In March of 2001, I embraced the idea that I should endeavour to run more power on HF. My confirmed DXCC total was by then well over 200, and those entities left un-worked tended to be somewhat rare, and on many people’s ‘most wanted’ lists. The problem there is that when a rare entity comes ‘on-air’, the whole world and his brother pounce on him. I needed an amplifier. One thing I had to be mindful of was the fact that my ATU is only rated at 300W. Initially I hadn’t considered building my own amplifier, and I began searching for ‘off the shelf’ proprietary kit . . . . . without success. It appeared that if I was going to buy one, I would have to invest in one capable of developing 800W at just over 1 UK pound per Watt! A ‘trawl’ of the Internet yielded roughly the same result. I was going to have to build my own!
A browse through the Motorola RF Devices data book reintroduced me to Helge Granberg’s designs and in particular application note AN758 which describes a 1.2KW HF Linear. This design employs a quad of 300W Push-Pull amplifiers in it’s final stage, combined to comfortably deliver just over 1KW. This particular design runs from a 50V supply, which might be looked on as being difficult to obtain, particularly when the current drawn by one 300W stage alone can be in the region of 14A. 50V power supplies are simply not readily available, particularly at that sort of rating. A similar amplifier running from a 28V supply would require around 20A and to develop that sort of power from a 13.8V supply would require an input current in the region of 50A!
I was going to have to build my own power supply anyway, so I figured that I might as well go for the 50V option. In June 1986, John Matthews, G3WZT published an article in RadCom describing a single stage linear amplifier for 50MHz. This article also details a very robust 50V high current PSU complete with ‘soft-start’, over-current and over-voltage protection. I found that power darlingtons are essential as pass transistors in this design and I was most impressed when I had smoke pouring off my power rheostat while the output voltage remained ‘rock steady’.
The January 1992 issue of RadCom described a 150W HF linear amplifier by Mike Grierson, G3TSO. This used another of Helge Granberg’s designs, with the actual amplifier being supplied in kit-form by Communications Concepts of Ohio, U.S.A. A quick e-mail to CCI confirmed that they were able to supply the AN758 units for a very reasonable price . . . . . and a look at their WebSite confirmed that they could also supply the parts for the necessary output filters. I ended up also getting the massive heat sink from them too.