In the first instance, it monitors the front panel push-button, cycling thru the four bands (LPFs) when pushed, writing the current band into EEPROM for persistence over power-down. I used a Nano as controller, overkill in the first instance, but able to support more control sophistication in the future. But I consoled myself with the thought that they would not be engaged when in ‘standby’ mode. 120mA is a bit much current draw for a pair of relays. The outputs also drive a front panel LED to indicate the selected band.
Homebrew amplifier driver#
Each pair of relays is controlled by a 2N7000 driver from an Arduino digital output. They are not identifiable but they tested out OK, drawing 60mA at 5V. I used small SPDT 5V relays, pulled from an old board. High voltage LPF capacitors are one of the more difficult components to buy these days, but I found some suitable ones from Minikits. This power level corresponds to 140Vp-p, and voltages go much higher into a high impedance load, so the capacitors would need to be 250VDC minimum, preferably 500V types. That meant a compact set of four relay switched Low Pass Filters, rated at 50 watts.
My amp would cover the four main HF bands of interest for SOTA work, 80, 40, 30 and 20m. In the pictures, the PA FETs are mostly obscured by the fan. This circuit, and the RF actuated T/R switch, were built ‘ugly style’ on the front of the PA board. An Arduino output keys the relay for 80m and 40m. The amp worked first time, once I had worked out that it needs a good 5 watts of drive before it sits up and takes any notice.īecause my homebrew transceiver’s RF output (IRF510) is highest at 80m and drops off significantly at 20m, I decided to add a 3dB pad, relay-switched in on the lower two bands to reduce the drive level below 10MHz. The input network is a simple FT37-43 transformer as per Don’s circuit. The FET tabs are the drains, so must be insulated from the metalwork (more on this later). There were no issues in its layout or construction. The PA board was built about 18 months ago, for a different enclosure, and is true to the W6JL design. This involved minor reconfiguration of the main board and the design and construction of a new LPF board. The idea was to sacrifice a few nice-to-have extras (a front panel meter, for example) in order to keep it all compact, and to use the heatsink channel as the case. Back in 2017 I had intended to use an on-hand commercial sheet steel enclosure, but after the project sat idle for twelve months, upon returning, I decided to squeeze the whole thing into an aluminium heatsink-shaped channel I had picked up in the intervening time. I made one of my ‘artisan PCBs‘ in the usual way, and construction was straightforward.
Homebrew amplifier full#
When a faint flicker of QRO interest returned recently, I fired it up to the full 50 watts into a dummy load on the bench, and that gave me the impetus to finish this project at last.
I started my build in late 2017 but my fickle homebrew energies got focused on other projects.
All reported a trouble-free build and a good 50 watts from the Class AB IRFZ24N FETs on my bands of interest, 80, 40 and 30m, with a bit less on 20m. This design got some traction with some SOTA activators a few years back, so I studied the stories of builds by Glenn VK3YY, Andrew VK3JBL and Glenn VK3PE. The FETs are commonplace and cheap - they can be bought for as little as 98c each! The amp offers a useful order of magnitude (12dB) power lift over QRP levels, and apart from the power FETs can be built from an averagely stocked junk box.
Don won the QST Homebrew contest in 2009 with this design. Don W6JL’s 50 watt FET amplifier is a popular afterburner for FT817s and other QRP rigs and exciters.
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