I wanted to drive my amplifier with my FT-950 radio. This radio can do 100 Watt, and gives 10W if you push the TUNE button.

This 100W is far too much to drive the 600W amplifier. If you push the power of the FT-950 down to the minimum, it gives 5W of output power, but if you forget this, you turn the amplifier in to smoke. So I wanted to work the FT-950 on the 100W ( also because the FT-950 is designed to be doing this, best linearity and impedance). Because the max input of the Amplifier is about 5W, we need to use an attenuator. I use the 100W, 20dB device of AVX. They are not cheap, so I hope it will work OK. The 100W, 20dB attenuator is recommended by Razvan. There are many types of it available on the internet.

Because this attenuator is producing nearly 100W of heat in the Amplifier, it is extra important to work with a very good cooling system. See the corresponding chapter.

 

It is now June 25. I found some time to update my documentation.

 

I changed the 20dB attenuator to a 3dB attenuator on the PCB of the Amplifier. And between the FT-950 and the amplifier I connected a box with a 10dB attenuator. So with input of 100W, I have 10dB which brings the power down to 10 Watt  and the 3dB attenuator brings it further down to 5 Watts for driving the FETs. Just according to the specs.

(Later, I put the 10dB attenuator on a cooling block and screwed the cooling block into the case, close to the back panel. Works fine.

Razvan is warning that some 100W radios produce a start peak of power, which can be significant! Up to a few 100 hundreds of Watts. That can kill your FETs ! I tested the FT-950 radio on this behavior. I found out that in the low power range (5-20W), the overshoot of power can go up to 20%. But in the high power range up to 100W it is getting less. At 100W, it is only a percent of 3-4.

During experimenting, make sure you measure the input power via the sense port. It will not kill your FETs if you do a short 6 Watts, but continuously going over the specs of 5W will probably damage the FETs bit by bit a bit,......HI.

In this pictures you see the 3dB attenuator on the amplifier PCB, and the experimental box with 3dB, 10dB and 20dB attenuators. They can dissipate a max of 100Watt.

Juli 4th: I discovered that above the 80Watt input the voltage on the FETs gates is flattening. The contribution to the output power is getting less. But again, be warned that you stop at 5 watts input ! And check the input power for all bands, the FT-950 can deviate about 10%.

**********************************************************************************************************

IMPORTANT UPDATE !!!!!!!!!

**********************************************************************************************************

After a defect FET (why?????) I checked again all connections on the amp PCB and I found out that I was one of the lucky guys that got a defect PCB!!! The C's from the drains to earth were NOT CONNECTED to earth. This is confirmed by Razvan. There was a small batch of defect PCB's,.........

I corrected this by soldering 2 times a rather big wire from the C to the earth, scratch the paint on the PCB(2mm thick). The result was that the amplifier was doing a much better job. Efficiency changed from 49% to 65% !!! The DC current went down from 22A  to 19 Amp at 550W.

Also the overall gain went up from 20dB to 23dB.

So I had to do something at the input  stage. More attenuation was needed and I decided to change the input attenuator from 3dB to 6dB. So if the ft-950 gives 100W, the input on the FETs is 100 minus 10 dB = 10W, minus 6dB =2.5W. Enough to produce 550W output (CW)!!

***********************************************************************************************************