Ignoring more exotic tubes like gyrocons, gyrotrons, TWTs, relativistic
electron devices, things that the Naval Research Lab is doing, there is
still vacuum tube improvement being done in the industry. Notice that I
didn't say research. As broadcasting and wireless has pretty much
embraced silicon devices (except for HF transmission at high power),
advances in power tubes are being made to support more demanding
applications such as scientific and military needs. Not amateur radio
and not AM/FM radio. Ham radio saw the peak of development 20-30 years
ago, although occasional variants of old tubes are introduced for
specific targets, like the somewhat successful 3CX1200 introductions
from Eimac that were supposed to replace glass 3-500Z sockets as well as
8877s. There are others too, that hams embrace here. Television has
converted to solid state where it is feasible, although IOTs (a linear
beam tube with a grid) have held ground in UHF for higher power due to
simplicity and lower cost.
IOTs, klystrons (multi gun/beam) and gridded tetrodes are still being
designed for various projects, such as resistive heating sources for
fusion machines and as sources for RF particle accelerators. There are
many accelerators built and planned to be built worldwide, serving
industrial to basic science applications. Not just big bang machines
like LHC at CERN.
Techniques like multiphase cooling (Hypervapotron is one mfr's name) to
exchange anode heat with water at much higher efficiency, pyrolytic
graphite for grids, and improvements to cathodes for higher loading are
all improvements that are working their way into big tubes. These make
tremendous increases in power density. In cathode loading, a tube with
thoriated tungsten (going back to Langmuir and others at GE) can deliver
as much as 3 Amps per cm^2 of electrons in a well designed tube to get
lifetimes > 20,000 hours. Figure the size of the filament basket and you
understand how the peak current can be 1000 amps overall. Thats just not
possible in a transistor junction, no matter what type. Combining lots
of transistors gets there, however, at a cost.
For example, the TH628 from Thales has over 1 MW plate dissipation, with
less water flow and high performance into the VHF. The 4CM2500KG from
Eimac has over 2 MW plate dissipation, with slightly lower frequency
ranges but very high CW power. I wouldn't say that the few remaining
tube manufacturers are sitting still, but they are not cranking out new
designs yearly either. They are making measured step improvements to
their existing products, a lot easier than starting over with a clean
page. These two examples are sort of the crowns for these two companies
to push performance limits.
Others mentioned cold cathode research with field emission and such.
Metal filament and oxide-type tubes and cathodes provide excellent long
life performance, if properly rated for the application. It's when
engineers try to exceed the ratings that lifetimes shorten. Or
manufacturers reduce quality (rising gas levels for example). Similar
lifetime-reductions happen in solid state devices when application
demands more than the device designer expected, except much quicker -
flash, pop.
As a designer of power amplifiers using both tubes and transistors, I
see a good trend in solid state, with 1200 watts or more per device. But
I don't expect to see a megawatt in a device, something that I can and
do see with new tubes. Ham radio will eventually quit using tubes except
for nostalgia reasons; we are still seeing a few 304TL and 450TH
amplifiers getting built due to having the parts and for fun. As long as
their is a market, the Far East will provide bottles for building simple
amplifiers. Don't expect any new tubes for this, however. HV power is
getting harder to come by due to cost of iron and copper. Switch mode
technology comes just in time.
73
John
K5PRO
N7CXI wrote:
> This discussion of "super semis" makes me wonder if anyone is still
> engaged in vacuum tube research these days. Could any of the basic
> tube wear points be improved on with modern technology? Could
> filaments be made to emit twice as long without degradation?
> I just looked at the CPI Eimac site and I can't see where they're
> motivated or have the resources to innovate at the basic level. They
> list "Amateur Service" as a product category, which can't bode well
> for volume sales... Assuming MOSFET technology passes tubes by at
> high power levels, will they just go quietly into the night?
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