERATION (Below 30 MHz) GRID DRIVEN Class AB Anode Voltage...... 20.0 kVdc Screen Voltage...... 1500 Vdc ABSOLUTE MAXIMUM RATINGS: Grid Voltage#...... -380 Vdc Zero Signal Anode Current*...... 20 Adc DC Anode Voltage...... 22.5 kV Single Tone Anode Current...... 86.5 Adc DC Screen Voltage...... 2.5 kV Single Tone Screen Current*...... 3.8 Adc DC Grid Voltage...... -2.0 kV Peak rf Grid Voltage*...... 380 v DC Anode Current...... 125 A Anode Dissipation*...... 505 kW Anode Dissipation...... 1500 kW Anode Power Output*...... 1225 kW Screen Dissipation...... 15 kW Anode Load Resistance...... 132.2 Ohms Grid Dissipation...... 4.0 kW Anode Efficiency*...... 70.8 %

* Approximate Value # Adjust for zero-signal anode current

RADIO FREQUENCY POWER AMPLIFIER TYPICAL OPERATION (below 30 MHz) Class C Telegraphy or FM (Key-down conditions) Anode Voltage...... 21.5 kVdc Screen Voltage...... 1000 Vdc ABSOLUTE MAXIMUM RATINGS: Grid Voltage...... -700 Vdc Anode Current...... 125 Adc DC Anode Voltage...... 22.5 kV Screen Current*...... 12 Adc DC Screen Voltage...... 2.5 kV Grid Current*...... 7.2 Adc DC Grid Voltage...... -2.0 kV Calculated Driving Power...... 7.0 kW DC Anode Current ...... 125 A Anode Dissipation*...... 530 kW Anode Dissipation...... 1500 kW Screen Dissipation...... 12 kW Screen Dissipation...... 15 kW Grid Dissipation...... 1.9 kW Grid Dissipation...... 4.0 kW Anode Load Resistance...... 85.5 Ohms Anode Power Output*...... 2158 kW Anode Efficiency*...... 80.1 %

*Approximate value

ANODE MODULATED RADIO FREQUENCY TYPICAL OPERATION: POWER AMPLIFIER Class C Telephony Anode Voltage...... 17.5 kVdc (Carrier conditions) Screen Voltage...... 1000 kVdc Grid Voltage...... -1000 Vdc ABSOLUTE MAXIMUM RATINGS: Anode Current#...... 95 Adc Screen Current*...... 8 Adc DC Anode Voltage...... 17.5 kV Grid Current*...... 4.4 Adc DC Screen Voltage...... 2.0 kV Peak Screen Voltage (100% modulation)..... 1000 v DC Grid Voltage...... -2.0 kV Peak rf Grid Driving Voltage*...... 1280 v DC Anode Current ...... 100 A Calculated Driving Power...... 6465 W Anode Dissipation...... 1000 kW Anode Dissipation*...... 279 kW Screen Dissipation...... 15 kW Screen Dissipation...... 8.0 kW Grid Dissipation...... 4.0 kW Grid Dissipation*...... 2.05 kW Anode Load Resistance...... 85.6 Ohms * Approximate Value Anode Output Power*...... 1384 kW Anode Efficiency*...... 83.3 %

2 *Approximate value 8974

RADIO FREQUENCY POWER AMPLIFIER Carrier Tube - Carrier Conditions: Doherty Amplifier Service Anode Voltage...... 19.0 kVdc ABSOLUTE MAXIMUM RATINGS: Screen Voltage...... 1600 kVdc Grid Voltage*...... -400 Vdc DC Anode Voltage...... 22.5 kV Anode Current...... 101 Adc DC Screen Voltage...... 2.5 kV Screen Current*...... 7.3 Adc DC Grid Voltage...... -2.0 kV Grid Current*...... 0.14 Adc DC Anode Current ...... 125 A Peak Grid Driving Voltage *...... 443 V Anode Dissipation...... 1500 kW Grid Driving Power*...... 65 W Screen Dissipation...... 15 kW Anode Power Output*...... 1380 kW Grid Dissipation...... 4.0 kW Anode Dissipation*...... 510 kW Anode Efficiency*...... 71 % Anode Load Resistance...... 102 Ohms

TYPICAL OPERATION (Frequencies to 30 MHz): Carrier Tube - Peak of Modulation

Peak Tube - Peak of Modulation: Peak Grid Drive Voltage*...... 668 V Peak Grid Driving Power *...... 1090 W DC Anode Voltage...... 19.0 kV Anode Power Output*...... 2750 kW DC Screen Voltage...... 1600 V Anode Load Resistance...... 51.5 Ohms DC Grid Voltage*...... -1.8 kV Peak Grid Drive Voltage*...... 2220 V Actual Load Resistance at Combining Point = .25.7 Ohms Peak Grid Drive Power*...... 10 kW Screen dissipation averaged over a sinusoidal modu- Peak Anode Power Out*...... 2750 kW lation cycle - Modulation Index 1 Anode Load Resistance...... 51.5 Ohms Carrier Tube...... 14.0 kW Peak Tube...... 8.5 kW

*Approximate value

NOTE: TYPICAL OPERATION data are obtained from direct measurement or by calculation from published charac- teristic curves. Adjustment of the rf grid voltage to obtain the specified anode current at the specified bias, screen and anode voltages is assumed. If this procedure is followed, there will be little variation in output power when the tube is changed, even though there may be some variation in grid and screen current. The grid and screen currents which result when the desired anode current is obtained are incidental and vary from tube to tube. These current variations cause no difficulty so long as the circuit maintains the correct voltage in the presence of the variations in current.

RANGE VALUES FOR EQUIPMENT DESIGN

Min. Max. Unit If, Filament Current, at 16.3 volts ac (Per Section)...... 575 650 A

Voltage required to cut off ib, EC2 = 1.5 kVdc, EB=15.0 kVdc...... -- -600 V

3 8974

APPLICATION

MECHANICAL sure during this process must be limited to 2 Bar (29 psi). Under no circumstances should an un-regulated UNPACKING – To insure safety of the operator per- air source be used for this procedure. forming the work as well as preventing damage to the tube, the following instructions should be followed The tube should be stored with a portable VACION when unpacking the tube: pump power supply connected to allow monitoring the tube’s vacuum properties (see section on VACION 1) Open the crate by removing the lid, first unlocking PUMP OPERATION for details.) the toggle bolts in 8 places. The original shipping crate with the shock mounts and 2) Attach a lifting hoist to the lifting loop and raise the hardware should be retained in a dry place for future tube slightly to support the weight of the tube. use such as moving a tube over a con-siderable dis- tance. 3) Remove 8 bolts securing the mounting brackets to the corners. MOUNTING - The 8974 must be mounted vertically, base down. The full weight of the tube should rest on 4) Using the hoist, lift the tube and place on blocks or the screen-grid contact flange at the base of the tube on a stand that supports its weight by the bottom of the and all lifting of the tube should be done with the lifting lower corona ring. eye which is attached to the top of the anode cooling jacket. 5) Remove the brackets from the tube. ANODE COOLING - The anode is cooled by circulat- ing water through the structure. The inlet and outlet HANDLING – This product contains a thoriated-tung- sten filament, and although of a rugged mesh design, connections are clearly marked on top of the anode it is relatively fragile and a tube should be protected cooling jacket and it is important they be connected from shock and vibration. A lifting eye is available at only as indicated. the center of the anode cooler and should be used any time the tube is to be lifted for moving, etc. A lifting Tube life can be seriously compromised by water con- device such as a chain hoist may be employed to lift dition. With contaminated water, deposits will form on the tube and should be capable of safely supporting the inside of the water jacket, causing localized an- the full weight of the tube (in excess of 200 lbs with ode heating and eventual tube failure. To minimize cooling water in the anode cooler) and should be op- electrolysis and power loss, water resistance at 25°C erated with great care, especially when lowering the should always be one megohm per cubic centimeter tube onto a resting place or into equipment. It is rec- or higher. The relative water resistance should be pe- ommended that a thick rubber mat or similar material riodically checked using readily available instruments. be used to absorb any undue shock that may occur if the tube is to be placed on a hard surface. Minimum water flow requirements for the anode are shown in the table for an outlet water temperature not STORAGE – If a tube is to be stored as a spare to exceed 70°C and inlet water temperature at 50°C. it should be kept in its shipping crate and all water System pressure should not exceed 100 psi. should be purged from the anode cooler and from the filament supports/connectors. ANODE WATER APPROX JACKET DISSIPATION FLOW PRESS DROP Prior to shipping, water should be removed from the (kW) (GPM) (PSI) tube’s anode cooler and filament structure. The an- ode cooler can be drained by inverting the tube. Water Filament Only 35 5 should be purged from the internal filament support 500 130 25 structure by applying compressed air to one of the fila- 1000 250 75 ment coolant ports and it is important to note that pres- 1500 300 100

4 8974

This cooling data is applicable to steady-state or tran- ELECTRICAL sient anode dissipation. At significantly reduced anode dissipation a lower flow rate may be permissible but ABSOLUTE MAXIMUM RATINGS - Values shown for using the maximum flow rate will protect the tube from each type of service are based on the “absolute sys- unforeseen events that may cause increased anode tem” and are not to be exceeded under any service dissipation. conditions. These ratings are limited values outside which serviceability of the tube may be impaired. In Cooling water must be well filtered (with the equivalent order not to exceed absolute ratings the equipment of a 100-mesh screen) to eliminate any solid materials designer has the responsibility of determining an aver- which could block cooling passages, as this would im- age design value for each rating below the absolute mediately affect cooling efficiency and could produce value of that rating by a safety factor so that the ab- localized anode overheating and tube failure. solute values will never by exceeded under any usual conditions of supply-voltage variation, load variation, EIMAC Application Bulletin #AB-16, “Water Purity Re- or manufacturing variation in the equipment itself. It quirements in Liquid Cooling Systems” is available on does not necessarily follow that combinations of abso- request and contains considerable detail on purity re- lute maximum ratings can be attained simultaneously. quirements and maintenance of systems. HIGH VOLTAGE - Normal voltages used with this tube BASE COOLING - The tube base requires air cooling are deadly, and equipment must be designed properly with a minimum of 50 cfm of air at 50*C maximum at and operating precautions followed. Design all equip- sea level, directed toward the base seal areas from a ment so that no one can come in contact with high volt- general purpose fan. It should be noted that tempera- ages. Equipment must include safety enclosures for tures of the ceramic/metal seals and the lower enve- the high-voltage circuits and terminals, with interlock lope areas are the controlling and final limiting factor to open primary circuits of the power supply and the maximum allowable temperature is 200*C. and to discharge high voltage when access doors are opened. Interlock switches must not be Temperature-sensitive paints are available for use in bypassed to allow operating with access doors open. checking temperatures in these areas before equip- Always remember that HIGH VOLTAGE CAN KILL. ment design and air-cooling requirements are final- ized. EIMAC Application Bulletin AB-20, MEASURING FILAMENT OPERATION – When cold, the resistance TEMPERATURE OF POWER GRID TUBES, covers of a thoriated tungsten filament is very low, therefore this subject in detail and is available on request. the initial starting (inrush) current when filament volt- age is applied can be many times the normal (hot) Water cooling of the filament and screen grid supports current; this can be detrimental to the longevity of a is also required, with inlet water temperature not to ex- filament structure. ceed 70*C. Each of the two filament connectors in- clude both an inlet and an outlet line, with the proper Filament inrush current should never exceed a value connector for the inlet water shown on the tube out- of twice the nominal rated current. Filament turn-on line drawing. Minimum flow for the F1 connector is and turn-off should be programmed. Filament voltage 2.0 gpm, at an approximate pressure drop of 12 psi. should be smoothly increased from zero to the operat- Minimum flow for the F2 connector is 4.0 gpm, at an ing level over a period of two minutes, and a motor- approximate pressure drop of 50 psi. The screen grid driven continuously variable auto- (such cooling water is fed by means of 1/4-18 NPT tapped as a VARIAC* or a POWERSTAT*) is suggested. Nor- holes shown on the tube outline drawing, with a mini- mal turnoff procedure should be a smooth decrease mum flow of 2.0 gpm required, at an approximate from the operating voltage to zero over a period of two pressure drop of 12 psi. minutes.

ALL COOLING MUST BE APPLIED BEFORE OR At rated (nominal) filament voltage the peak emission SIMULTANEOUSLY WITH THE APPLICATION capability of the tube is many times that needed for OF ELECTRODE VOLTAGES, INCLUDING THE most applications. A reduction in filament voltage will FILAMENT, AND SHOULD NORMALLY BE MAIN- lower the filament temperature, which will substantially TAINED FOR SEVERAL MINUTES AFTER ALL increase life expectancy. The correct value of filament VOLTAGES ARE REMOVED TO ALLOW FOR TUBE voltage should be determined for the particular appli- COOLDOWN. 5 8974 cation. It is recommended the tube be operated at full this device is to allow monitoring of the condition of the nominal voltage for an initial stabilization period of from tube vacuum, as shown by an ion current meter. 100 to 200 hours before any action is taken to operate at reduced filament voltage. The voltage should then With an operational tube it is recommended the VA- be reduced gradually until there is slight degradation in CION pump be operated full time so tube vacuum may performance (such as power output or distortion). For be monitored on a continuous basis. A reading of less operation the voltage should then be increased sever- than 10 uAdc should be considered as normal, indi- al tenths of a volt above the value where performance cating excellent tube vacuum. In addition to other in- degradation was noted. The operating point should be terlock circuitry it is recommended that full advantage rechecked after 24 hours. Filament voltage should be be taken of the VACION pump readout by providing closely regulated when voltage is to be reduced below circuitry which will shut down all power to the tube in nominal in this manner, to avoid any adverse influence the event the readout current exceeds 50 uAdc. In the caused by normal line voltage variations. event of such a shutdown, the VACION pump should be operated alone until vacuum recovery is indicated Black heat operation (a reduction of filament voltage by a reading of 10 uAdc or less, at which point the tube to 50% or less of nominal voltage during standby pe- may again be made operational. If the vacuum cur- riods) is not recommended. During standby periods, rent rises again it should be considered as indicating forced air cooling and water flow must be maintained a circuit problem such as some tube element may be on the filament supports to ensure the ceramic/metal over-dissipating and outgassing. seal temperature does not exceed 200°C. In addition, anode cooling water flow must be maintained at a rate The VACION pump requires a positive voltage applied that ensures the outlet water temperature never ex- to the center pin of approx. 3000 Vdc to operate prop- ceeds 100°C. See cautionary information regarding erly. hot water on p.10. Serious damage and personal harm can result if water flow is interrupted while power is One source for VACION power supplies is Varian, applied to the filament therefore system interlocks are Inc. Varian’s web site www.varianinc.com has several necessary to remove all power to the tube if coolant models which may be suitable for use with the 8974. flow is not present for any reason. Varian model 9290200 (120 volt ac line) or 9290201 (220 Volt ac line) appear suitable. Alternatives for Var- Filament voltage should be measured at the tube ian power supplies are: HeatWave Labs http://www. base, using an accurate rms-responding meter. cathode.com/ and Duniway Stockroom http://www. duniway.com/ Where hum is an important system consideration, it is permissible to operate the filaments with dc rather At the tetrode a is attached to the VA- than ac power. Contact CPI Power Prod- CION pump and comes attached to the tube as deliv- ucts EIMAC Operation, Application Engineering for ered. This cable assy. includes a that prevents special precautions when using a dc filament supply. the filament current from being shorted to ground. The end of the resistor has a solder lug that is attached Care should be exercised to keep any rf power out of using one of the small screws to the magnet assy at the filament of the tube, as this can cause excessive the VACION pump. The other end of this cable has filament temperature. Both sides of the filament must a female receptacle (type MHV, also designated mil. be bypassed to avoid rf resonance in the filament. UG-1016A/U or Amphenol type 27075).

This tube is designed for commercial service, with only To plug onto this receptacle a male plug type UG- one off/on filament cycle per day. If additional on/off 932/U, also supplied with each tube, is normally cycling of filament power is anticipated it is recom- used for making up an extender cable of the required mended the user contact CPI Microwave Power Prod- length. The other end of the extender cable goes to ucts EIMAC Operation, Applications Engineering for the VACION power supply; the Varian supplies require additional information. a Kings plug 1065-1 (not supplied with the 8974). For info see: http://www.kingselectronics.com/ and cable VACION* PUMP OPERATION – The tube is supplied type RG-58A/U or Belden 8259 is recommended for with an ion pump permanently mounted on the filament this connector. structure at the base (stem). The primary function of 6 8974

In the case of a tube being held as a spare, it is recom- (such as regulator service or lower power and low im- mended the VACION pump be operated continuously pedance “tuning” conditions) can, as a result of the if possible, otherwise it should be operated periodical- screen and grid voltages chosen, lead to anode dam- ly to check the condition of tube vacuum and operated age and subsequent failure from spot heating as a re- as long as necessary to achieve a reading of 10 uAdc sult of focusing effects in the tube. If operation under or lower. such conditions is necessary CPI Microwave Power Products, EIMAC Operation’s Application Engineering Figure 1 on p.10 shows the relationship between tube should be contacted for assistance in selection of op- vacuum and the ion current reading. Electrode voltag- erating parameters. es, including filament voltage, should never be applied if a reading of 50 uAdc or higher is obtained. In the GRID OPERATION - The maximum grid dissipation is event that poor vacuum cannot be improved by opera- 4 kilowatts and protective measures should be taken tion of the VACION pump the user should contact CPI to insure that this rating is not exceeded. Grid dissipa- Microwave Power Products, EIMAC Operation and re- tion is approximately equal to the product of dc grid view the details with an Applications Engineer. current and peak positive grid voltage. A protective spark gap device should be connected between the If the tetrode is grid-driven the cathode (and therefore control grid and the cathode to guard against exces- the filament power supply) is generally referenced to sive voltage. dc ground potential, that is no bias voltage or other voltage is applied to the filament. In this case the VA- Under some operating conditions the control grid may CION power supply may be used with no isolation and exhibit a negative resistance characteristic. This may the shield and connector on the VACION cable should occur with high screen voltage when increasing the be grounded for reasons of electrical safety. If howev- drive power results in a net decrease in grid current. er the tetrode is to be used in grounded-grid (cathode- Large values of negative grid current can cause the driven) configuration as is typical in the case of VHF, amplifier to become regenerative. The driver stage then rf drive applied to the tube’s cathode/filament will must be designed to tolerate this condition. One tech- also be present on the VACION cable and therefore nique is to swamp the driver so that the change in load, good rf isolation must be provided to prevent rf power due to secondary grid emission, is a small percentage from flowing on this cable. The system designer must of the total driver load. therefore incorporate rf filtering on the VACION cable to keep rf energy from passing back into the VACION SCREEN OPERATION - The maximum screen grid power supply and its metering circuit; ferrite chokes dissipation is 15 kilowatts. With no ac applied to the around the cable may be suitable. The filament sup- screen grid, dissipation is simply the product of dc ply should be grounded as any dc voltage that is ap- screen voltage and the dc screen current. Anode plied between the filament to ground will interfere with voltage, anode loading, or bias voltage must never be proper VACION operation. removed while filament and screen voltages are pres- ent, since screen dissipation ratings will be exceed- ANODE OPERATION - The maximum anode dissipa- ed. Suitable protective circuitry must be provided to tion rating of 1500 kilowatts should not be exceeded remove screen power in case of a fault condition. A even for very brief periods during setup or tuning. protective spark-gap device should be connected be- tween screen grid and the cathode to guard against Anode current which flows at high anode voltages excessive voltage. with no rf, such as interpulse idling current, must be avoided by such means as reducing screen voltage or Operation of the 8974 at high screen grid current, even increasing control grid bias during the “idling” period. if under low duty pulsed operation, can result in suffi- Current flowing at high anode voltage generates sig- cient screen grid heating to cause significant screen nificant levels of X-Ray radiation. At typical Class AB grid current. Such operation will not cause tube dam- idling currents, X-Ray intensity is very high and repre- age provided proper procedures are followed; howev- sents a significant potential hazard to personnel in the er, the screen grid power supply should be designed vicinity of the tetrode. See X-RADIATION HAZARD on with this characteristic in mind so that the correct op- p.9 for more information. erating voltage will be maintained and tube life will not be compromised. Operation with low values of anode current under some conditions of high instantaneous anode voltage 7 8974

under all conditions. Dangerously high anode current stored energy in the load system, care must be taken may flow if the screen power supply exhibits a rising to avoid excessive return energy from damaging the voltage characteristic with negative screen current. A tube and associated circuit components. current path from the screen to cathode must be pro- vided by a bleeder resistor to absorb the reverse cur- MODE SUPPRESSION CONSIDERATIONS – Large rent without allowing the screen grid voltage to rise ex- high-power gridded tubes, including the 8974, have cessively. A series-regulated power supply can only be natural circular resonance modes of oscillation which used when an adequate bleeder resistor is provided; must be suppressed during initial equipment develop- a shunt-regulated power supply is also very effective ment. towards meeting this requirement. The short compact stem structure of EIMAC tubes PULSE OPERATION - The thermal time constants of provides easy access for mode suppression tech- the internal tube elements vary from a few millisec- niques. It is recommended that short pulse testing be onds in the case of the grids to about 200 milliseconds used to detect this phenomenon and to evaluate the for the anode. In many applications the meaning of effectiveness of the suppression techniques used. duty as applied to a pulse chain is lost because the in- terpulse period is very long. For pulse lengths greater The 8974 has been found to exhibit internal circular than 10 milliseconds, where the interpulse period is mode oscillations in both L-band and S-band frequen- more than 10 times the pulse duration, the element cy ranges. These TE11 modes must be suppressed dissipations and required cooling are governed by externally to prevent damage to the tube and to pro- the watt-seconds during the pulse. Provided the watt- vide stable operation in the intended application. One seconds are less than the listed maximum dissipation technique which has worked to suppress these circu- rating and sufficient cooling is supplied, tube life will lar modes is using ferrite tiles. The ferrite tiles can be not be compromised. cemented (using General Electric RTV-102 or equiv- alent) to the conical and flat surfaces of the “screen FAULT PROTECTION - In addition to the normal an- deck” around the base of the tube. The size of the tiles ode over-current interlock and coolant interlock, the can be up to approximately one inch square or rectan- tube must be protected from internal damage caused gular and 0.1 to 0.3 inch thick. The ferrite must have by any arc which may occur. A protective resistance properties such that the material is effective at the cir- should always be connected in series with the grid and cular mode frequency but not excessively lossy at the anode to help absorb power supply stored energy if fundamental frequency otherwise excessive heating an arc should occur. An electronic crowbar, which will of the ferrite may occur. One source for the ferrite discharge power supply capacitors in a few microsec- material is: National Magnetics Group, Inc. in Beth- onds after the start of an arc, is required. A maximum lehem, PA, USA. For further information contact CPI of 50 Joules is permitted during an arc but this should Microwave Power Products, EIMAC Operation. be limited to less than that amount if possible. The pro- tection criteria for each supply is to short each elec- In some instances, a compromise between tube ef- trode to ground, one at a time, through a vacuum ficiency and anode load impedance by way of modi- and a 6-inch length of #30 AWG copper . fying the output matching circuit will prevent circular The wire will remain intact if proper criteria are met. mode oscillations from occurring while a tube is de- livering peak output. Adjustment of both the screen As noted under GRID OPERATION and SCREEN voltage and grid bias voltage while testing for circular OPERATION, a protective spark gap should be con- mode oscillations is also predicated, fine adjustments nected from the control grid to ground and from the in both parameters will often reduce or eliminate circu- screen grid to ground. CPI Application Bulletin #17 lar mode oscillations. titled FAULT PROTECTION contains considerable de- tail and is available on request. Changing a tube’s operating conditions from those ini- tially established for a given application may also lead LOAD VSWR – Amplifier load VSWR should be moni- to the need for re-checking a tube for circular mode tored and the detected signal used to operate the oscillations prior to commencing operation under the interlock system to remove anode voltage within 20 new conditions. milliseconds after a fault occurs. In the case of high

8 8974

X-RADIATION HAZARD - High-vacuum tubes op- INTERELECTRODE CAPACITANCE - The actual erating at voltages higher than 15 kilovolts produce internal interelectrode capacitance of a tube is influ- progressively more dangerous X-ray radiation as the enced by many variables in most applications, such voltage is increased. This tube, operating at its rated as stray capacitance to the chassis from the tube ter- voltages and currents, is a potential X-ray source. minals and associated wiring. To control the actual Only limited shielding is afforded by the tube envelope. capacitance values within the tube, as the key com- Moreover, the X-radiation level may increase signifi- ponent involved, the industry and military services cantly with tube aging and gradual deterioration, due use a standard test procedure described in Electronic to leakage paths or emission characteristics as they Industries Association Standard RS-191. The test is are affected by the high voltage. X-ray shielding may performed on a cold tube, and in the case of the 8974, be required on all sides of tubes operating at these with no special shielding. Other factors being equal, voltages to provide adequate protection throughout controlling internal tube capacitance in this way nor- the life of the tube. Periodic checks on the X-ray level mally assures good interchangeability of tubes over a should be made, and the tube should never be op- period of time. The capacitance values shown in the erated without required shielding in place. If there is test specification or technical data are taken in accor- any question as to the need for or the adequacy of dance with Standard RS-191. shielding, an expert in this field should be contacted to perform an X-ray survey. In cases where shielding The equipment designer is cautioned to make allow- has been found to be required, operation of equipment ance for the capacitance values, including tube-to- with interlock switches “cheated” and cabinet doors tube variation and strays, which will exist in any nor- open in order to be better able to locate an equipment mal application. Measurements should be taken with malfunction can result in serious X-ray exposure. mounting which represent approximate final layout if capacitance values are highly significant in the design. RADIO-FREQUENCY RADIATION - Avoid exposure to strong rf fields even at relatively low frequency. Ab- SPECIAL APPLICATIONS - When it is desired to op- sorption of rf energy by human tissue is dependent erate this tube under conditions different from those on frequency. OSHA (Occupational Safety and Health listed here, write to CPI Microwave Power Products, Administration) recommends that prolonged exposure EIMAC Operation ATTN: Applications Engineering; to rf radiation should be limited to 10 milliwatts per 607 Hansen Way, Palo Alto, CA 94304 USA. square centimeter.

9 8974

OPERATING HAZARDS

Proper use and safe operating practices with respect to power tubes are the responsibility of equipment manufactur- ers and users of such tubes. All persons who work with and are exposed to power tubes, or equipment that utilizes such tubes, must take precautions to protect themselves against possible serious bodily injury. DO NOT BE CARE- LESS AROUND SUCH PRODUCTS.

The operation of this tube may involve the following hazards, any one of which, in the absence of safe operating practices and precautions, could result in serious harm to personnel.

HIGH VOLTAGE – Normal operating voltages can be HOT WATER – Water used to cool tubes may reach deadly. Remember the HIGH VOLTAGE CAN KILL. scalding temperatures. Touching or rupture of the cooling system can cause serious burns. LOW-VOLTAGE HIGH-CURRENT CIRCUITS - Per- sonal jewelry, such as rings, should not be worn when HOT SURFACES – Surfaces of tubes can reach tem- working with filament contacts or connectors as a short peratures of several hundred °C and cause serious circuit can produce very high current and melting, re- burns if touched for several minutes after all power is sulting in severe burns. removed.

RF RADIATION – Exposure to strong rf fields should MATERIAL COMPLIANCE - This product and pack- be avoided, even at relatively low frequencies. age conforms to the conditions and limitations speci- CARDIAC PACEMAKERS MAY BE AFFECTED. fied in 49CFR 173.424 for radioactive material, ex- cepted package-instruments or articles, UN2910. In addition, this product and package contains no beryl- lium oxide (BeO).

Please review the detailed Operating Hazards sheet enclosed with each tube, or request a copy from CPI, Eimac Division Application Engineering at 650/592-1221.

10 8974

11 8974

12 8974

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607 Hansen Way, Palo Alto, CA 94304 . Tel: 650/592-1221 . Fax: 650/592-9988 5/12 Printed in USA

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