On 1/24/2014 10:58 AM, Ralph Mowery wrote:
"Jerry Stuckle" wrote in message
...
And yes, many inexpensive "modern" receivers suffer from poor front
ends. And since transistors (especially bipolar) typically generate more
noise than tubes,
noise can be a worse problem now than in the 60's. You can get low noise
transistors, but these are more expensive.

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How did ou come up with transistors generate more noise than tubes ?


Almost 50 years of experience, including studying both tubes and
transistors in my EE courses back in the 70's. Plus measurements of
both received and transmitted signals, using lab-grade test equipment.

The easiest way of seeing it is looking at the output of both tube and
transistorized transmitters on a spectrum analyzer. You will see much
more hash on the transistorized transmitter.

Back in the 70's, I ran a CAP repeater from my home. Transmit and
receive antennas were separated by about 25' vertically. It was a
surplus Motorola tube rig, running 25W. I was able to run it without
any desense without duplexers. Yes, the channel spacing was 4.25Mhz,
but you can't do that with a transistorized rig.

Most of my research into low noise has been above 50 mhz and the beter tubes
generate much more noise than transistors. The older U310 devices have a
noise figure less than 2 db at 150 mhz and a 6cw4 will have around 3 db.
Cheap gaasfets have noise figuers less than 1 db.


Yes, nowadays, there are transistors with lower noise figures. But they
are relatively expensive, and you won't find them in the less expensive
receivers.

I seem to remember that most tube sets of the old days were stating about .5
uv senstivity on ssb, and many of todays ham trasceivers are way less than
that.


Plus or minus, that is about right. But that wasn't because of the
tubes; they could have done better but it would have required more
amplification and higher cost. Plus with a decent antenna, the
atmospheric noise was higher than that, so there was no need for more
amplification. It would have just been lost in the AGC circuitry.

Even back in the early 70's, commercial tube VHF radios could easily get
..15mv for 20db S+N/N ratio. Not much different than the transistorized
versions today.



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