[Amps] CW and High Voltage

[David Kirkby]

R.Measures wrote:

> I define "full drive" as what is required to bring the  cathode-I up to 
> the point where linearity is on the verge of decreasing.  For a young 
> 3-500Z, this is c. 420mA.  If the anode-V is c. 3000, this roughly 70w 
> per cathode.

I don't think that is a good definition. For a start, the subjects says "CW" for which you don't need a linear amplifier anyway.

But consider the amplifier is for SSB use, for which it should be linear. Linearity will almost certainly start decreasing at 25% or less than what the tube is rated to produce. i.e. I suspect a 4CX250B (rated by Eimac at 300W output) will start decreasing in linearity from 75W or even less. Do you care if the 3rd order products have changed from -60 to -59 dB down? That is a decrease in linearity, but not likely to be one that would worry most amateurs (assuming they know what it means). 

Professionally the maximum output power of many amplifiers is defined in terms on the 1dB compression point. At the 1dB compression point, the amplifier is clearly less linear than at the 0.5dB compression point. 

Perhaps a better definition for maximum output power of a tube operated in amateur SSB service would be defined in terms of the 3rd order intermodulation products being 35dB below one tone of a two tone test. Once you have defined what maximum output is, only then you can start defining maximum drive. But with maximum output defined as  "the point where linearity is on the verge of decreasing" I think you are on *very* shaky ground. 

35dB would probably be considered adequate for amateur service. I suspect many will be happy with it poorer than that. For some professional services, it would be inadequate. Hence the definition would depend on the service to what an amplifier was to be put. 


-- 
David Kirkby, 
G8WRB

Please check out http://www.g8wrb.org/ 
of if you live in Essex http://www.southminster-branch-line.org.uk/