On Mon, 30 Oct 2000 17:49:31 -0500 "Tom Rauch" <w8ji@contesting.com>
writes:
Hi Tom,
>My experience is HV switching transformers, let alone KW level
>high frequency transformers, are the worse part of the system to
>get right.
>
I have a some friend's who've been doing this kind of work for years --
one to drive big movie strobe lights and the other an 8 KW tesla coil.
>Once again, I can't imagine anyone wanting to run a tube at 250
>watts output (500 watts input)!
QRP is sometimes fun.
>How'd we get to 250 watts RF output?
That's where the discussion started -- I was just suggesting a way to
build the supply at that size.
>I don't think so. That (incorrectly) assumes the rest of the
>equipment is perfectly shielded, and the antenna is a long distance
>from the transmitting equipment and receiver. Of course I guess
>you could shut the supply off when receiving, and delay the TX on
>startup, but who would want that?
If you can keep the 2nd & 3rd harmonics of your KW amp from futzing up
the neighbors (or your own TV's!), then you can certainly deal with the
20th - 300th harmonics of the switcher.
You could actually shut down the output (phase controlled converter)
section of the power supply in receive -- while leaving the PFC running
in a discontinuous mode. The plate supply would come up before the
antenna relay can switch, so there would be no noticeable delay.
>Shielding a high-power high-frequency switcher from a receiver
>connected to an antenna that might be fractions of a wavelength
>away is a royal pain.
It works fine when I build it here.
I have two "factory made" switchers in my 4CX1500B amp (one encased,
the other open frame). They were both noisy as hell in the open
monitoring with a receive antenna a couple feet away -- you could hear
them all the way up to 30 MHz! In the cabinet, and properly line
filtered, you can't hear a squeak out of them.
The only devices I have any RFI trouble with here are my stupid
computers!!!! -- but, I didn't design, package or test them -- HP made
my latest wideband noise generator! and they said they divested the test
equipment division... :-)
>If size and weight are the targets, a solid state PA makes the most
>sense.
This project wasn't about making the most sense.
>The design problems of the supply would exceed the design and
>cost of the PA section. Might as well put that money and time into
>a solid-state PA that can directly operate off the mains with HV
>FET's.
Yep, the $$$ do add up. No one said that this was an economic
approach.
I'm using the same PFC section as I would with the tube PS system we've
been discussing plus a buck regulator in my transistor KW under
construction. No transformer but, just as much core cross section in all
the output filtering.
73,
Marv WC6W
*
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