Elenos T30000, Handbook

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Hand Book T30000 -
9
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TECHNICAL DESCRIPTION
The T30000 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.