Re: Elecraft KPA500 vs Tokyo Hy-Power 1.1Kfx

Posted by Joe Subich, W4TV-4 on Aug 18, 2012; 12:10am
URL: http://elecraft.85.s1.nabble.com/Elecraft-KPA500-vs-Tokyo-Hy-Power-1-1Kfx-tp7561226p7561234.html


> 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
>>
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