Hi ye all,
>> 10 pounds, 2KW - It looks promising. I am just wondering,
>> how come no manufacturers use them?
> 75% efficiency
> 750 watts heat at 3kW load
> 17% regulation
I knew about its price and the lack of regulation, but I didn't know
that it has such a low efficiency! That's of course unacceptable for a
switching power supply. I wouldn't settle for anything less than 90%
efficiency, and even this is still poor for a high voltage switcher, in
which the secondary rectifier diode drop is negligible! Low voltage
switchers easily achieve 92%, and the largest loss is the diode drop in
the secondary rectifiers, so it stands to reason to expect an efficiency
near 95% for a high voltage switcher.
Years ago, shortly after my 13.8V 40A switcher was published in QST, I
was approached by someone wanting me to develop a high voltage switcher
for a tube type linear amp. While the project was attractive, I didn't
have much time, and I didn't know (and still don't know) what problems
one would find with dV/dT issues, which made it hard to estimate the
time required for development. So I declined. But sometime I might take
up this project again.
In any case, a switching power supply without voltage regulation is
pretty stupid. Adding voltage regulation costs almost nothing, compared
to the cost of the power components which need to be there in any case.
This is very different from a linear supply, in which the addition of
regulation means adding lots of parts, which are expensive and generate
lots of heat and thus lots of loss.
> The main advantage of tubes is a
> regular transformer can be used.
I don's see that as an advantage. A REALLY advantageous active device
would be one that can work directly off the line voltage! Insulation
from the line would then be accomplished in the input and output RF
transformers, making the whole amplifier MUCH lighter and less expensive.
Such devices might be tubes with large cathodes and small interelement
spacings, but I fear that the high cathode current would require an
excessive amount of power put into the filaments. So I have much greater
hope placed in MOSFETs. There are some available off the bench, and
quite inexpensive, to produce legal limit power with a single pair, fed
directly from 300VDC, which can be obtained by simple rectification of a
220V line or by voltage-doubling a 110V line. This would be very
attractive, IF these devices would be able to work in linear mode, with
good stability, no hotspotting, etc. But apparently this isn't the case.
In linear mode they need a much lower voltage to remain safe and
stable, and so we still need a power supply - either a switcher or a
large, heavy transformer affair.
By the way, based on long experience, I don't share the apprehension
about fragility of semiconductors. I did professional repair of
communication equipment between 1982 and 1989, and since then I
occasionally repair equipment for ham friends. Speaking about 100 Watt
final ampplifier stages, I have replaced too many failed tubes to even
try to count them, but only very few transistors. And this is despite
tha fact that tube output stages in this power class are few compared to
solid state ones! Clearly in the field the transistors are more
reliable. And in the 1kW class, I have had to replace about two dozen
tubes so far, but never even one MOSFET! I admit, of course, that solid
state high power amps are still rather few, but if they were unreliable,
I would expect at least one or two to have found their way to my workshop.
The reason for this reliability is of course mostly in the protections.
Solid state equipment always includes protective circuitry, while tube
type equipment very often doesn't, or has only very basic protection.
Then there is the issue of more difficult operation. Many tubes I had to
replace were 6146A and sweep tubes, with their bulbs molten down, a
result of improper loading of the radio. Many gassy 3-500Z might have
failed for the same reason. Of course, a ham should know how to load his
equipment, but it happens even in the best ham families that the little
son moved some knobs, dad thinks his amp is still loaded on 20 from the
last evening, goes on the air without checking the laoding, and the
tubes are done in before he has a chance to notice.
I have built lots of equipment, mostly solid state but also some with
tubes. I have never burned an expensive RF power transistor, or even a
large switching transistor for that matter. Working with expensive and
supposedly "fragile" devices, one simply has to apply basic safety
measures: Current-limited power supplies, voltage clamps on the drains
or collectors, a scope to catch any self-oscillation before it causes
damage, and doing only quick, short tests until the stability of the
circuit is guaranteed.
Being a solid state person, I apply some of the same precautions to my
tube equipment, and as a consequence I have not even made a tube go red,
much less yellow, by mistake. But I have done it on purpose a few times,
and it looks mighty beautiful! I admit this is something one can't do
with transistors! :-)
Amps mailing list