Date: Thu, 5 Dec 2013 03:03:44 +0100
From: "Peter Voelpel" <dj7ww@t-online.de>
To: <amps@contesting.com>
Subject: Re: [Amps] PARALLEL CAPS IN OUTPUT
It?s quite simple without measurements.
Lets assume the amp is tuned for full legal output into a low SWR on 80m
which needed 1000pf of load capacitance.
Then suddenly the SWR goes sky high because the coax connector went open.
How can the voltage across the load capacitor become 5KV when the reactance
of the load capacitor is just 45 ohms on 3.5MHz?
That would require about 110 Amperes of RF current through that capacitor.
And where is that big current coming from in a 1.5KW amp, not in a 100KW?
73
Peter
## back in the late 1970s.....with my single 4-1000 GG amp....it did aprx
1900w pep out.
It used a clunky 3PDT mech relay. One day the output side didn’t quite make
contact, from
all the hot switching etc. 130w of drive applied.....and NO ant connected for
a split second.
The result was the 1500 pf cardwell air variable load cap was welded shut.
It had aprx 1500 V
spacing. I used a flat ignition file to grind the weld free. It happened a
2nd time months later..same deal.
## I found the safest way to ensure the expensive vac load cap never opens up
is to install an adjustable
spark gap across it. Then adjust it for a break down V that is less than what
the peak V rating is on the
vac load cap. In normal operation, the amp will shut down when ant SWR
exceeds 2:1 or whatever
threshold I desire. The spark gap is adjusted for the eq of say a 4:1 SWR. If
the ant fails...and coax goes open,
and IF the high swr protection doesn’t shut down the amp 1st.... the simple
adjustable spark gap will ensure
the vac load cap will survive.
## If you have seen how the mech screw adjustments are on any vibroplex keyer,
the adjustable spark gap
is very similar. Just take a pair of 10-32 machine screws....and grind the
ends to a fine point. Make one fixed,
and the other adjustable, complete with lock nuts to maintain a precise gap.
Some bar stock etc, can serve to
support the machine screws.....just tap threads in em.
## For folks with PI-L networks in their amps.... the load cap is typ at a
200-400 ohm point already..and peak
V across the load cap is already a helluva lot higher than the load cap in a
PI net.
## For folks with a PI net, make sure you use a piece of 50 ohm coax between
the load cap and the TR relay.
And with shield bonded to chassis at BOTH ends. If wire or cu strap is used
instead...then u end up with a huge Z bump.
That alone will put the load cap at something higher than a 50 ohm point...
increasing the peak V. On 10m band, the
stray L of a piece of wire instead of coax will make the PI now appear as a
PI-L.... putting the load cap at a much
higher Z point. same deal peak V increases. Now with a wide open on the
ant, coax cable etc, the load cap
will weld shut.
## as far as the current ratings on vac caps go.... read the various graphs
very carefully. They only handle max
current when FULLY meshed. Most graphs will depict a peak at some freq..like
4 mhz. Below 4 mhz, the current ratings
drop way off. At lower freqs, the XC will increase for a given cap value.
Current drops off because of either the
cap is being used at less than max C. and or the freq is below 4 mhz. Both
cases will cause an increase in XC.
Below 4 mhz the cap is said to be V limited. IE: the V would have to be
increased well above max V ratings to
increase the current. Above 4 mhz, the vac cap is now said to be current
limited...again when fully meshed.
IE: you cant run max rated V on freqs higher than the peak depicted in the
various charts.
Jim VE7RF
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