|Theory of Operation|
|Manual and Schematics|
Alpha 8406 6 Meter Linear Amplifier
The ALPHA 8406 is fully assembled and tested at the factory prior to shipment to a customer. Because of the mass of the AC transformer and the concern of damage to the chassis during shipment, the unit is shipped in two packages - one containing the power transformer and the second containing the rest of the fully assembled amplifier. After unpacking, the transformer is installed by the user by mounting it inside the cabinet into four pre-drilled PIM-reinforced mounting holes and is connected electrically to the Amplifier using two factory-installed Molex-type connectors. No other assembly is required by the end-user.
After installation of the A/C transformer by the user, the entire Amplifier is contained in one enclosure. The Amplifier is designed to operate as a "single band amateur VHF amplifier" within band segment from the 50-54 MHz Amateur band. The output stage of the amplifier is designed around a 4CX1500B (8660) (Alpha P/N VTX-X120) tetrode, a vacuum tube manufactured by Eimac in San Carlos, CA.
The amplifier has input and output bypass switching which permits the driving source (transmitter or transceiver) to be directly connected to the load. This allows reception of signals if the load is an antenna and the source a transceiver, or direct transmission from the transmitter without the amplification afforded by the ALPHA 8406 if the source is a transmitter. An input matching and input RF detect board detects (in an analog fashion) the presence or absence of drive power from a nominal 50 ohm source, and matches this power to the drive requirements of the tube within the band 50-54 MHz. The 4CX1500B tube amplifies the RF signal, and the output matching network matches this power to a nominal 50 ohm impedance.
The output power is low-pass filtered and returned to the output bypass relay. This power, and any reflected from the load, is monitored by an output power sensor. The power for the operation of the tube and all other circuits is supplied from the AC line by a transformer-coupled power supply of conventional design. A controller module establishes correct operating conditions for the tube and provides operating power for the ancillary circuitry. A front panel display allows the operator to correctly set up the amplifier so that it can be operated in compliance with the FCC requirements. The amplifier does not require any alignment or set-up functionality other than the routine front-panel tuning of the output tank circuit that is required to match the amplifier output to the output load over the 50-54 MHz band.
DC operating Conditions
The final tube is operated in Class AB1, with an electronic bias switch (EBS) to switch the tube to its nominal bias setting only when RF is present at the input. The tube plate voltage is approximately 3.00 kV under no-load conditions, and approximately 2.70 kV when tuned up correctly and delivering full output power. The current drain from the high voltage supply is in the range 0.9 to 1.2 Amps under full load. The grid current is less then 20 mA under normal conditions. The control grid (grid 1) voltage is also regulated, and is set during manufacturing to values that give the correct idle current for the tube which is about 300 mA. The approximate control grid voltages are: receive, -65 V; operating idle, -8.0 Vdc; operating active.
Special Design Considerations
Care has been taken in the design of the ALPHA 8406 to ensure compliance with the applicable FCC requirements set forth in Part 97 and Part 2 of the Commission's rules. Examples of these features include:
Minimal gain at 27MHz
FCC rules severely restrict gain in the 11m (27MHz) band. This is frequency from 26-28 MHz. The ALPHA 8406 has a standard design feature which makes achieving useful gain in this frequency range impossible. The frequency of the driving signal will not pass through the amplifier when enabled due to the tuned input and output circuits being resonate only in the 5054 MHz band. The first stage of the RF sample circuit is resonate at 50-54 MHz only and will not sample any RF out side the 50-54 MHz spectrum. This is necessary to take the amplifier from Stand-by to Operate. In the design engineering of the amplifier the amplifier gain at 26-28 MHz has gain of -25 dB.
Gain in the intended amateur ham bands of between 50-54 MHz is limited to no more than 15 dB by means of the way in which the tube is biased and operated. Increasing the gain beyond this is not possible by any simple modifications.
The output filter provided in a Pi C network impedance matching for each amateur band segment. Typical second through tenth order harmonics attenuation is better than 72 dBc due to the Q of the matching networks and the addition of a high Q low pass filter in the output network.
There is no computer, crystal oscillator, switch-mode power supply or other source of spurs non-harmonically related to the driving RF source. The tube circuitry has been designed in accordance with well-established procedures, including the use of cathode resistance to stabilize gain and bias. Each section of the amplifier design is compartmentalized, and adequate filtering is used for connections between sections.
The cabinet is closed by multiple fasteners, which are spaced close enough to prevent accidental leakage of radiation from the seams. The power cord is bypassed to prevent radiation and has RF beads installed on the AC leads into and from the AC power supply. RF cable connections are provided with standard "N" Female connections. Key line interface is also filter with RF beads and by-pass capacitors.
There are no user-accessible modifications which allow the amplifier to exceed it's design specifications. The tube is operated in a socket specific to that type of tube, and there are no tubes of greater capability which will fit into that socket.
There are no power supply voltage taps or other DC voltage or current adjustments which increase output power. The power supply has been designed such that the amplifier is reliably capable of sustained operation at full power. In addition, the amplifier power supply will sense the input voltage and automatically select the proper input voltage tap on the transformer so the power supply will always have the proper tube anode voltage.
There is no input attenuation of the RF input signal. The input circuit is constructed on a printed wiring board, and it would be difficult for a user to modify this in any way to exceed the specifications without risking consequential damage to the amplifier. The amplifier is a tetrode swamp grid driven design with a resistor terminating the input RF. Almost all of the RF input power is dissipated as heat or loss in the input circuit and drive for the tube. Changing its value or removing it will result in amplifier instability which will likely damage or destroy part of the amplifier. In addition if the drive is increased beyond an acceptable level the amplifier will fault, automatically removing the amplifier from the RF path.