I'm drifting off topic a little but is it not preferable for heat transfer
purposes to run several lower power transistors in parallel to reduce hot
spots? There is also the opportunity to have redundancy, ie if one in a
string dies, the amplifier can still work. The transistors will also be
significantly cheaper. Matching input and output will be more difficult but
the robustness obtained is more desirable than minimum parts count. The
layout will be more of a challenge to achieve the highest frequencies.
Perhaps from a commercial point of view this is less desirable but for home
builders easier to achieve high power in a small space, which will become
the dominant restriction. Water cooling is a lot quieter and smaller than
fan cooling : - )
David
G3UNA
>
>
> On 10/17/2010 1:37 AM, Baruch Zilbershatz wrote:
>> The DL guys that built that gizmo had it clamped with copper bars and
>> screws both sides to a copper heat sink .looks great.
>> One guy made his own Al heatsink from folded Al strips and on top of it
>> had a copper heat spreader.
>
> I hope you mean "on top of it" as meaning "in addition to". Copper, the
> better heat transfer material goes *under* the finned aluminum heat sink.
> Putting the Aluminum first is like installing a resistor as it would
> limit the ability to transfer heat.
>
> Actually this is an ideal place for water cooling and it doesn't take
> much. In addition you can even use chilled water.
> The heat sink is at ground potential so the problems associated with
> water cooled vacuum tubes running high voltage are not encountered.
>
> The best heatsink compound goes between the transistor and copper heat
> sink. Using an all copper heat sink makes more sense than using a
> sandwich of copper and then finned aluminum except to save money and
> when you get into a project with expensive transistors, the cooling
> system is certainly not a place to try to save money. It's a place to
> save expensive transistors. Of course if copper is difficult to obtain
> in the required dimensions I can understand using a finned aluminum heat
> sink over the copper. This might even be a good place to use the
> multiple "heat pipes" in an exotic configuration.
>
> 73
>
> Roger (K8RI>>
>>
>> Hi Dan,
>>
>>
>>
>> You are right, heat and control are the primary issues to solve, and they
>> can be solved in a solid state design just as they are in a tube design.
>>
>>
>>
>> When the QEX article came out, I contacted JA1DJW about reproducing his
>> article. He was very kind and sent me many emails with details, pictures
>> and some of the critical parts for the amp. I think there are two
>> fundamental reasons the 2.5k isn't rated for full power output in
>> continuous mode.
>>
>>
>>
>> The first reason is the power supply isn't rated for continuous duty and
>> is maybe a bit under sized for the application. A secondary reason
>> would be the heat issue. Nobuik sent me a heat spreader but I opted not
>> to use it since the heat spreader is about 3.5" x 3.5" x 3/8" copper. I
>> opted to use a 12 " x 8" x 1/2" heat spreader with "copper fins"
>> attached and replicated his machine work layout onto it. Efficiency is
>> more along the order of 60% at the worst, usually better though.
>>
>> From: Dan Levin<dan@andlev.com>
>> Subject: Re: [Amps] New 2 PA design
>>
>> The big issue with parts like the ARF-1500 is managing the heat. For
>> high duty cycle use, like contesting or RTTY (or worse yet, contest
>> RTTY :-) ), you are dealing with dissipating something like 1500 watts
>> of heat (assuming 50% efficiency running class AB) being generated in
>> an area about one inch by three inches.
>>
>>
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