> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of Carl
> Sent: Sunday, February 07, 2010 12:58 PM
> To: Jim Garland; amps@contesting.com
> Subject: Re: [Amps] Vac Relays for B+
>
>
> ----- Original Message -----
> From: "Jim Garland" <4cx250b@muohio.edu>
> To: <amps@contesting.com>
> Sent: Sunday, February 07, 2010 12:44 PM
> Subject: Re: [Amps] Vac Relays for B+
>
>
> >
> >>
> >> I don't understand why putting the 25 Ohm resistance in one
> >> transformer
> > secondary
> >> lead wouldn't serve all needs, turn on surge (cap. charge), control
> arc
> > and shorted
> >> condition overload and current limit under shorted rectifier or
> >> capacitor.
> >> Like the 10 Ohm 1/4 Watt resistors in series with the plates of 6X4's
> >> they
> > are fuses.
> >> --
> >> Ron KA4INM - He who attacks the fundamentals of the American
> > broadcasting industry
> >> attacks democracy itself. -- William S.
> Paley,
> > chairman of CBS
> >>
> >> ************************************
> > The 25 ohm resistor functions both as a fuse (in case of extreme
> overload)
> > and partly a current limiter, e.g., when the capacitor bank is
> charging.
> > Putting it in the transformer secondary lead is probably okay, but I'd
> > prefer to put it after the diode rectifiers, but before the filter
> > capacitor. Note that this resistor, while desirable, doesn't protect
> > against an arc or short circuit, since
> > a fully charged capacitor bank can momentarily supply hundreds of amps
> of
> > pulse current into a short circuit. Thus one also needs some
> protection
> > after the filter cap, such as the Drake .82 ohm 2W composition
> resistor
> > used
> > in the L4B.
> > 73,
> > Jim W8ZR
>
> Jim, Drake and Dentron both use a similar tiny resistor acting as a
> fuse. I
> guess they were trying to save money.
>
> The purpose of a properly sized HV resistor is to both limit a tubes
> fault
> current during an arc but to also survive until the primary fuse or
> breaker
> does its job.
> A true vitreous enamel wirewound works well for this.
>
> OTOH, that tiny resistor can AND WILL sustain an arc under the right
> conditions and result in some serious damage to other components.
>
> Carl
> KM1H
>
I agree with Carl. A fuse in the high voltage, whether it be a resistor or a
regular fuse will sustain an arc when it opens and can let more energy thru
than you want.
The glitch resistor can not be just any old resistor either. It has to
withstand the full high voltage potential of the power supply. When there is
a short, full high voltage appears across the glitch resistor. Sometimes
more than one vitreous enamel resistors are put in series to get the voltage
rating needed.
73
Gary K4FMX
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