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This is what you need (with a bit of re-work)
.....
VK or Bust !!
FL2000 , with 2 x 813 ? nice
amp, my skanti trp5000 has
2 x 4cx250-b in the PA runs well
1.8/30 Mhz , I use 2
combined skanti 250 watt solid state 50v linear amps
with the ma1723 for LF>Hf , wide band ,
just change the ATU hi ..... could do
with a simple class e/d for 500 as the
ab1 amps are a little power hungry ..
G..
Sent: Wednesday, June 13, 2012 11:59 PM
Subject: Re: LF: RE: Class E etc
Hi Stefan and Graham
Thanks for the comments. None of my MF/LF tx.s run more that
160watts although by seriesing a couple of
PSUs one with a floating ground I can run
about 280Watts o/p from the TX. I don't like seriesing the PSUs so
only do it during experimenting not on the air. Need to build a new high
voltage PSU 100V at a good current
rating. The rigs are FET PA IRF840 etc mostly xtal controlled or xtal
mixer type master oscillator (but do have DDS), into a FET driver chip. And set them as described
earlier.
I have played with High power Audio amps on 136kc/s transforming the
Low impedance to 50 ohms+/- using Ferrite cored transformer to drive the antenna
tuner. Front end currently popped!!
Using it for QRSS overnight.Will get it
back on soon.
My interest in Valve or Tube TX is a side issue and am not advocating
them for serious modern MF/LF work.
However there is no reason why not.
I have made a couple of TUBE QRP rigs one for 160/80 about 4/5 watts out Choke modulated AM
and CW and FSK. Worked most of UK on AM
on 3.615megs. The second is a copy for 500kc/s was originally VFO controlled but now is again xtal mixer vxo. They use 6CL6 video tubes in the PA. The 500kc/s tube
tx only runs a watt o/p yet to figure out
why, it should be about 5watts. How ever on my new found Opera4
have been reported in PA0 land (on the limit).
I have built and still got one or two 807 PAed rigs for 80m knocking about. Plus a Louis Varney design made by an unknown amateur in the
1950s and a Codar T5 and a push pull 807
rig on the stocks.... but probably won't get finished. Plus an FL2000B. This to
show my thermionic credentials.... hee!
For shear interest I would like to try and make a high efficiency
TUBE PA but probably won't get around to it. But that 1960s RCA AM BC TX design
looks interesting but probably very dodgy to set up, stability and all. Graham
your arc tubes is all very well but what I have read is that they have a
tendency to blow up...... and so do FETs
ok so no change there then.
My current limitation to power Stefan is lack of a single stable
PSU with a couple of KW output. The
antenna is now reduce to 35ft up and 128ft top.
I don't have a good RX for MF/LF what I have are deaf by others standards and this is my
main area of interest at the moment. I have used a TS50 and a TS690s full
filters plus a couple of direct conversion HB RX with active filers and notch.
Front ends have gone from dual gate mosfet to straight into the mixer but S/N is still poor evidenced by WSPR and Opera reports I give to other station who are being
received much better elsewhere. If any
one can help me with that then it would make life here on MF/LF a lot better.
Keep the bands
open and may get to work you Stefan now you have 472/9....... why the delay in the UK????
73 es GL Pete M0FMT IO91 UX
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/ DK7 FCAm 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 IO91 UX
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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