> 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.
In solid state broadcast transmitters, there is always APC (automatic power
control). The exciter or IPA power output is varied dynamically, usually
under microprocessor control, to keep the final TPO at the setpoint. It is
not uncommon for the final amplifiers to drop in gain as they come up to
normal operating temperature. 1 dB wouldn't be unusual, 2 dB probably would
be.
> OTOH, solid-state devices have
> been used in
> broadcast transmitters since the late 1970s (e.g., Harris MW-1).
The Harris MW-1...now there's a transmitter I'd like to forget...
> 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.
The obvious solution here is liquid cooling.
> 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.
High volume/low velocity only works when you have a suitably large surface
area of heatsink. Heatsinks aren't cheap, and, if the goal is to maintain a
small form factor, you have design criteria that are mutually exclusive.
Again, liquid cooling makes good sense.
> Another factor affecting users of some VTs is that SS devices
> are just as
> prone to go obsolete.
Very true. However, modern SS manufacturers are usually very responsible
when it comes to putting out EOL (end of life) notices for devices well in
advance. That doesn't seem to be the case with hollow state.
> If the MRF150 was to go obsolete
> tomorrow, probably
> enough existing Asian sources can pick up that demand
Be very wary of buying SS RF devices from overseas. There is so much
counterfeiting, re-labeling of devices, selling of e-waste, etc., going on,
that you never know what you're going to get. I've been burned several
times.
When it comes to selecting devices, learn from what we've seen in the tube
realm. The devices that have a high usage in commercial fields will likely
be around longer than those that only the hams are using. 8877's are still
being made, and likely will continue to be made until the commercial world
stops using them. Same with LDMOS FET's. MRF151G, BLF278, SD2932, etc. are
still used in large quantities in broadcast, medical imaging, etc., so
expect them to be available for quite some time. Picking the latest and
greatest kilowatt-class ruggedized FET from Freescale or NXP might seem like
a good idea initially, but unless the device gains acceptance in the
commercial world, don't count on it being around 10 years down the road...
--- Jeff WN3A
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