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The 8791V1 Tube

The type 8791V1 tube, which is used in the aural and Visual IPA stages of the FL-Line of television transmitters, was selected for this application because of certain characteristics which made it suitable for the requirements.  This tube type has a unipotential, oxide coated matrix type cathode which plays an important role in determining these characteristics.

The main advantage of an oxide coated cathode are high emission efficiency and high peak current capability.  A characteristic of this type of cathode is a gradual change in cut-off characteristics over the first few hundred hours of life, as compared with a tungsten cathode.  It is very probable that the change in characteristics has led many people to believe that their tubes have reached the end of their useful life prematurely.  It has been determined that the average life can be extended appreciably by adhering to the following set-up and operating procedure for the visual IPA stage in all FL transmitters.

It should be noted that the procedure requires dual screen supplies to allow balancing of the static plate currents between the two push-pull tubes.  Even with the older, single output screen-supply, it is possible to raise the screen voltage and possibly extend the useful life of some tubes; however, it may be necessary to select tubes for a reasonable balance.  Also, the voltage adjustment control is not conveniently located on the front panel of the single supply as are the controls for the dual supply.

It is suggested that the following procedure be initiated immediately with tubes that are presently in service.  Do not wait for new tubes to be installed.  It is even possible that some of the tubes which were previously removed from service might be returned to service for several thousand hours.  

Visual IPA - 8791V1 Set-up procedure

  1. When new tubes are installed in the visual IPA (Modulated Amplifier), measure heater voltage at P201 and adjust to a maximum of 5.5 volts RMS.

  2. Modulate the transmitter with sync only (no setup) while making the following adjustments and recording meter readings.  Feedback clamp switch 15S2 must be set to INTERNAL.

  3. Turn off the exciter(s) and observe the visual IPA bias, multi-meter switch position 11, and adjust PEDESTAL level control for a reading of 40 volts.

  4. Observe the visual IPA cathode (1), multi-meter switch position 12, and adjust the IPA screen voltage  (1) for a reading of 0.25 Amps, using OUTPUT 1 VOLT ADJ on the screen power supply.  See note 1

  5. Repeat step 4 for tube #2 using position 13 of the multi-meter switch and OUTPUT 2 VOLT ADJ on the screen supply

  6. Note the reading on VIS MOD AMP PLATE current meter (1M5 or 4M5) and trim the screen voltages, if necessary, to obtain a reading of 0.50 Amps, while maintaining a balance of the cathode currents.

  7. Turn on the exciter(s) and adjust the VISUAL EXCITATION control for an average power output equal to peak power divided by 1.68.  Allow the transmitter to operate under these conditions for one-half hour before continuing.  See Note 2

  8. Turn off the exciter(s), rechack the static currents and repeat step 6 if they have changed.

  9. Turn on the exciter(s) and readjust the EXCITATION, as in step 7, if necessary.

  10. Record the meter readings, and then check the set-up of the transmitter for differential gain and differential phase.

  11. Record the meter readings weekly.  If the static currents have changed, adjust the screen voltages as in Step 6.  Be sure that the transmitter has been in operation long enough to become stabilized before making adjustments or recording meter readings.

  12. If screen voltage adjustments are made, record a new set of meter readings.

  13. After the maximum screen voltage has been reached for a particular tube, additional hours of life can be attained by adjusting the heater voltage upwards in 0.2 to 0.3 volt increments.  Do not increase the heater voltage to compensate for a decrease in RF drive or video modulating voltage.

  14. When a tube is to be replaced, the heater voltage should be reduced to 5.5 volts or less, and the screen voltage should be readjusted to its minimum value.  Failure to do this could result in catastrophic failure of the tube as a result of an extremely high static current and plate dissipation at turn-on.

Note 1.  Screen voltages as high as 700 volts are permissible; however, it may be necessary to increase the spring tension on the LV overload relay 2K26 (3K26) for a trip setting of 400mA to prevent nuisance tripping due to turn-on surges.

Note 2.  Operate the tubes with the lowest heater voltages that will furnish full power.  A considerable increase in life can be realized from the tubes by operating them at reduced heater voltage.  If the tube is in the aural IPA, follow Step A, below,  If the tubes are in the visual IPA (modulated amplifier), follow Step B, below.

A.  Reduce the heater voltage in 0.1 volt increments until a reduction in aural PA grid current is noted.  Then increase the heater voltage 0.1 to 0.2 volt above this point.

B.  Modulate the visual transmitter with black picture at 100% power output with 25% sync.  Reduce heater voltage in 0.1 volt increments until sync starts to compress.  Then increase the heater voltage 0.1 to 0.2 volts above this point.