[Amps] 4CX1500 class AB1 vs. AB2

[Peter Chadwick]

W1NR said:
>I know that it is designed for class AB2 service for very low
distortion. This seems to limit the tube to 1100W output. I am considering
adding a switch to change the screen and grid bias to allow me to run it in
AB1 service as well for 1500W output.<

Generally, there's more output in AB2 than in AB1. The data sheets I've found suggest you can get 1800 watts with 4kV on the plate in AB1. AB2 requires a low impedance RF drive, because you don't want the source impedance to be high enough that you get distortion when grid current starts, annd you need a stiff, preferably shunt regulated, grid bias supply. Are you sure about running AB1 for more output? I found a Penta data sheet on G8WRB's site: that definitely shows AB1operation.

Losing screen volts with plate volts still on is no problem, BUT losing plate volts with screen volts on definitely is. Likewise, losing grid bias isn't nice, and an interlock is definitely suggested. I'd also suggest an interlock for the air supply, too - and that interlock should also cut the heater if the air fails.
73
Peter G3RZP