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earth lift resistors

I have previously been advised to use 100R as earth lift resistors, and have been using wirewound cerramic cased ones rates at 3W with success.

Now an engineer proposes to install a l meg (one meg) 'earth leak' resistor in my valve amp. He says he will bypass this with a 0.1mf capacitor.

Can anyone advise:

i) whether the l meg is as safe as the 100R?
ii) what the purpose of the 0.1uF bypass is and whether it increases safety?
iii) what wattage rating and type of resistor should be used in a valve amp, assuming that 1 meg IS appropriate?
eguth
4 responses Add your response
herman
2,253 posts
 
What does your engineer say when you ask him those questions?

I've never heard the expression "earth lift resistor." Where is it installed and what does it do?
aball
2,016 posts
 
I agree with your engineer (but then, I am one too). A 1Meg is a much smarter choice than a 100 since it will cut down on chassis currents rather significantly. I am assuming your amp has chassis ground.

The small cap would be there to filter out noise from the power rail. Your amp probably already has a few of them here and there so one more can't hurt.

You definately don't need a high power rating. 3W would be way overkill - 0.5W is plenty. However for the cap, make sure its voltage rating is twice your rail voltage.

Arthur
eguth OP
10 posts
 
Aball

Thanks. That clears it up nicely. I was worried about 'underkill'. I go for overkill as a matter of routine- especially with regard to anything with a safety aspect. I was under the illusion that a very high resistance would be equivalent to almost no earth at all; obviously I was mistaken. Yes I can confirm that the amp is chassis grounded, and now via a 1 meg resistor with bypass.
aball
2,016 posts
 
A high resistance is infinitely lower resistance than no earth at all! Electrons know that very well. Glad the info was helpful to you.

Arthur
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