----- 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
>
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