I think I was not clear and am possibly being misunderstood. First, I own a KPA500 and couldn't be happier. Second, in no way am I saying there is anything wrong with the KPA500 design. The question was if the power levels are the same and the individual device capabilities are similar, why would one use 2 or 4 devices. I merely reported what one experienced designer told me would cause him to use 4 devices. Was he right or wrong? Probably neither. As I indicated he was used to designing for a different market with a different cost/reliability model. If you are cost driven you will run closer to device maximums. If you wish to go for long life and a lower chance of failure you derate devices.
David K0LUM At 8:10 PM -0400 8/17/12, Joe Subich, W4TV wrote: > > With only two of these devices he says you either have to run the >> voltage too high or have the current density too high in order to get >> 500 watts. > >Since the VRF2933 is *designed* for the current necessary to produce >300 W each with proper cooling, one can not say the current density >is "too high". The THP design with its four ST2933 is effectively >the old Granberg 4xMRF150 amplifier and no different than an Ameritron >ALS-600 with more expensive FETs that are not being used to their full >capability. > > > He was designing military/aircraft gear and had to allow a big safety > > margin. > >The military can afford to pay for redundancy - particularly in mission >critical applications - but such overkill is wasteful in commercial and >amateur applications. > >73, > > ... Joe, W4TV > > >On 8/17/2012 7:31 PM, David Christ wrote: >> I myself am not qualified to judge the designs, but a friend of mine >> with decades of design experience at Rockwell Collins has been saying >> from the first revealing of the KPA500 design that he would have used >> 4 devices not two. If I understand his comments it relates to >> current density and voltage. With only two of these devices he says >> you either have to run the voltage too high or have the current >> density too high in order to get 500 watts. He does agree that the >> KPA500 is a nice amp but that he would have made different choices. >> He was designing military/aircraft gear and had to allow a big safety >> margin. >> >> Hopefully a real engineer can tell me if this makes any sense. >> >> David K0LUM >> >> At 6:20 PM -0400 8/17/12, Robbins, James wrote: >>> I am wondering if anyone has an explanation for why the Elecraft >>> KPA500 amplifier (500 watts) employs 2 VRF2933 FETs (Microsemi) in >>> the final PA where the THP uses 4 ST2933 FETs (STMicroelectronics) to >>> generate very similar power levels. The data sheets for each FET >>> indicate that both brands are rated at 300-400 watts, support similar >>> mismatches and run at similar voltage maximums. They have very >>> similar specs all together. >>> >>> Perhaps our amplifier gurus could weigh in here. Much appreciated. >>> >>> 73, >>> Jim Robbins >>> N1JR >>> >>> ______________________________________________________________ >>> Elecraft mailing list >>> Home: http://mailman.qth.net/mailman/listinfo/elecraft >>> Help: http://mailman.qth.net/mmfaq.htm >>> Post: mailto:[email protected] >>> >>> This list hosted by: http://www.qsl.net >>> Please help support this email list: http://www.qsl.net/donate.html >> >> ______________________________________________________________ >> Elecraft mailing list >> Home: http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:[email protected] >> >> This list hosted by: http://www.qsl.net >> Please help support this email list: http://www.qsl.net/donate.html >> >______________________________________________________________ >Elecraft mailing list >Home: http://mailman.qth.net/mailman/listinfo/elecraft >Help: http://mailman.qth.net/mmfaq.htm >Post: mailto:[email protected] > >This list hosted by: http://www.qsl.net >Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
