> Is there anyone out there
>who makes an amplifier who's design is not based upon TLC and the operator
>never making a mistake?
Having worked for a major communications company designing components to
feed forward linear amplifiers, I must state, that even in the major
commercial companies, poor amp designs exits. On one project I was
involved with a module was designed by an engineer who had never done an
RF power amplifier project before. His module was a piece of junk. Kept
on blowing up. In fact, it was the number one most unreliable product
that we shipped to the customer. I was going to re-design the module
myself after about 18 months of failures. However, management decided
that due to the limited life of the product (a new generation was being
developed) it would be cheaper to just give the customer an unlimited
warranty and replace all failed units rather than spend the money on a
re-design. Sad to say it, but good amplifier designers are a rare
commodity.
>
>Is it unreasonable to expect a tube to last 10-15 years in normal Ham
>service?
>It would seem to me that any failed tube with less than 8,000 hours on it,
>probably came out of an amplifier with an inherent design defect.
I would agree with this. I don't see any reason why a tube shouldn't
last that long. Amateur service is much less demanding than commercial
service and they should last just fine.
>
>I hear very little about solid state amps on the reflector. Is that because
>they are much more reliable than vacuum tube amps? It seems to me (keep in
>mind, I'm not an RF type) that designing a tube amplifier to cover 1.8 - 30
>MHz is a very tricky proposition, made particularly difficult by the demands
>placed upon the output matching network -- a problem which is simplified with
>solid state designs.
Well, I think the main reason is that it's just easier to get the kind of
power you need with a tube amp. I don't know of many solid state HF amps
that really deliver the full legal limit at a 100% duty cycle. It could
be done, but I think in the end, modern tubes still end up being more
efficient than a solid state device. Additionally, tubes are much more
rugged than solid state. A tube won't blow up transmitting into a 4:1
VSWR. A transistor just might. So, one has to add foldback circuitry to
cut the power down at bad VSWR. Believe it or not, I think power supply
design is simpler with a tube! In a tube you run high voltage but
relatively low current. With a 27 Volt transistor circuit with approx
65% efficiency and 1500 watts out you would have an input current of
approx 85 amps! That's one heck of a power supply (Input power =
1500/.65 Current = input power/27).
Solid state designs require output matching networks as well. I don't
think there is much difference in complexity here. Also typically, solid
state devices have very low input impedances (on the order of a few
Ohms). This can make input matching more difficult. Also solid state
designs need large heatsinks and very efficient cooling systems. You
can't just blow air over a transistor like you can a tube.
Bottom line: tubes are more rugged and efficient. That's why they are
still the dominant type of amplifier in HF service.
73,
Jon
KE9NA
-------------------------------------
Jon Ogden
KE9NA
http://www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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