75.6% efficiency shows the devices are biased deep into class C, not
pulsed. Linear should be 60 to 65%. With 4 devices running 1500 to 2500
PEP total out the heat and junction temperatures should not be a
problem. Even linear / digital at 1500 shouldn't be a problem as they
are running less than 400W per device..
These appear to me to be a major step in the right direction. OTOH we
should not forget that a tiny (short duration) voltage spike can take
out a SS device, while tubes are relatively forgiving. Very forgiving
when compared to SS devices
73, Roger (K8RI)
On 4/23/2017 2:21 AM, Roger (K8RI) wrote:
On the parallel, PP, the convention has been PP, parallel. Each unit
runs two devices, PP with the outputs into a combiner.. Those new 65
Volt devices,
http://www.richardsonrfpd.com/Pages/Product-Details.aspx?productId=1241241
rated at 1800 watts ea (one page lists Max as 2KW Carrier) at $250 ea.
4 would be $1,000 and only 375W per device. Less than a quarter of
their ratings which should require far fewer efforts at the most
efficient cooling per device.
Actually with 4 of these, 3 KW out is still less than 50% leaving them
running well away from the "1% knee"
NOTE the base, rather than being insulated is the source, so the
copper spreader would be at 65 VDC
Running at those levels would require less protective circuitry and an
ability to handle higher SWR. Of course, with that much overhead
there would be those who would want every watt they could get out of
it even though the circuits were optimized for the legal limit, or
relatively close to it.
With 4 devices at $1,000, we are very close to the cost of tubes
capable of running any mode at the legal limit. Even at the 1800 W
limit we're looking at 7200 Max which 4 devices should do on SSB. How
ever you look at it these new LDMOS are capable of working the legal
limit from 160 through 440 although the LP filters could get kinda
messy, but ALL bands with one amp! Now there's something to think
about. OTOH the layout for HF and low VHF wold probably be a problem
at high VHF and UHF.
73, Roger (K8RI)
On 4/22/2017 10:08 PM, Jim Thomson wrote:
Date: Sat, 22 Apr 2017 16:53:50 +0000 (UTC)
From: Catherine James <catherine.james@att.net>
To: <amps@contesting.com>, Manfred Mornhinweg <manfred@ludens.cl>
Subject: Re: [Amps] Price per Watt Conversation
Manfred,
So would it be better to use 4 or or more devices with somewhat
lower ratings for linear operation?
Is there a cheaper option to use simpler heat sinks that don't
have to be carefully machine, but push them less hard by spreading
the power over a larger number of devices? What's the practical
upper limit of devices that could be used before creating other
design problems?
73,
Cathy
N5WVR
## why not use 4 x BLF188XR or 4 x MRFX1K80H ? Is it even
possible to use 2 x devices in parallel in each half of a push-pull
amplifier ? If it is possible, then the heat could be extracted
over 4 x devices.
The TX imd, with 4 x devices run at 1.5 kw, on paper, should be good,
since each device is running at 375 w pep output. Toss in
pre-distortion, and IMD could be reduced further.
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