Using RM Italy KL-Series Linears On HF

I’ve been using an RM Italy KL-400 linear amplifier on HF for a couple of years to boost the reach of my Yaesu FT-917ND. There are quite a few modifications and considerations that are needed to use these linears on HF.

I see these linears as a ‘component’ rather than a ‘product’. Whilst the manufacturer’s web page says 400W and 3-30MHz, in reality it’s a 14-30MHz 150W amplifier. You must add a fan in all circumstances and also add external low pass filter(s) to use on anything other than 10m & 11m. It’s an inexpensive way to go QRO from an FT-817.

Whilst I see that RM Italy appears to have discontinued this particular model, they are still selling similar models. Also I note that they have introduced quite a few new models which are very similar to the KL-400, but they have fans bolted on to the top of the heatsink, no surprise in that development!

If you are considering buying or already have one of these RM Italy linears, I expect you could apply the same principles to your own device, even though the model number might be different.

Low Pass Output Filter

Here’s my 20m low-pass filter enclosure – hewn from solid 100x100mm extruded 6000 series alloy bar stock. At 1.3kg it weighs more than the radio, I believe.


I made air-core inductors with 1mm enamelled wire (not a great idea, they spring about alot and so the inductance value is vague, at best), some silver mica caps and manhattan construction. Here’s the circuit for 20m:


KL-400 PTT Modification

The VOX activation of the linear by 5W output from the FT-817 works as long as you don’t pause for breath. With less than 5W input the VOX operation is unusable. It’s handy to be able to reduce the input power to the linear to scale back to 20-50W, but this means adapting the KL-400 to accept a PTT input from the FT-817 ACC interface. I got some tips from Bert van Lelieveld PA3EKA (with the help of the Wayback Machine since his ham web pages went down some years ago) and compared his RM Italy KL-500 information with the KL-400 schematic. I hope Bert doesn’t mind, here’s his photo of his KL-500 Rev 3.00 mod with a phono socket to the back of the amp:


There’s an NPN transistor that’s used to switch the power amp side of the circuit via a relay. If you short the collector of that transistor to 0V that switches you to transmit mode. In the case of the KL-500 version 3.00 board that’s TR3. In the case of my KL-400 Rev 4.10 board, that’s T4, a BC547. Pull the point marked with the red dot below to 0V with the PTT output from the FT-817 and then you can forget about VOX feature and switch the SSB / AM switch to AM. I did this mod by adding a wire to the pad for R23 (green dot) which is a no-fit and also connected to the collector of T4. It works just fine. While I was at it I also added a 13.8V out switched from the main amp power switch to switch the external fan. To finish, I boxed up a relay and flyback diode to the 13.8V and TX GND of the FT-817 ACC port. It’s not that I don’t trust RM Italy not to fry my precious Yaesu radio but…


Here’s the linear with some new wiring for the PTT input. While I was about it I also added a 12V output that’s switched with the linear to make a neat fan connection.