Scott Townley
Raritan, NJ

I think the 4CX250B (and many of its bretheren...the 4X150A/D, 4X250, 4CX300A, 4CX350A, etc.) are misunderstood.  Further, I think that most of the misunderstanding is that much of the original handbook and magazine literature on these tubes was, well...wrong.

All the information here is openly available, but is scattered in different publications of different vintages and difficult to assemble in one spot.  This is an attempt at consolidation.

NX7U

Cathode-Driven and "Grounded-Grid" operation

No, they are not the same thing.

Screen Regulation

Is terribly critical and nearly never done correctly, particularly in articles of 1950s-1960s vintage.

Heater Operation

Absolutely, Positively No More than 6.3 V...and preferably less

Cooling and Blower Requirements


These tubes are not easy to cool.

There is no such thing as a "1 kW 4CX250B" amplifier.  Period. 


A "1 kW 4CX250B splatter generator", maybe.  But certainly not a linear amplifier.

Class AB1 means NO GRID CURRENT


And a Class C amplifier is not a linear amplifier.