[Amps] 4CX1500 class AB1 vs. AB2

[Tom W8JI]

> I have decided to build a 4CX1500 amp.  I have a couple of 
> tubes and sockets
> already. 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.

What particular 4CX1500 do you have? The A  or the B? There 
is a huge difference.

The 1500B generally should be used at zero or very near zero 
grid current.

> Is there anything else I need to look out for?  The specs 
> do not show AB1
> service, but I assume it can be set up the same as a 
> 4CX1000.  Am I wrong?

You better not run the 4CX1000A anything but sub-class 1 (no 
grid current).

If you limit the screen dissipation properly, you won't need 
to worry about interlocking the screen. If you don't limit 
dissipation, you will eventually cook a tube even with the 
interlock.

73 Tom