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.

I decided to utilize a EF Johnson roller inductor I had an EF Johnson plate capacitor, shown. It sits atop the loading cap 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.

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 (C9, L2, C10) for 75 meters, using a toroid, one compression cap and a fixed silver mica. The values will need to be determined for each band. You need this to get max output as the input to the amp is not a good impedance match for solid state rigs. This is not necessary when using a radio with tube output.

You can purchase tuned inputs from Ameritron. You also may find a complete assembly on Ebay from another amp, such as a Heathkit SB-200.

The key is to feed the cathodes which are all tied together. Some sockets I've found have the cathodes, pins 2, 4, 6, 8, connected internally. This makes it much neater. Otherwise they've got to be connected with heavy wire. Next the screen grid, pin #1,  must be grounded. The easiest way is just to solder it to the metal socket. Make sure that all of the pins except pin #1 are removed from ground. Many sockets come with some of the pins fastened to ground thru tabs. These have to be cut. I used my Dremmel tool.  The control grid, in order to make current flow, must connected to the cathodes 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. A 4X250A or a 4X150A work just as well.