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DOCNUMBER 8410
Document Issue 1.1
August 2015
Page 2–7
RKR Designs LLC
Alpha 8410 Linear Amplifier User Manual
Amplifier Components and Specifications
The tubes are operated in Class AB1, with a plate voltage of 2800 V
(nominal, full output, key down), a grid 1 voltage of –50 to –60 (minus 50
to minus 60) V, and a grid 2 voltage of 200 V. They have a low-
inductance resistor in series with their cathodes. This resistor stabilizes
the tube bias and provides negative feedback, which improves linearity
and hence intermodulation distortion (IMD) performance. Electronic bias
switching (EBS) increases the negative grid 1 voltage in pauses in speech
or between Morse code elements. This reduces the standing bias on the
tubes, resulting in less waste heat, longer tube life, and higher overall
amplifier efficiency. The artifacts of EBS are not noticeable under normal
communications conditions.
The tubes are operated as a “swamped grid” tetrode design. The tube grids
are tied at RF to a 50-ohm swamping resistor that absorbs most of the
input-drive power. The RF voltage across this resistor is added to the
grid 1 DC bias to provide the net low-impedance tube grid 1 bias. The RF
impedance represented by grid 1 and its capacitance is compensated for
by a series inductance to provide SWR <2:1 on each band at the
amplifier’s input. At higher frequencies, a relay switches in a separate
matching network. This relay is under microprocessor control and is
actuated according to the band.
NOTE
To prolong tube life, refrain from cycling AC power on-off-on-off
repeatedly. It is less stressful to leave equipment in standby for
several hours than to cycle it on and off repeatedly.
The tube deck is a mechanical assembly enclosing the tube sockets and
the tube-deck board. The tube sockets contain the integral screen grid
(grid 2) RF bypass capacitors as well as contacts for the screen, heater,
and filament of the tubes.
2.5 Specifications
The Alpha 8410 linear amplifier specifications are as follows.
Table 2-2
Alpha 8410 linear amplifier specifications
Parameter
Value
Frequency coverage
1.8–29.7 KHz
Input-drive level
50 W nominal
Power output
1500 W
SWR tolerance
3:1
Duty cycle
100%
Tubes
2 4CX1500B (Alpha part number VTX-X120
