> 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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