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People keep rating transistors based on the NF on a spec sheet when really each amplifier design should be looked at from the perspective of the components and impedance driving it and the device operating point.

The front end common base amps in old Uniden CBs are not ideal designs but if you want to improve the noise figure of the complete amplifier then really this should be done either by trial and error with real test equipment or (much better) the design should be simulated on a computer and the biasing and input components optimised for source impedance and operating point to get best noise figure. I'd guess you could get a useful improvement in noise figure just by keeping the 2SC1674L and swapping the parts around it. I'd be very surprised if the input transformer LA279 was ideally optimised for NF for the 148GTL-DX radio.

I've not tried the 2SC2999 swap but I'd also be very surprised if people saw much benefit unless other parts were swapped to optimise the design as above.

Also, there's some really dodgy information being handed out on this thread about suitable diodes for the front end TR/AGC diodes without any appreciation of how these parts were spec'd by the manufacturer.

<blockquote data-quote="G0HZU" data-source="post: 277816" data-attributes="member: 19109">People keep rating transistors based on the NF on a spec sheet when really each amplifier design should be looked at from the perspective of the components and impedance driving it and the device operating point.The front end common base amps in old Uniden CBs are not ideal designs but if you want to improve the noise figure of the complete amplifier then really this should be done either by trial and error with real test equipment or (much better) the design should be simulated on a computer and the biasing and input components optimised for source impedance and operating point to get best noise figure. I&#039;d guess you could get a useful improvement in noise figure just by keeping the 2SC1674L and swapping the parts around it.&nbsp; I&#039;d be very surprised if the input transformer LA279 was ideally optimised for NF for the 148GTL-DX radio.I&#039;ve not tried the 2SC2999 swap but I&#039;d also be very surprised if people saw much benefit unless other parts were swapped to optimise the design as above.Also, there&#039;s some really dodgy information being handed out on this thread about suitable diodes for the front end TR/AGC diodes without any appreciation of how these parts were spec&#039;d by the manufacturer.</blockquote>

[QUOTE="G0HZU, post: 277816, member: 19109"] People keep rating transistors based on the NF on a spec sheet when really each amplifier design should be looked at from the perspective of the components and impedance driving it and the device operating point. The front end common base amps in old Uniden CBs are not ideal designs but if you want to improve the noise figure of the complete amplifier then really this should be done either by trial and error with real test equipment or (much better) the design should be simulated on a computer and the biasing and input components optimised for source impedance and operating point to get best noise figure. I'd guess you could get a useful improvement in noise figure just by keeping the 2SC1674L and swapping the parts around it. I'd be very surprised if the input transformer LA279 was ideally optimised for NF for the 148GTL-DX radio. I've not tried the 2SC2999 swap but I'd also be very surprised if people saw much benefit unless other parts were swapped to optimise the design as above. Also, there's some really dodgy information being handed out on this thread about suitable diodes for the front end TR/AGC diodes without any appreciation of how these parts were spec'd by the manufacturer. [/QUOTE]

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