Paul
It was on this board a while back that someone commented that any
amplifier built to be on a desk top was not going to be gutsy enough to
do constant power modes indefinitely (or words to that effect). Packing
more in a smaller box can be a problem not limited to HF amplifiers.
Look at modern servers. Actually it is not necessary to look as they can
be heard from a distance. New MOSFET technology is helping by offering
lower internal thermal resistance thus getting more heat from the
junction to a heat sink. It is still up to the engineer to develop a
robust heat sinking system to remove that heat. Liquid cooling as used
in gaming computers is an option. Several BC TX manufacturers have gone
to that method in the TV and FM lines.
On 4/30/12 6:56 AM, Paul Christensen wrote:
>> One thing that seems to be missed in this discussion about home brewing
>> amps was clearly pointed in a comment in this thread -- the key to solid
>> state amp design is THERMAL design and protection. Those are very
>> different skills than most amp designers of our generation learned, so
>> at least in part, it's an "old dog new tricks" problem, not a limitation
>> of the technology.
> Jim,
>
> Based on my experience, I agree with your comments above. First, the
> solid-state amps I've used are not very power-stable with PA deck
> temperature. In the broadcast industry, output power must generally stay
> within a reasonably tight tolerance. The thermal drift from my SPE amp
> (-15%) would not qualify. OTOH, solid-state devices have been used in
> broadcast transmitters since the late 1970s (e.g., Harris MW-1). My
> experience in college with the MW-1 is that it was very temperature stable
> and was able to be cooled at a noise level no greater than that of a 1KW VT
> transmitter. So, perhaps in our quest to make SS amps as compact as
> possible, manufacturers are placing emphasis on size rather than noise and
> high duty-cycle heat conduction properties. By making cabinets smaller,
> compromises are made that affect performance.
>
> Seeing output power change with temperature is not that big of a deal as
> long as the amp remains reasonably linear. But it's an extreme annoyance to
> me as is the sound of jet noise coming from the amp when I'm engaged in a
> 3-5 CW exchange. IMO, the greatest improvements to be made to ham-class
> amps is in the area of cooling and thermal power drift.
>
> In VT amps, we can rely on high-volume designs to keep air moving at low
> sound pressure levels. Perhaps someone can create a similar design to
> create a finned heat that allows for a high volume, but low velocity of air
> to keep the PA deck cool. No doubt, that will mean an increase in box size
> at least until more efficient methods can be developed. I believe there was
> a recent QEX article that discussed next-generation SS cooling.
>
> Another factor affecting users of some VTs is that SS devices are just as
> prone to go obsolete. If the MRF150 was to go obsolete tomorrow, probably
> enough existing Asian sources can pick up that demand since there are so
> many of those devices out in the hands of commercial, industrial, and
> military users. But contrast that against the two-brick Tokyo Hy-Power
> design (Microsemi ARF1500). I'm not sure there will ever be enough of those
> devices deployed where it would make economic sense for an off-shore company
> to pick that up. Moreover, there may be protected intellectual property
> contained within the bricks that preclude the manufacture of the devices --
> at least lawfully until such time that an active patent expires.
>
> Paul, W9AC
>
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