[Milsurplus] trany to tube transplant

[Bob Camp]

Hi

The real part of the input and output impedance of a Pentode at RF is  
monstrously high. Effectively it's a vacuum dielectric capacitor and  
vacuum isn't very lossy. The equivalent numbers on a MOSFET are not  
quite as good. There you are looking at the loss in a silicon dioxide  
capacitor or a doped silicon capacitor. The tube numbers are at least  
an order of magnitude higher.  Leakage current probably is higher on  
the tube, but that's not important at RF ....

Bob


On Aug 31, 2007, at 2:59 PM, Hue Miller wrote:

>
>
> > Sensitivity is the easiest thing to get when doing a solid state
> > "conversion". You have *way* more gain to work with using FET's than
> > with tubes. The problems come with the more difficult to measure
> > stuff like selectivity and overload immunity. Anybody who has bought
> > a cheap SWL radio knows exactly what to expect .....
> >
> > Bob
>
> MOSFETs certainly have higher input and output impedances than
> bipolar transistors and are similar in a way to tetrode vacuum tubes.
> Maybe better - there will NEVER be any grid current in a MOSFET.
>
> If you look at the schematic for a cheapo 10-bands for $19.95 radio
> you'll see that they use ONE chip most often, and some outboard parts
> like coils hung on it.
> I saw one recently in an ad for some vitamins supplement, the radio  
> was
> free  for the "First 500 Responders" to the advertisement. The  
> description
> said that besides for example hearing guerilla communications, you  
> could
> hear satellites in deep space. Since they provided a postage-paid  
> envelope,
> i took the opportunity to tell them what i thought of their "gift"  
> radio,
> their product, and the quack doctor  who was out to mine the rubes'
> wallets.
>
> I think i recall reading that the performance of the 75S receiver  
> with the
> Tubesters in, was very similar, with the sensitivity slightly  
> greater than
> before. However, much less drift. Some folks would pine for that good
> old-time drift. -Hue
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