4CX250B Grounded Grid Amplifier

I was in need of a small rf amplifier for my vintage SSB station using a Ten-Tec Triton lV as the driver.

Mr Amplifier, Jerry - K7LFE, suggested I build one using 4CX250B ceramic tubes. I wasn't too crazy about having to develop grid voltage from another supply source in addition to plate voltage and filament voltage. He said that it wasn't necessary. I'd read in numerous places that it just couldn't be done. Here is the proof that it indeed can.

I got some brand new 4CX250's from Bob, N7UA. I purchased a couple of sockets and chimneys from Fair Radio and then it was just a matter of arranging things to fit the case that I had.

                             

                           

Finished

I decided to utilize a EF Johnson roller inductor I had as well as an EF Johnson plate capacitor, shown, and add the loading cap on top of it with an insulation plate in between. You can see the 6.3 vac filament transformer also installed along with a National plate choke and doorknob blocking caps.

                               

I put the two tube sockets into their own small chassis which I then mounted on the back, over the two holes already conveniently made. Since I used a small box from my junkbox, there were numerous holes that had to be plugged. I could then attach a squirrel cage blower to the outside and pressurize this chassis to cool the tubes. It worked nicely and the entire amp rarely gets even warm to the touch running about 650 watts on SSB.

                           

Here is the finished amp. It has only one meter, to monitor the plate current. There are toggle switches on the front for "Filaments", "In/Out" amp, and one other which I installed thinking I'd need it and I don't. You can see this amp on my shack page.

Here is the schematic showing everything. As you can see, there is a tuned input using a toroid, one compression cap and a fixed silver mica. You need this to get max output as the input to the amp is not a good impedance match for solid state rigs.

There is a bifilar choke in the filament leads which doesn't really need to be there. You aren't feeding the filaments so there's n o real reason to isolate from rf.

The key is to feed the cathodes which are all tied together.( Make sure that all of the pins are removed from ground. Many sockets come with the pins fastened to ground thru tabs. These have to be cut. I used my Dremmel tool.) Next the screen grid must be grounded. The control grid, in order to make current flow, must be at a different potential than the screen grid and therefore it is above ground thru a 100 ohm resistor.

That's it. Down and dirty!

This system runs day in and day out at about 600 - 700 watts peak output with about 1750 volts on the plate. The 4CX250 tubes seem to like this arrangement just fine.