[Amps] 4CX1500 class AB1 vs. AB2

[Ian White GM3SEK]

Tom W8JI wrote:
>> 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.
>
The manufacturers recommend these tubes are used with an ALC system to 
keep them in class AB1. However, the most practical way to derive the 
ALC signal is to detect the grid current that starts to flow when it 
crosses the boundary into class AB2.

The current needed to activate the ALC on peaks of modulation only needs 
to be about a milliamp, or even less... but it isn't zero, so strictly 
speaking we have crossed the line into class AB2. This creates a 
confusing situation where the data sheet has to say "AB2" to be correct, 
but this hides the more important fact that the tube spends almost all 
of the time in class AB1.

Contrary to the ham folklore, edging over the border into AB2 doesn't 
have to be an IMD disaster. The two possible causes of IMD when crossing 
from AB1 into AB2 are a sudden change load impedance on the exciter, and 
a sudden change in DC grid bias. The change in load impedance is 
minimized in modern amps that use a grid swamping resistor, and a well 
regulated bias supply will keep the voltage constant.



-- 
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek