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Re: [Amps] Resistor Types

Subject: Re: [Amps] Resistor Types
From: Manfred Mornhinweg <>
Date: Fri, 22 Jun 2007 12:29:40 -0400
List-post: <>
> Hi Garry,  Bryan,

> Likely, the biggest concern is the voltage rating of
> the replacements.

I agree with Bryan on this. One of the most unreliable type of 
components is resistors operated at high voltage. In my amp, I 
discovered bad voltage equalization resistors when the electrolytics 
started failing. The resistors had changed their value over a wide 
range, despite being operated well withing their ratings.

And I have lost count of how many times I have replaced high voltage 
resistors in my Tektronix oscilloscope. They just go open circuit, 
despite operating at less than one tenth their power rating and less 
than one third their voltage rating. A went so far as to replace 1/4 
watt resistors by 2 watt ones, in the hope that the bigger size will 
help them survive. Nope. They go open just the same!

When I overhauled my amp some months ago, I eliminated the equalization 
resistors across the rectifier diodes. Half of them were open anyway, 
but the diodes were all still good! Modern diodes just don't need 
equalization resistors, because they will act like high voltage zeners, 
equalizing the voltage well enough to survive, without any help from 
resistors, and even when there are some good and some open resistors!

By the way, if anyone can point me to an explanation as to why my 
oscillocope resistors keep going open circuit, I would appreciate that! 
One particularly problematic group is a string of five 10M resistors in 
series, with 2kV applied to the string. Even 1000V rated resistors keep 
going open!

For the filter capacitor voltage equalization resistors in my amp, I 
used some glassed ones, which so far are working well.


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