Cobra XL 450 Linear Amplifier From Mexico

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.

Cobra XL 450 Face
Cobra XL 450 Side View.
Cobra XL 450 Rear View.
Cobra XL 450 Inside View.
Cobra XL 450 PC Board View.

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

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