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120w 13 cm Amplifier

2304 is one of those bands where you can find some surplus components to build with.

This amplifier uses two 60w Spectrian amplifier boards with the usual input and output hybrid coupler and combiner.

Spectrian amplifiers and their components can be found in the usual online auctions, and occasionally offered as surplus elsewhere.

These two amplifier boards are not terribly efficient, requiring a total of 26a @ 26.5v at full output, or almost 700w in for 120w out. But they do the job.

A Dow-Key high-power transfer switch handles the changeover, and is controlled by a sequencer in the driving transverter, which is pictured below the amplifier in this photo.

As in the 60w 23cm amplifier, the rear panel connections allow for expansion. The transfer switch can be bypassed, and the internal amplifier accessed directly via the bulkhead N connectors on either side of it. These are normally jumpered to the transfer switch as shown.

The sequencer in the transverter supplies the trigger for the transfer switch, and enables the amplifier boards shortly after (the control connector is the interface link).

Here's the layout inside.

The driving transverter has decent power (25w), so I use an input attenuator to drop the drive to about 2.5w before the input splitter.

The attenuator is an 8db terminated hybrid coupler (the copper PC board at upper left). This feeds the input coupler mounted on the vertical bracket to it's right.

On the other side, the outputs are combined in the coupler shown at lower left. All couplers were made from Teflon PC board material.

The small PC board mounted atop the transfer switch is it's latching relay driver.

Well, I guess everyone discovers something else that is needed after construction is complete.

After I had the amp in use for a few minutes, I noticed I underestimated the cooling requirements - the heat sink got too hot - so I added the small fan driver you see here.

The Spectrian boards have a temperature sensor output, and this little board turns on the fan when the sensor indicates 95f.

I used a quiet (22dba) PC fan mounted atop the heat sink, and it plugs into the amp via a small DC power connector I added to the rear panel.