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Stupid question regarding chokes
Stupid question regarding chokes
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Old 15th July 2019, 06:55 PM   #1
Smoky Black is offline Smoky Black  United States
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Default Stupid question regarding chokes

I知 building a spare parts amp, roughly based on the Zen, EL84 single ended, 6sn7 driver.

I知 realizing, the choke I have on hand is slightly mismatched, it痴 80 ma and I think I知 going to be in the 110 ma range.

I also realized my plate voltage is going to be a bit high.

The original zen uses a 1k resistor, rather than a choke.

I figured, with my power transformer, I壇 need around 400 ohms to get the plate voltage where I want it.

My question is, if I put a resistor in series with the choke, say a 250 ohm resistor in series with a 150 ohm choke, in addition to lowering the plate voltage, will it also reduce the current going through the choke? I知 thinking most of the current would be dissipated in the voltage dropping resistor.

Thanks!
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Old 15th July 2019, 07:08 PM   #2
jan.didden is offline jan.didden  Europe
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Since the resistor and choke are in series, the current through them is identical.
The resistor and the choke do divide the voltage between them, and with that the dissipation, but the full current is still going through the choke.

Jan
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Old 15th July 2019, 07:12 PM   #3
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Look up Kirchhoff's law;
Kirchhoff's current law (1st Law) states that current flowing into a node (or a junction) must be equal to current flowing out of it.
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Old 15th July 2019, 07:18 PM   #4
dhaen is offline dhaen  Europe
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A schematic might attract some positive ideas to solve your problem.
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Old 16th July 2019, 11:22 AM   #5
Smoky Black is offline Smoky Black  United States
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Thanks, for the responses! Much to learn.

I’m just going to use a resistor and since i’ve already got the choke mounted, just use it for the driver tube. Maybe I’ll buy a bigger choke later down the road.
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Old 16th July 2019, 12:09 PM   #6
jan.didden is offline jan.didden  Europe
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That'll work.
One more trivia on the choke: the max current spec is not just for it to get too hot, that normally is not the issue. But if you exceed the max current spec, the flux in the core gets so high that it (the induction) collapses, and then you have nothing left than a length of wire and no induction ... ;-)

Jan
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Last edited by jan.didden; 16th July 2019 at 12:15 PM.
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Old 17th July 2019, 03:57 AM   #7
PRR is offline PRR  United States
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The inductance falls off very slowly in over-current. Heat is often not terrible. I would just build it, check the ripple, and in an hour check the heat.

If it isn't happy, and you are limited to spare-parts, the alternative is to back-down the main load (power tube) to nearer 80mA, and suffer the slight loss of power output.
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Old 17th July 2019, 10:11 AM   #8
Molenaar is offline Molenaar
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You could put the resistor in parallel with the choke to share the current and still get some smoothing. Adjust the resistor value as necessary to ensure the choke only has to endure 80 mA.
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Old 17th July 2019, 12:22 PM   #9
jan.didden is offline jan.didden  Europe
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For that resistor to bypass some of the DC current it will need to work against the copper resistance of the choke wire, and therefor be a very low resistance. That will negate most if not all of the choke impedance against ripple.

Jan
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Old 17th July 2019, 12:36 PM   #10
20to20 is offline 20to20  United States
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Quote:
Originally Posted by jan.didden View Post
For that resistor to bypass some of the DC current it will need to work against the copper resistance of the choke wire, and therefor be a very low resistance. Jan
It needs to be 2x - 3x the choke's DC resistance.
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