Hi Pete, LF,
On 137 kHz or 500 kHz you can quite easy achieve 95% efficiency with a
class E PA. BTW a ferrite transformer is not needed! So if your antenna
provides stable resonance and impdedance and if you like to play
arround a bit to obtain the best efficiency (not the highes output
power), a well designed class E PA is definitifely a good choice! 1 kW
RF (CW, key down!) is possible with a single IRFP MOSFET.
I hardly expect that you will get such an efficiency with a tube amp
but i can understand the fascination of a glowing tube in a dark shack,
although i'm not from the tube generation ;-) The first power amplifier
i heard of was a 2N3055 in the late 80s as i was 12 year old or so :-)
73, Stefan/DK7FC
Am 13.06.2012 16:51, schrieb M0FMT:
Well shoot me down
with my britches on fire!
Mal
I have just found an article on an enhanced Class C in
Tube amplifiers By RCA.
I have heard of higher
efficiency Valve amps but never gave them any time because I thought
they used special valves to achieve the fast response. But apparently
not, it relies on a tuned circuit in the anode and cathode. The cathode
LC being set at the third harmonic of the anode one. By combining the
drive wave with the third hamonic you get a very fast rising pulse
type wave at the drive frequency in the PA tuned circuit as with a
class D/E solid state amp. URL is ........ http://nrcdxas.org/articles/bta5t/
I think the point
about this is that it works OK for fixed frequency TX's like broadcast
transmitters and the like. And probably will work OK for us on LF with
such narrow allocations where we may get away with one set up.
Now to see if it is
scalable to lower power video tubes.
73 es GL Pete
M0FMT IO91UX
Pete es Co
The class C tube amplifer is the nearest you
can get to class D or E and well designed would produce 70% efficiency.
The class D or E is not really an amplifier
in the convential sense it is merely an RF switch at the frequency of
interest and the voltage generated is increased by a step up
transformer with probably 80% to 90% efficiency if you are lucky.
Like some have said a tube amp is more robust
and tolerant of mismatch to antenna without smoke. Fets are good but
can go bang if one is not careful about matching to load.
I have some Dentron amps for the HF bands and
they have been going for years.
Fets are probably more manageable, compact
and safer regarding voltages, beware of tubes with voltages of a few
K/volts at high current, like an electric chair!!
73 de mal/g3kev
----- Original Message -----
Sent: Tuesday, June 12, 2012
11:15 AM
Subject: Re: LF: RE: Class E etc
Hi Ken
I believe
you can operate some thermionic devices in higher classes than C but
they generally lack the frequency response of a modern FET. I
won't go into figures
but just think about it. The high efficiency
output stage has to operate like a switch it is not actually an
amplifier. The faster it changes state from fully on with ultra low
resistance to fully off (which means a device that will operate well
into the VHF) the higher the efficiency. If you have a device that is
barely rated to say a couple of megs its not going to be much good (in
terms of what we are talking about) at 500kc/s. Remember you are trying
to get the magnetic field in the inductor to rise and collapse at a very
fast rate in-order
to generate a high electromagnetic field at the drive frequency. We are
not talking sine waves here as a scope on the drain will show. Although
a scope on the output side of the drain inductor will show quite a
clean looking sine wave. Try it.
That chart at the end of that PDF file
is very clever and is the guide I use to bring a PA into optimum
performance.
What I was trying to say is that the math is all very
well but if you are trying to make a practical and efficient CW TX for
LF then there is enough empirical
data from following the guide lines lines shown on the sites I have
posted that will get you a good TX. If you want a doctorate in
theoretical physics to bamboozle
you colleagues
then that will detract from the time it takes to build a very interesting high
power, efficient TX design well within anybodies capabilities. The GW3UEP
design being IMHO a very good (Scalable) example with no RadCom style
unobtainium
in the components listing.
73 es GL Pete M0FMT IO91 UX
Hi Pete.
About 40 years ago my integration was
quite good, liked doing 2nd order differential
equations, partial integration and integration by parts, the one thing
I had big problems with was 3 dimensional integration of a point in
free space . Looking at some of the sites my Class C MOSFET
PA is probably more like Class E by default. The reason I like valves
is because they are very forgiving devices unlike semiconductors, also
the keep the shack and the cat warm.
73s
Ken
M0KHW
Ha ha Stefan, how good is your Integration
Ken?
Here are a couple of sites that should
answer your questions. You need a good dummy load and a good
oscilloscope. Setting the conditions for the amplifier is to do with
A/ the correct driver preferably using a FET
drive chip (like TC4422 non and TC4421 inverting) that will turn the FET
fully on and fully off on each cycle. Very fast rise time with
accurate on off timing (mark / space). Ensuring the max permitted gate
voltage for the FET is not exceeded otherwise they
pop, look out for transients, a diode clamp helps. B/ Setting the
shunt capacitance and inductance in the drain circuit can be done empirically
See article below. Then follow it with a low pass filter, although the
output wave will be surprisingly
clean. Calculating the values is difficult as you don't necessarily
know all the starting parameters and in any case the result is going to
wide of the mark. The site below will gives some starting values. Best
to use 'Scope measurements
to set the output conditions. You are looking for fast, very fast On /
Off rise times. Gate drive and understanding the drain circuit is the
key.
These amplifiers are capable of
incredibly good efficiency well above a dodgy class C amp. This means
the heat dissipation from the FET is very low at high power giving
cool heat-sinks!
The GW3UEP (do
a web search) single ended design is capable good efficiency and
high power depending upon the FET used and Drain supply voltage. I
have modified one to run reliably at 140watts and is capable of almost
twice that into a dummy load with a heat sink that is fairly cool.
Adjusting L and C empirically
using the 'scope to get the correct drain wave shape for max efficiency
is the way to go. The article below shows you what you are looking for.
I set my TX up into a 50 Ohm dummy load adjusting L and C to get the
correct drain wave shape on the 'scope then measuring the peak voltage
across the Dummy load. I then transfer my connector from the Dummy load
to the input of the tuned transformer in the antenna up-lead.
I adjust the coupling link to give the same peak voltage across the
tuner link turns. You are not looking for maximum peak just the sames
as across the dummy load indicating a 50ohm match. This is done by
either adding or reducing the number of turns (about two turns) along
with bringing the the transformer to resonance by measuring the peak up-lead
current, here you are looking for the maximum peak.
Oh yes and WIMA
high voltage high value capacitors from Maplin
work well.
Read the articles and search info on
class E amplifiers.
For a good practical circuit for a
single ended amp that is a scalable
surefire design, no need to reinvent the wheel.
For antennas and antenna feed methods
a "look no where else site"
73 es GL Pete
M0FMT IO91UX
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