Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Electron Tornado on April 18, 2019, 09:06:24 am
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Reference this schematic: http://www.muzique.com/news/images/Epi1.gif (http://www.muzique.com/news/images/Epi1.gif)
What is the purpose of R11 (220k to ground) in the power supply?
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It's called a bleeder resistor and is used to drain the voltage from the filter caps when the amp is turned off. It has no purpose while the amp is on.
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Hey Sluckey can I assume that the 220K must be a lot higher impedance then the impedance looking into the remaining nodes so that it doesn't cause any negligible voltage drop when the amp is on. :w2:
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Hey Sluckey can I assume that the 220K must be a lot higher impedance then the impedance looking into the remaining nodes so that it doesn't cause any negligible voltage drop when the amp is on. :w2:
Sure. I'd rather assume in resistance than impedance since this is a DC circuit. :wink:
That bleeder will always cause a small voltage drop either just by loading, or in this case, voltage divider action. If you pull all tubes from that vj, you'll see that 220K works with the 220Ω to make a simple 1000:1 voltage divider. So, if the B+ was 1000V you would only get 999V across the 220K. Of course, this all changes a bit when you add the load of the tubes.
But the idea of that bleeder is to drain the caps fairly quickly without changing the B+ rail significantly.
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Impedance is an ac thing; you're talking dc here, so resistance. That being said, its resistance is relatively high. But that it does not have a function with the amp on, does not mean it doesn't do anything. It flows current (typically in the 1mA region) and drops voltage (as much as the voltage of the node it's on, together with whatever is in parallel). Nonztheless, its effect on other supply voltages in the amp will be negligible, hence it is a good safety feature.
--edit: simultaneous post
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Makes perfect sense. Thanks guy's
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Thanks, sluckey!