tubes age 2
* Replace the 3TF7 with a diode that is rated to handle the voltage and
current. This creates a pulsing DC voltage of the proper value for the
filaments.
Nolan Lee provided some background on the 3TF7: " The resistance wire
is usually iron, and the glass envelope is filled with either hydrogen or
helium gas for heat conductivity. The glass envelope runs about 160
degrees. Current regulation is usually within plus or minus 1%. They
work with either AC, DC, or pulsating current. When the current in the
circuit is increased to a high enough level for the regulating function to
start working, only a small portion of the filament will glow. As the voltage
across the ballast increases, more and more of the filament will glow.
When the entire filament is glowing, you're at "max" and any additional
increase will overheat the tube and shorten it's life. The rated life
expectancy when operated as recommended within its ratings is 2000
hours. Run it at "max" all of the time and it's only 1000 hours. Run it at
80% of max and it's 5000 hours. If a steady voltage of a value in the
middle of the operating range is applied to the tube continuously, it's life
will be tens of thousands of hours. Opening and closing the circuit with
the resulting expanding and contracting of the filament greatly reduces
the life of the tube. Also, as in incandescent lamps, turning the unit on
and off many times will reduce it's life especially if the unit if operated
near it's maximum voltage.
If full voltage is applied to the tube, the circuit may be opened and closed
only a few hundred times before the current is outside of the limits or the
filament is burned out. Thus the life of the tube will be determined entirely
by it's duty
cycle."
Two 26Z5 tubes are used in the R-390A. The 26Z5W is a high voltage
rectifier tube.
1. V801 is located in the Power Supply subassembly, where it
forms one half of a full-wave rectifier. V801 appears in
figures 47 and 80 of TM-11-856 and in figure 28 of
TM-11-5820-358-35.
2. V802 is located in the Power Supply subassembly, where it
forms one half of a full-wave rectifier. V802 appears in
figures 47 and 80 of TM-11-856 and in figure 28 of
TM-11-5820-358-35.
Solid state replacements were installed in some R-390A receivers by the
military. Often, The shield bases on the tube sockets were bent inward
and flat with the chassis to prevent tubes from being inserted into the
sockets. This modification can usually be reversed by removing the solid
(14 pages)
(10 pages)
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