[Ronja] Vacuum tube in Ronja preamplifier

[Karel Kulhavy]

On Fri, Oct 12, 2007 at 02:26:01PM +0200, Gullik Webjorn wrote:
> 
> I worked a lot with mobile radio when I was 13-20 years old.
> Most installed base VHF and UHF radios had a dual triode cascode
> as first RF input stage and were beeing replaced with solid state x-ceivers.
> 
> Noise factor was not so good in these designs, so a common "fix"
> we did to "beef up" old sets was to replace the dual triode with
> a dual gate mosfet. The lower noise figure gave new life to old
> radios. Often this could be done "airwire" by soldering the fet
> to the tube socket, and changing bias and DC. Sensitivity could
> go from 1 uV @ 20 dB SINAD to 0.2 uV , more than doubling range.
> 
> To me this indicates there were other sources of noise, a guess
> thermal from the 800 deg cathode electron cloud.
> 
> In these cases, the Cin of the tube was of course resonated with the
> input L/C circuit, so the capacitance did not disturb, whereas in Ronja
> it will create a pole with the PD resistor.
> 
> Are you trying to get more speed or lower noise figure?

Lower noise figure. The input capacity doesn't have an effect on speed
in the Ronja design.

CL<