Hi Rob - Agree with most all your comments - As I dig into the Centurion
deeper , there are a lot of corners cut that I would have done differently.
For instance the meters are lousy - and I am sure for another $20 they could
have bought nicely damped meters - but the $20 probably translates into $40
at selling point- which I would glady pay ! The fan is marginal and for
virtually zero extra cost they could have almost doubled the cooling with a
fan from the same vendor that fits the exact same space and mounting ( their
answer to this is that then they get too many complaints about noise) - My
25 year old L7 handles that with a 30 cent thermostat in the airflow and a
30 cent resistor.
I have often wondered to myself what the L7 would have been like if I had
designed it - I was asked to come to work at Drake part time and do the L7
amp design as a first task - but turned it down becaue I was working at AVCO
Electronics in Evendale, Ohio and was working long hours plus was not too
enamored of driving to Miamisburg that often - I missed my chance!)
But it seems TT design philosophy is to do it as cheaply as it can possibly
be done in some respects , but in other areas , it looks like they made it
about as hard on themselves as possible! But I guess that comes from years
of making my living designing RF stuff to make a profit. Everybody has their
own approaches.
I have not measured the input swr across all the bands with a MFJ , but the
Orion internal SWR (subject to some suspicion) -shows better than 1.4 over
the cw portions which is all I use 99 percent of the time. I do notice some
shift in swr at low power (300-500 watts out ) versus 1000-1300 watts out
but I think thats to be expected. The amp runs with something around 2800
volts fully loaded so the gain is sufficient that the Orion easily drives it
to 1200 watts or so. Anyhow , have not measured the input swr with a MFJ due
to the shift with level.
RE: Varying the length of exciter-amp jumper is an okay solution
if you spend your entire ham existence operating on only one band.
As I said , at least right now on CW , it works OK - just for grins when I
can't find anything else to do , I think I will try a even longer length to
see what happens - I suspect at the "wrong" length , even longer , the phase
angles will add up wrong again. Since its so easy to drive to a KW which is
about all I usually run , I toyed with putting in a 2 or 3 db resistive
pad - (would probably help SSB IMD too )
> Modern solid-state output MF/HF transceivers use a broadband push-pull RF
> output stage. In order to meet FCC requirements, Butterworth and/or
> Chebyshev pass band filters are used to suppress spurious emissions. Such
> filters introduce inductive reactance or capacitive reactance within their
> pass bands. In other words, the output impedance of a modern transceiver
> is
> seldom 50 ±j0 ohms. When driving a tuned input in a grounded-grid
> amplifier,
> filter reactance interacts with the input reactance in the tuned input.
> The
> length of the coax between the driver and the tuned input affects the
> interaction.
And something VERY few people realize is that because of exactly these
things - the performance of an external low pass filter or other harmonic
suppression tactics - can vary significantly-it comes down to what the
harmonic source impedance is - thats why I prefer short interconnections and
a filter scheme that starts with a series L , not a shunt C .(or an output
network with a final series L) Also a filter with a shunt C at input can
really screw up the efficiency of the amp because you are seeing lower
impedance at the second and higher harmonics - again narrow band you can
get away if you use a quarter wave at the second harmonic - still affects
thirds but so be it . The guys running the web site on solid state amp
design have apparently realized this as I see they are using complimentary
harmonic ` filters so they terminate the harmonics in a more or less 50
ohm load.
I worked at Motorola Semiconductor in the RF section for many years and
spent a lot of time with Helge Granberg discussing these sort of things.
99 percent of what you say - you are preaching to the choir !
I spent a few years at Gates Radio in the AM broadcast section and that was
an interesting experience - the big beasts are somewhat of a different
animal - but it was all AM . Did a 50 Kw MW AM for the Army that fit in two
helicopter transportable shelters. They wanted to be able to manually retune
to any freq in the 550 -1600 range in less than 5 minutes.
Was pretty much a snap - but my design was a lot different than the classic
Gates designs which were designed to a customer frequency and stayed there
forever. The old timers gave me a hard time - but it worked like a charm.
(The basic PA design approach was based on a 304TH amp I had built as a kid
in the 50's !)
I had fun working with the guy who started Gates (Harris ) transfer into
solid state and who built the first 100 KW switching mode transmitters .
That all happend about as I was leaving for another job so I missed the real
stuff.
Anyhow - good reading - Thanks for the reply --
Rgds-- hank K7HP
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