On 1/24/2014 2:24 PM, Ralph Mowery wrote:

"Jerry Stuckle" wrote in message
...
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.


I think you are mixing apples and oranges. For transmiters the tubes
usually have less broad band noise. One reason is not the tube, but the
tuned circuits are much more selective. With the high impedance of the
tubes it is easy to be very selective due to the circuit Q.
For a receiver, it is still all about the noise figuer and having enough
gain (which is not usually a problem) to overcome the noise of the other
parts of the receiver.


No, I am not mixing apples and oranges. Sure, the transmitter tuned
circuits have a higher Q, but that does not affect noise on nearby
frequencies (like 4.25Mhz apart on 2 meters - less than 3% of the
transmitted frequency).

Remember also that receivers also have tuned circuits for input; many of
the older receivers had preselectors to tune the input to the desired
frequency (and these circuits typically had higher Q than transmitter
output circuits).

Yes, you could run the CAP repeater with seperate antennas with tubes where
you could not with the transistors. As above the circuit selectivity has
alot to do with it. Tube circuits are much more selective when it comes to
broad band noise. Many transistor receivers are broad band in the first few
RF stages. That gives two problems to over come. Broad band noise for the
transmitter (which I am not talking about) and the broad RF stages of the
receiver (Not noise of the transistor/fet but poor selectivity). The old GE
Mastr ll is one of the few that has a fairly narrow front end. I have one
of those on 2 meters.


No, tubes themselves generate less noise, especially when running in a
non-linear mode such as Class C. But transistors definitely generate
more noise, as can be identified on a good spectrum analyzer.

What do you call relative expensive for a transistor/fet that has a noise
figuer of around 1 db ?


For manufacturers, anything over a couple of cents per device. But also
GAsFETs are also more susceptible to static charges from the antenna,
requiring additional protective circuitry at the front end.

Even in some of the old ARRL repeater handbooks they are putting fet preamps
ahead of the tube receivers.


Yes, and they also put tube preamps in front of the tube receivers.
FETs were real popular back then, mainly because they weren't tubes -
and didn't have the high power requirements associated. They also were
new, making them ripe for experimentation (quite popular at one time).

What tubes are you talking about that has an under 2 db noise figuer at 150
mhz ? I think there was a 417 and maybe a 416 that might make it, but they
were very expensive, especially the 416.


I don't remember tube numbers any more - that was over 40 years ago, and
I haven't touched a receiver tube circuit in at least 30 years But I
also remember having to design low noise RF circuits - and make them
work. Not easy to do even in the lab; much harder for manufacturers.
And when we were doing solid state amplifiers, it was much harder to get
a great noise figure.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================