To: <amps@contesting.com>
>Date: Thu, 11 Dec 1997 09:07:15
>To: "Ian White, G3SEK" <G3SEK@ifwtech.demon.co.uk>
>From: Phil Clements <philk5pc@connect.net>
>Subject: Re: [AMPS] parasitic suppressors
>
>At 07:25 AM 12/11/97 +0000, you wrote:
>
>>
>>Rich is obviously correct in saying that the bias zener is not in the
>>path of a surge current loop that goes B+ - anode - grid - chassis - B-.
>>However, what happens if the grid is connected to the chassis by a choke
>>or resistor which then burns out?
>>
>>When the grid floats, wouldn't the electron stream then switch to the
>>cathode? That would blow the bias zener, unless something else such as a
>>fuse blows
>
>You are correct, Sir.
>
>I think of the enclosed RF deck as the inside of a fuse. There have been
several
>discussions here about how the vapor from a blown fuse continues to conduct
>after the metal is vaporized in a HV DC application. I visualize the inside
>of the RF deck in the same manner, in which Ian is very correct in saying
that
>the fault path will switch about; following in succession the paths of least
>resistance. No components are safe in this environment. In Eimac's paper,
>"Fault Protection" (Rev. 1987) They say that endangered components with an RF
>deck fault are:
>
>1. The tube(s)
>2. External Circuitry (I translate this to mean EVERYTHING!)
> (It goes on to list capacitors, RF chokes, meters, and....DIODES!)
>Let me also add that if the vapor "cloud" is potent enough, even the
>bearings in the blower shaft/motor are not exempt from the wrath, and
>they certainly are not in any normal current paths.
>
>The bottom line is that all bets are off when a fault occurs. Normal
>paths are no longer are in the picture. It is the ABNORMAL paths that now
>take over. As my old ground instructor at Braniff used to say: "Sources,
>forces, and courses".
>
>Eimac states that an amount on energy in excess of four Joules will
permanently
>damage EITHER the cathode OR the fine wire grid structure during repetitive
>arcing in typical circuits used by amateurs. This is what the anode glitch
>resistor we have discussed here on the reflector is for. Every case is
>different in designing the glitch resistor, depending on how many Joules are
>available from the power supply when a fault occurs. It is not just the
energy
>stored in the filter caps, but the follow-on charging from the A.C. mains
during
>the time between fault onset and primary contactor opening also will
represent
>a value of equivalent Joules that must be considered in the design of the
glitch
>resistor. The faster the A.C. main circuit breakers/fuses are, the fewer
Joules
>we need to worry about. Since most of us do not have the capability of
measuring
>total available energy at the output of our power supplies, Eimac has
devised a
>test to see if the available energy to the RF deck has been reduced to
four Joules
>or less by the glitch resistor. A length of #40 soft copper wire is placed
from
>the anode lead to ground. Full anode voltage is then applied. If the #40 wire
>does not melt, you have reduced the energy level to > four Joules. If the
resistor
>blows up, the wattage was not great enough, or your A.C. mains C.B. or
fuses are
>too slow/large a value. All these perimeters can be juggled around by
trial and
>error to design the correct glitch resistor for your particular situation.
It is
>not rocket science like the parasitic suppressors have turned out to be!
Of course,
>if test equipment and a calculator is available, all this can be done
successfully
>on the first try.
>
>Tube manufacturers would love for you to omit the glitch resistor, as a
fault is
>not covered in the warranty!
>
>For you "tubes-with-handles" guys...please disregard all the above...you are
>allowed 50 Joules!
>
>(((73)))
>Phil, K5PC
>
>
>
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