> Along the same line, different amp:
> Tom, have you tested any of the Mirage VHF amps?
> I have a few which don't seem to do well. Never tried to
> measure IMD, but they're not anything close to linear once
> operated at maybe
> 50% of rated output power. 1 dB compression occurs at
> maybe just over 50%
> rated output, and at 100% rated output they're in heavy
> compression (CW
> signal, 14Vdc power). Not just one, but a few different
The one of the last things I did was redesign the Mirage 160
watt amp. The old Mirage amps were a disaster. They had
.01uF disk caps from collector to ground to attempt to
stabilize them, and ratty bias systems. They were parallel
transistors through a half-xxx combiner. It was sometimes
hours to get one stable.
I laid out a strip line board with the same push-pull
transistors and put an active bias system in, but the
problem is the transistors are running totally out of steam
at 150-160 watts. That's why they are linear only up to 70
or 80 watts. Any transistor amp I've seen has to be about
half or 3/4 of saturated power to be decent, but all the
manufacturers like to identify them by the saturated power.
When I turned the manual over to them I'm positive I advised
people not to run more than 70 or 80 watts PEP on linear
modes, but I have no idea what the manuals are like today. I
don't even know the model number of that amp, but it should
be easily identified by the oval race track stripline with
12 ohm semi rigid coax soldered to it.
By the way, a new trend at HF is for people to get into 100w
rigs and crank up power limit pots to run 120-150 watts.
That of course turns already marginal radios into real
splatter boxes. People fail to realize that unlike a tube
that goes into compression really fast, transistors slope in
over a wide power range. You can run a tube amp optimized
for 1500 watts close to 1500 watts without problems, but
not bipolar solid state. These moronic mods are on the web
at ham sites. While receivers are getting better we are
doing our best to ruin transmitters.
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