Hmmm...There seems to be some confusion here. The tubes in question are
Eimac 4CX1500B tubes. According to the data sheet it lists and recommends
AB2 operation for lowest IMD in RF linear service. This is in stark
contrast to the 4CX1000A which states running it into the AB2 region is a
bad thing. Running the 1500 in AB2 offers 1100W output in typical
operation. My main question is can the tube be operated in AB1 according to
the 4CX1000A specs for additional power output (screen voltage at 325V vs.
225V)? Does AB2 operation with the 4CX1500B really offer a noticeable
advantage in IMD over running it in AB1?
I think the bottom line is that if running the tube at AB2 (225V screen)
offers no significant advantage over AB1 (325V screen) then I have no need
to make it switchable.
Thanks,
Mike, W1NR
> 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.
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