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Gosh me too !
A switching valve cct and something that ''
maintains circulating current within each resonator. Since the resonator is
designed to store high KVA'' lumped delay line ....
with a little -E voltage (-100's !) to the
grid showing on the cct , any big tube can be used in pulse (modulator)
service ..... and with 50% modulation
at 15 KHz audio , that was going to be
some hi-fi broadcast system ,
must be some air defence ww2 / 1950
radar guys about to fill in the gaps ! ,
I think you could use 807 valve Pete , in
pulse service , should do a good job , lots of -Ve
grid bias and a ringing drive cct ....... you
could use a old 'valve' pulse generator for
experimentation , or a new one with a (pulse)
transformer on the o/p .but with the valve into grid
'amps' on peaks , the drive ctt will need to
be stable as the load will swing from
10K to .. 100's ohms at conduction ?
asking before tx 'Is this spectrum in use ?'
G.
Sent: Wednesday, June 13, 2012 3:51 PM
Subject: Re: LF: RE: Class E etc
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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