> On Sep 12, 2005, at 10:32 PM, Tom Rauch wrote:
>
> > The spitting is either a bad part like a bad or burnt
plate
> > on a capacitor, misaligned relay contacts (the load
contact
> > should mechanically close slightly before the input
contact)
> > , or you have it loaded too lightly on 80. Sounds like
it is
> > loaded too lightly. Open the load control up more.
>
> I resolved the "spitting" problem. After carefully tuning
up the amp
> for 80m, using as little loading C as possible to get the
maximum
> output, there was no spitting into the dummy load. Then I
switched it
> over to my doublet, which is fed through a Murch UT-2000A
tuner.
>
> Then I got the spitting, even though the tuner was tuned
up for a
> good match. A little detective work revealed that the
spitting sound
> was coming from the tuner, not the amp.
Must be an oscillation in the tuner. Probably the tuner
needs nichrome!! :-)
Seriously, T-network or Pi-tuners (or any other tuner with
three variable components) can be adjusted for a variety of
network Q's into a given load impedance...all of which
transfer energy efficiently and present a low SWR.
The lowest possible loaded Q results in the lowest possible
stress on components and highest efficiency. To obtain the
lowest Q possible in a T network use the settings that use
the maximum possible capacitance and minimum possible
inductance to match the load. This will provide maximum
possible power capability of the tuner.
> It's really curious that a tuner rated for 2 kW arcs so
easily at
> just 600 watts of CW.
The power rating of MOST tuners is still stuck in the 60's.
They are rated by plate input power, not RF output power.
When the FCC changed measurement requirements and power
regulations from SB220-era plate input power of 1 kW DC on
all modes, most tuners did not change. They used some
half-baked nearly meaningless "if the capacitor is this
spacing the dc plate input power rating is this". Probably
because most tuner designers never really understood how the
tuners actually work. They were designed with a little
technical ability and a whole lot of guesswork.
For example? Did you know relocating a choke balun from the
output of a tuner to the input, when the network floats,
does not change the balun core stresses or balun impedance
and core requirements at all?? Did you know the amount of
capacitance available has as much or more to do with power
rating on 160 and 80 meters than the capacitor plate spacing
does in a series capacitor network?
73 Tom
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