Cobra XL 450 Linear Amplifier
This is the other unit that my customer brought in for
test. The heatsink and front panel layout looks very familiar. It is a
copy of the old Palomar Magnum linear amplifier. The Palomar folks no
longer use the Palomar name or produce linear amplifiers since they were
busted many years ago. They managed to get off with a small fine, and they
don't ever want to go through that ordeal again.
Before and after the bust many linear manufacturers used
the Palomar name for their products because it was the most recognizable
for it's quality and performance. Very few people new that long before the
bust they had come to an agreement with Palomar Engineers to no longer use
the name even though they were the original holders of the name. They
didn't want to be dragged though court and defend the production of these
amplifiers, even though they sold them to countries were they were legal.
They started to use the Magnum name alone as this was recognizable as
their top of the line band. Few people new the way to identify a counterfeit,
was by the Palomar name still being displayed on it.
These counterfeit units were of inferior quality and
design. They didn't incorporate the filtering and regulated biasing or
even the logical layout of the originals. Many of these units created a
mess on the air with all types of harmonics and distortion. Most of them
didn't last long either. In a black market, beggars can't be choosers, so
many buyer went along and hoped they got the real McCoy. And sellers
appreciated having a name that commanded top dollar.
This Cobra is a duplicate of the Magnum but with Motorola
MRF 454 transistors, which are a true sideband device. This unit has 4 of
these devices and is Class AB1 biased. The unit has the original negative
feedback and lowpass filter circuits. The layout is the same as the
amplifier built back in the 80s. The four stage power selector switch is
also the same configuration of the original. The oversize low profile
heatsink is the same as well. I made a phone call to double check if these
were produced under license from Palomar and was told they had no
connection to the units in any way. From my description it was determined
that someone has copied an old unit.
There are two jacks on the rear of the amp that were
unfamiliar. One labeled KY and the other labeled RX. I was told the RCI
2950 it is used with has been modified with two cables coming out of the
back of the radio that plug into the XL 450. The one labeled KY is tied
into the PTT circuit which keys the amp in any mode as soon as the mic is
keyed. The other is connected to a switch on the front panel so the
receive preamp can be controlled from the radio face. Great features that
I don't know who should get credit for.
On the bench, the amplifier had an output of 400+ Watts
PEP on sideband at 28.300 MHz into a dummy load. This is a high drive unit
that will take 25 to 30 Watts peak power. On AM with a 3 Watt carrier
input the output carrier was around 100 Watts with plenty of modulation
swing to around 375 Watts peak. The receiver preamp was of high gain,
around 20 dB with very low noise. The keying circuit seemed to key the amp
with power less than a 10th of a Watt, which is perfect for sideband. The
sideband delay is perfect. The four stage power selector gives a good
spread of selection.
This amp is probably made in the states, from the outside
it resembles some I've seen around. I'm not sure if the units sold in the
U.S. have the MRF 454s or use the Toshiba transistors. Illegal or not,
it's a pleasure to see quality amplifiers. Bob F