While Eimac suggests a value of R in the screen ckt, experience
indicates that a value of R in the screen ckt during a flashover will
allow the screen voltage to ramp above the rating of the screen bypass
capacitor. My experience is in VHF and UHF using the 4CX250 and 4CX300
tubes. Thus sockets with screen bypass capacitor built in is
necessary. The end result, get the check book out, cause a new socket
is needed.
73
Bob K4TAX
Phil Clements wrote:
>
> Let us see what Eimac says about HV glitch protection:
>
> In cases where operation involves oxide cathode tube types
> up to about 1500 watts dissipation, the addition of a series
> resistor in the anode and/or screen grid high voltage lead
> will limit peak currents and provide a means of dissipating energy
> in the event of a tube or circuit arc.
>
> This resistance should be as HIGH as is practical without
> dissipating an excessive amount of power. Typically, a resistance
> of 20-50 ohms is sufficient to protect the internal tube structure
> in the event of an arc. It also serves as protection to the other
> circuit components such as bypass capacitors, RF chokes, meters,
> and diodes. For oxide cathode tubes a maximum of four Joules
> total energy is permitted to be dissipated in an internal arc. An
> amount in excess of four Joules will permanently damage either
> the cathode or the fine wire grid structure during repetitive
> arcing. In a case involving a 2000 volt power supply with a 2mfd
> filter capacitor a total of four Joules is stored in the capacitor
> alone, not including the energy resulting from FOLLOW-ON current
> from the power supply. (Follow-on energy is the energy being delivered
> from the power supply from the time the fault starts until the primary
> C.B's/fuses trip/blow. This energy must be added to the energy stored
> in the filter capacitor(s) in calculating the proper HV glitch
> resistor.)
>
> Most power supplies employ much larger filter capacitors than 2 MFD,
> resulting in even more stored energy. With no series resistor all this
> energy is dissipated in the tube structure in the event of a tube arc.
> Typically, there is about 50 volts across a tube arc and the addition
> of
> a series resistor of 50 ohms will limit the peak fault current to 40
> amperes.
> With 1950 volts across the resistor and 50 volts across the tube arc,
> less than 5% of the total energy is dissipated in the tube with 95%
> being
> absorbed in the series resistor. Note that a MAJOR portion of the
> fault
> energy may come from the FOLLOW-ON current if the primary C.B.'s/fuses
> take excessive time to trip or the overload relay(s) take too much
> time to
> open the primary contactor. Without protection, several low-energy
> arcs
> may occur before serious degradation in tube performance is noted.
> However, in cases where sufficient energy is involved, a single arc
> may
> completely destroy the tube.
>
> As a test for the amount of energy delivered in an arc, EIMAC suggests
> that the power supply be short-circuit tested by causing an arc (in
> air)
> from a short lead connected to the tube terminal to the surface of a
> grounded sheet of 0.025mm (0.001in) thick aluminum foil. If total
> energy
> delivered is less than four Joules, the hole burned in the foil will
> be
> no greater than 3mm (0.120in) in diameter.
> An alternative test which will verify tube protection is to
> short-circuit
> test the power supply through a 6 inch length of 0.079mm diameter (#40
> AWG) soft copper wire. If the total energy delivered is less than four
> Joules the wire will remain intact. This test must be run at full
> operating
> voltage and may be performed by using a vacuum switch or other
> suitable
> high voltage relay to apply a short to the supply through the aluminum
> foil
> or the copper wire.
>
> Most tubes employing thoriated tungsten filaments are capable of
> withstanding
> higher energy arcs than are the oxide cathode types. However, some of
> the smaller types such as the 4CX1500A, 3-500Z, 5CX1500A, as well as
> some larger high-mu triodes such as the 3CX3000A7, 3CX10000A7, or
> 3CX20000A7 while employing thoriated tungsten emitters, have very fine
> wire grid and cathode structures and should be protected in much the
> same manner as the smaller oxide cathode tubes.
>
> The above design criteria and testing should take less time to perform
> on a power supply than is spent measuring for proper anode cooling
> when
> building up an amp. If the proper C.B's/fuses and glitch resistor are
> selected, you will not even destroy your 6 inch #40 wire!
>
> (((73)))
> Phil, K5PC
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