How to select a solid-state HF amplifier ? (I)
If you like digital gears and if complain about performances of your antenna system, in this case the purchase of a solid-state HF amplifier can be a good solution to work DX stations in better conditions.
I do no mean that your amplifier might replace a defective antenna. Far from me this idea. Do never use a high power to compense the poor performances of an antenna. This is indeed the best way to create QRM. Build or purchase first a better antenna !
If fundamentally a solid-state amplifier ensures the same functionalities as a tube amplifier, its architecture is quite different as well as the intensity of voltages and currents flowing in its circuits.
Tube and a solid-state amplifier : differences
Not only currents and voltages are differents in a tube and a solid-state amplifier but the load resistance differs also in both designs. So if a tube amplifier requires a RF tank circuit (Pi-L network) to transform the load resistance in 50-ohm, due to the low load resistance of a solid-state PA module, a broadband architecture is requested using ferrite-core transformers. As the input-transformer primary presents a 50-ohm resistive load, tuned input networks are no more necessary because the exciter always sees a 50 ohm resistive input; on other words, an auto-tuner is not required to drive a solid-state amplifier.
To get 1 kW PEP output, you need first twice that power in DC input (see below), then several pairs of RF power devices, usually MRF-150 MOSFET power transistors, each of them being a larger version of the PA stage that we find usually in HF transceivers. If a tube amplifier uses only one PA module, the tube(s), a solid-state amplifier is always push-pull to reduce even harmonics (due to low Q).
In a kW-class amplifier we find an hybrid-transformer power-splitter. The splitter and combiner can for example be 5-ports circuits, 1 input and 4 outputs, the combiner having 4 inputs and 1 output.
Contrarily to the tube amplifier, in which a tuned circuit like a Pi-L network helps reducing harmonics, a solid-state amplifier uses low-pass filters (LPF) behind the combiner output to remove harmonics and spurious emissions to a level less than –46 dBc (up to -60 dBc). Its output is routed via the output T/R relay to a reflectometer for security purposes, and then to the antenna connector. This last entry is wired via the input T/R relay to the input port of the splitter. All this building makes a broadband, self-tuning solid-state HF amplifier.
For short, a solid-state amplifier is constituing of the following modules : a PA stage, a controller board, an ALC, low-pass filters, an optional (automatic) antenna tuner, a band-switcher, input interfaces, and the power supply unit. Let's review each of these modules.
Linearity and efficiency
As explained in pages dealing with amplification classes, the main limiting factor of power devices is the constancy of power gain over the entire power-output excursion. Generally, MOSFET transistors will exhibit superior linearity as compared to older bipolar junction transistors (BJTs).
Due to the higher voltage excursion, amplifiers powered between 40-50V DC (thus desktop models like Yaesu VL-1000 Quadra) exhibit also considerably better linearity, and thus lower IMD, than units powered on 13.8V DC, usually portable, because the longest is the output/drive power curve, the better is the linear portion. In addition, it is more difficult to design a power supply for a 13.8V amplifier due to high current requirements (e.g. typically 80A peak for a 500W PEP amplifier like Ameritron ALS-500MX). So usually a tube amplifier is more linear than any solid-state model.
At last the power output is determined by the maximum ratings of RF power transistors, and by the linearity of these devices. Usually these latter are arranged in push-pull pairs, each module being able to sustain 150W or 250W output.
Due to these specifications, you will find 4 push-pull pairs of 150W power transistors in the Yaesu VL-1000 Quadra or Tokyo Hy-Power HL-2KFX amplifier to get 1.2 kW PEP output (for 2.1 kW DC input).
The power gain of a RF transistor, defined as the ratio of RF output to RF drive power, decreases as frequency increases, MOSFETs offering a power gain a bit higher than BJTs (at 14 MHz, respectively 12 dB vs 10 dB). That means that to drive a 1 kW amplifier, BJT's requests input power levels of 100W against 65W only using MOSFETs. If the second model is more efficient, in all cases any solid-state transceiver will drive a kW-class amplifier to full rated power.
A solid-state class AB amplifier yields an efficiency of 45-50%. In a model rated at 1200 W PEP there are only 600 W output, the "missing" watts being transformed in heat (like a car engine). Due to the high power of RF devices, in emission this heat is dissipated in the amplifier case. Therefore a quality PA stage requires a very efficient cooling system, able to maintain the case at nominal temperature.
To respect good RF practices and to prevent an automatic shutdown of the amplifier, the PA cooling system must maintain the amplifier case at nominal temperature, around 80°C (176°F) for at least 30 min in SSB or 10 minutes in key-down CW transmission at rated output. This is accomplished using a heatsink or a heat-dissipator system, both blowing forced-air to maintain transistors at low temperature. In a tube amplifier the heat generated by tubes is simply dissipated in the air and blow out by a fan.
If you mainly use your amplifier for contests and hunting DX stations, or at high ambient temperatures (tropical climate) you might need a
CCS vs ICAS
Continuous Commercial Service (CCS) covers applications involving continuous operation in which maximum dependability and long life are the primary considerations.
Intermittent Commercial and Amateur Service (ICAS) is defined as a service including the many applications where the transmitter design factors of minimum size, light weight and considerably increased power output are more important than long tube life. In this service, life expectancy may be one-half that obtained in Continuous Commercial Service.
However we should all known that above the "maximum ratings", it usually result in catastrophic failure. Therefore power devices running all the time should refer to the more conservative CCS ratings. In the same way amateurs should never force the power stage of their amplifier too high to preserve its lifetime. Keep in mind and we will repeat it again, that the nominal power of your transceiver (usually 60-80W) is far enough to drive your amplifier at the rated power without over-heating the system.
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