Home made

“Hi DXing,

I read your spiel on the Chinese Tubes and that’s my experience too. I’ve repaired quite a few of the Rudi EMTRON linears using these tubes, and I can tell you the problem, they don’t get enough cooling…

These tubes are a copy of the Eimac 4CX1500b, if you alter the filament voltage and substitute a second hand Eimacs, no more problems. On the subject of cooling if you look down the fins of the Eimac tube you will see that they have internal stampings that give about 30% more radiation of heat into the air. The Chinese tubes are plain fins. In my personal linear that I made in 1979, I’m using a 4CX1000A , BUT as it uses the same socket I can accommodate a 4CX1500B and I have a switch to change the screen volts from 350 down to 225.

The 1000a is capable of 1740W output, the 1500B being a later design tube and optimized for SSB is a little bit cleaner, i.e. better 3rd order intermod. I’m still using the original tube in mine that I bought from a technician who maintained ABC country broadcast stations, I paid $20 in 1979.

They used to change the 4CX1000As in the modulator when audio distortion got above 6% but for RF service these tubes work like a new one, I actually bought a 20 litre bucket full of these tubes… I enclose a few pictures of my build and my home made plate transformer on the tail gate of my then 60 series Land Cruiser. My linear is all home brew, I even made the roller inductor and got it silver plated, I used to grid drive this linear, but I was testing a tube for a friend and it shorted plate to screen and blew the SG Bypass cap which is integral in the socket. SO I have earthed the screen and I’m using the Collins drive system “SUPER cathode drive” which is to say I’m driving the cathode and the control grid also [with a small sample] both drives in phase, I now introduce the screen grid supply to the cathode. I’ve been doing this for the last twenty years. The screen supply is a string of 68V 10W Zeners. I have a 400V Mov across the screen supply to protect it and the SG supply is interlocked with the plate supply. I’m using a sensitive 28V relay 1mA coil in series with a 3M ohm resistor when the 3Kv builds and the AIR interlock operates the relay picks up and starts the screen supply. NO ELECRONICS everything is done with Relays!

Enjoy your DX discussions too, cheers,

Bernie N”