I decided that this was going to be a ‘quality’ project and set out from the start do design something that not only looked good, but also had one or two features not generally found on ‘home-brew’ kit.
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Getting the 50V into the box was my first concern. 50V is a high voltage, sufficient to give you a fright if you’re not careful, and more than sufficient to cause expensive damage if the polarity were reversed!. I didn’t want a heavy on/off switch on the front panel, but I did want front panel switching. At these high currents, it’s tempting to forego any switching and simply wire the power right into the unit. However I came up with a way to combine reverse polarity protection and DC input switching by using a high current relay, a diode and an attractive illuminated switch. When the supply is connected, a small green LED illuminates, indicating that the PSU is on. Pressing the large red button on the front of the amplifier energises the high current relay via a diode, connecting the 50V (48V, actually) to the rest of the circuitry. The diode serves to prevent the relay energising in the event that the polarity is reversed.
300W Amplifier - Front Panel
Initially, I wasn’t going to have any metering on the amplifier since I already have a VSWR/Power Meter. However in light of the expensive nature of the output devices (a matched pair of MRF429s), I felt it was prudent to include a Collector Current meter.
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The rear panel, has four rather than the usual two SO-239 sockets. The AN758 typically requires between 15 and 18 watts to drive it to it’s maximum output. The majority of transceivers these days deliver around 100W while a much smaller number will deliver up to 200W and a few others offer 30W or less. Even if the output power can be adjusted by way of a front panel control, it is still very easy to accidentally over-drive the amplifier and send the PA devices into melt-down. So I decided to make provision for an external attenuator which can be omitted and replaced with a length of coax if not required. Phono sockets are provided for PTT input and ALC output and the DIN connector is for interfacing to the Auto Band Selector Unit.
300W Amplifier - Rear Panel