[AMPS] connectors and spark plugs

[Ian Roberts]

"John T. M. Lyles" wrote:
> Both of the topics I see discussed here are things I had to deal with
> also in the past few years. The type N connectors kept blowing up at
> 4 kW peak RF, 200 MHz, when VSWR crept up during tuning. THis is
> about 900 Watts of average power. This was in the interstage
> connection from Driver to IPA, and the driver was an old RCA cavity
> amplifier with 7651 tetrode in it.


Half of the problem with break down of cinnectors is probably
contamination of the dielectric with solder flux/vapours when soldering,
or the centre pin with solder itself.
Hams, and some professionals, don't know that any solder on the centre
pin will oxidise with time, weakening the contact. Probably gases off
when really hot breaking down the insulation. Solder flux and similar
contaminants on the insulation may be OK for a while, but attract
moisture with time, then cause the connector to say goodbye when hot.

For example, a BNC to N to UHF adapter will do 1 kW at a low VSW at 30
MHz, intermittent duty, but the spring contact must be good.

> We use some common spark plugs (no i don't know or really care if
> they are antifouling as long as they ionize and fire) as protectors
> for the screen to cathode of 4CW250,000Bs. I have two on each tube,
> and I purposely file off the bridging wire (ground side of gap) to
> make the voltage more predictable and repeatable. With it, they
> breakdown fairly easily, at a few kV. With the wire gone, they seem
> to fire at 4 kV or more. This is easily verified with the hipotter,
> and at our altitude (7000 feet above sea) it is important to test
> every one first. I pulse that screen grid at up to 2500 Volts, so I
> need some margin here.

Spark plugs are meant to handle high voltages with just about zero
current. Additionally some spark plugs feature carbon-composition or
similar inners aimed at suppressing RFI (rise time of the spark).
Presumably these are a no-no for any sort of protective device where
some current needs to be passed.

Ian ZS6BTE

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