> Roy questions whether it is appropriate to use a current-limiting resistor
> (in my case, 25 Ohms) between the transformer/rectifiers and the filter
> capacitor (50uF in my 8877 amp). His point is that doing so is equivalent to
> using a cheap transformer with a 25 Ohm "Equivalent Series Resistance" and
> thus negates the benefits of using a stiff (and expensive) transformer. This
> is a reasonable objection, but I don't fully agree. Here is the
> counterargument. First, there are two benefits of the resistor; it limits
> the inrush current when the hV supply is switched on, since without it the
> rectifiers would see a dead short from the uncharged capacitor. (of course,
> a step-start circuit in the xfmr primary is another way of limiting the
> inrush current) Second, it protects the transformer and rectifiers from a
> shorted filter capacitor. Many amps use banks of electrolytics for the hV
> capacitor and a short circuit in one can lead to a cascade of failure along
> the entire bank. (My 8877 amp now uses oil caps, but the original design
> used electrolytics.).
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
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