Elenos T25000, Handbook

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Hand Book T25000 -
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Technical Office TEL: +39 532 829 965 FAX: +39 532 829 177
TECHNICAL DESCRIPTION
The T25000 unit is composed of various functional elements: power amplifying cavity which houses the
thermionic tube, (this is a triode with direct cathode heating, configured in common grid mode with input
signal to the cathode and output from the anode); low-pass output filter for the suppression of harmonic
components of the carrier frequency; analog filter connected to the input circuit; three phase high voltage
power supply for the thermionic tube's anode; cathode bias circuit; AC stabilised power supply circuit for
the filament/cathode; safety circuits for the protection of the unit and service personnel; power supply for
the motors which control the amplifying cavity's tuning. The unit is designed to function with the
thermionic tube at full power in class C mode, at lower powers in class B and very low powers in class
AB. The cathode bias circuit (see card E20123.x) is automatic and senses cathode current. The
cathode/grid voltage is obtained via a chain of power diodes connected in series with each other. The very
high voltage gain of the thermionic tube allows the class of amplification to be changed with small
changes in bias voltage. The tube filament AC supply is kept constant by a phase-angle switching power
supply with stability of better than 1.5% (see cards E20127.x and E20128.x). Activation of the filament
power supply is conditional upon the pressure of cooling air within the amplifying cavity; if regular, the
filament supply is enabled. The working voltage is reached progressively to avoid overcurrent to the tube's
socket. A regulator circuit is available on card E20127.x which allows the filament voltage/power to be
varied in order to extend the life of the thermionic tube if the RF power required is less than the nominal
value (see technical notes on the filament). The high voltage anode power supply (7500 volts DC) is
generated by a six phase rectifier (see card E20144.x) with an inductive filter followed by a capacitive
filter. A high power, insulated resistor is connected in series with the tube's anode supply output in order
to limit the energy dissipated in the event of accidental electric arc within the amplifying cavity or the
thermionic tube. The high power transformer is inserted electrically in 2 steps, the first connects a series
resistor to limit overcurrent, the second connects the transformer directly to the line supply. This avoids
inadvertent power loss due to the action of the local supply's safety cut-outs. The principal electronic
circuit card (E20127.x) contains all the user interface circuits (see also card E20131.x) and the circuits
which monitor all the important variables of the amplifier. Control of the unit includes the facility to
disable operation in the event of variables exceeding their safety limits, with various automatic reset
modes. For a detailed description of the unit's protection logic, consult the chapter entitled "user
interface". The low-pass output filter is housed within the unit, above the amplifying cavity. It is essential
that this filter is both connected and functioning perfectly; in case of malfunction it is not possible to
guarantee the suppression of spurious, out-of-band RF frequencies and, furthermore, dangerous internal
overheating of the equipment may be caused. The control of the RF tuning of the cavity is performed by
2-speed electric motors. This technique , besides being convenient, ensures the safety of personnel
responsible for tuning the equipment. There are two amplifier cavity adjustment controls for the input and
two for the output; one of the two matches impedance, the other tunes the frequency.