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Old 22nd January 2003, 05:28 AM   #1
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Default Questions about a choke

OK...I have what may sound like a dumb question, but here goes:

Regarding chokes and their position in the circuit, what exactly is it used for? I understand (basically) that it is a filter of some sort, but I am confused. Most chokes I see in schematics come on the rail feeding the (negative) cathode, but it doesn't seem to make sense to me. Why would it not come on the B+ rail, ie from the center tap of the transformer?

Regards,
Tony
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Old 22nd January 2003, 09:02 AM   #2
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Well actually most amps have the choke, like you said in the b+. But the b+ is not always the center tap.

A choke has highish resistance to high frequencies and a low resistance to low frequencies...and therefore holds back high frequencies..and lets through the low frequencies...

Which is incidentally exactly the opposite of what a capacitor does.

Therefore it is used in a powersupply to keep out the ripple and some other rubbish. This is made more effective if you connect a capacitor from b+ to earth (after, or before the choke) because it will pass the high frequency noise and channel it to earth....so your b+ will be freed from the rubbish.
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Old 22nd January 2003, 09:52 AM   #3
dhaen is offline dhaen  Europe
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Default Well described

Hi,

Quote:
and lets through the low frequencies...
Such as..DC..a frequency of 0Hz.

Anything a cap does in parallel, a choke will do in series, and vice versa.

It's a wound component. Normally copper wire wound round a ferrous core.
This, as you know is the recipe for an electomagnet.
But in the case of a choke, the core is wrapped round on itself. This makes it more efficient at storing energy, which is stored as a magnetic field.
Caps store energy as an electrostatic field.

I'm sure ther's 50 ways of describing it...that's mine.

Cheers,
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Old 22nd January 2003, 07:20 PM   #4
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Default Explaination

OK, thanks for the explanations so far; I understand what the choke is used for now...But I am still unclear about the placement.

The image that I have attached is a reproduction of one from the Practical Audio Design Series @ http://www.audioxpress.com/resource/...s/ga399ac.pdf. (I added the red "+" and "-" to indicate what I think is going on at the transformer.)

In it, I think the choke is wired in series with a rectifier tube's cathode. If I remember what I've read so far, the cathode only works when it is negative with respect to the anode. Am I correct thus far?

And since that is the case, the choke would have to be on that negative rail as well, correct?

Now I also understand that the current is alternating at the transformer, but should maintain one polarity (ie become direct current) after passing through the rectifiers.

So, two things: how is the B+ in this case apparently electrically common to the cathode, and with respect to the choke, it would appear to me that the choke as wired now would be on the 'B-' if there is such a thing. Why then would it not be wired to the center tap in this case?

Hope I am not making myself look bad....

regards,
Tony
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Old 22nd January 2003, 07:27 PM   #5
Joel is offline Joel  United States
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Tony, the diodes only go into conduction at the tops (+) of the waveform. B+ is taken off the cathode/filament (in a DH rectifier).
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Old 22nd January 2003, 09:50 PM   #6
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Tony,

Sometimes, rarely, the choke is placed on the center tap to ground. This is done in at least one old radio I have, where the choke pulls triple duty.

1) it filters ripple (not totally effectively).
2) it is also the electromagnet for the speaker.
3) it provides a negative bias voltage for the output tube.

But the standard practice is to place it after the B+ at the output of the cathodes. As Joel pointed out, the tubes conduct on the positive half cycle, so the "output" of the cathode is +. The choke due to internal resistance will drop a few volts, but the output will still be positive.

The other thing is that you need to look at it from a complete circuit point of view. Electrical flow is from negative to positive, no matter where the tube is. One could have the load at the plate (anode) of the tube or at the cathode. As long as the cathode "points" to the negative and the anode "points" to the positive, it will work.

So, again as Joel implied, when the anode sees the positive half of the AC, the tube will conduct. But... the load sees the cathode as being positive with respect to ground, but the cathode will see the load as negative with respect the transformer, during its positive half. So in that sense the cathode will be negative with respects the anode, but only during the positive half cycle.

Understand? No? Neither do I!

Gabe
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Old 22nd January 2003, 10:13 PM   #7
Joel is offline Joel  United States
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Quote:
Originally posted by Gabevee
So in that sense the cathode will be negative with respects the anode, but only during the positive half cycle.
Understand? No? Neither do I!
Clearly, because that makes no sense!
The cathode is at 5V. Remember, it IS the heater.
On the negative half of the cycle, the plate is negative - the electrons don't go to it. On the positive half, it's positive - the electrons go to it. That's all there is to it.
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Old 22nd January 2003, 10:14 PM   #8
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it doesn't matter whether you put the choke in the ground return or in series with the cathodes of the rectifiers. run it in a simulation program and you will not that the amount of ripple is the same.

putting the choke in the ground return, does however, reduce the possibility of insulation breakdown
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Old 22nd January 2003, 10:22 PM   #9
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Default IT DOES MATTER.

Hi,

You lot should go back to school.

Ever heard of send and return?

Damn...I'm so fed up with ignorance I may just as well take on Phred's identity.

Jeez...;/.
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Old 22nd January 2003, 11:25 PM   #10
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Default It Does Matter?

run the simulation, I just did a 7000 datapoint simultaneous simulation of both -- It doesn't make any difference.
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