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Old 15th March 2013, 07:29 PM   #1
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Default Choke? Where to begin?

I want to add a choke to my Shuguang I-25 (EL34 P-P) tube amp.

Been trying to do some research but feeling less confident the more I read.

Where do I start? What specifications of my amp do I need to know in order to choose a choke with appropriate specifications? What are the critical specifications of a choke that make it suitable for one amp and not another?

Any help at all getting me pointed in the right direction would be greatly appreciated!
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Old 15th March 2013, 07:43 PM   #2
TheGimp is offline TheGimp  United States
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You can start with Peak input voltage at the cap before the choke, and current drawn by the amp at the output of the filter section.

You need to know the peak input voltage to insure the voltage rating of the choke is sufficient to insure it won't break down.

You will want to minimize the DC voltage drop across the inductor to keep from upsetting the design.

This assumes your intention is solely to reduce ripple voltage and hum.
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Old 15th March 2013, 07:44 PM   #3
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Simply adding a choke will not help.

A choke is usually a part of the filter network in the power supply, and should be understood as such. Not as an isolated 'add-on' that improves the sound.

For a first step, find a schematic or trace out the power supply by yourself. Then understand, with the help of this forum, how it works and whether a choke would improve it.

For playing around then, the PSUD II software will be helpful.

Andreas
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Old 15th March 2013, 07:47 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

The choke for a CLC supply needs to be rated at maximum supply current.
For fairly obvious reasons they are more useful in pure class A designs
with a constant supply current rather than pushpull amplifiers.

As long as it doesn't saturate due to current any decent value
will help in a CLC supply, how much depends on how big it is.

rgds, sreten.

I'm assuming voltage rating is completely irrelevant, because it is.
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Last edited by sreten; 15th March 2013 at 07:50 PM.
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Old 15th March 2013, 07:59 PM   #5
TheGimp is offline TheGimp  United States
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Wrong on the voltage rating.

Inductors are specified for Maximum DC voltage rating.

They are not guaranteed not to break down above the rating. While most will be tested at higher ratings, it is a bad idea to ignore the specifications.

Check the Hammond site for an example:

Hammond Mfg. - D.C. Filter Chokes - (153 - 159 Series)
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Old 15th March 2013, 08:15 PM   #6
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Quote:
Originally Posted by TheGimp View Post
You can start with Peak input voltage at the cap before the choke, and current drawn by the amp at the output of the filter section.

You need to know the peak input voltage to insure the voltage rating of the choke is sufficient to insure it won't break down.

You will want to minimize the DC voltage drop across the inductor to keep from upsetting the design.

This assumes your intention is solely to reduce ripple voltage and hum.
Thank you. The goals are to reduce ripple(?) and hum.

However, it is obvious to me based on your response and others that I am in over my head.

I have been told several times on various forums that I should install a choke.

Perhaps that is just hot air?

Perhaps a better approach is to describe exactly what I am hearing that I do not want to hear.

Low level hum: this is not too bad, and doesn't increase with volume. Easily tolerated.

Variable buzz: sounds like noise, sort of crackly, is variable, and increases with volume. After hours of hot running, starts to cycle (oscillate) slowly about once every 0.8 seconds. (is this ripple? RF?)

Last edited by cogitech; 15th March 2013 at 08:24 PM.
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Old 15th March 2013, 08:21 PM   #7
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No, I don't think this is hot air.

A choke in a CLC or other filter configuration can significantly reduce hum. But it is not a 'plug-and-play' thingy like some fancy cable.

A choke needs to be dimensioned according to the specifications of the amp and the power supply, and probably the supply needs some redesign to accommodate the choke.

Again, a schematic would be a good starting point.

Andreas
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Old 15th March 2013, 08:35 PM   #8
tomchr is offline tomchr  United States
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Assuming your current circuit has a rectifier followed by a capacitor, then you can get some ripple rejection by adding the inductor/choke (L) followed by a capacitor (C) to ground.

The inductor will need to be rated for the peak current drawn by the load (the amp in your case). The peak current of a single-ended amp is roughly twice that of the current at idle. Recall, in single-ended class A, the current in the output stage swings from the idle value to near zero and up to nearly twice the idle/quiescent value.
You'll probably find that the inductor current is specified as the RMS value rather than the peak value. If my math is correct, for a single-ended amp, the RMS current will be roughly 1.71*Iq, where Iq is the quiescent current.
For push-pull amps, you'll have to find the peak output current from the datasheet of your output tubes. The peak current is usually way higher than the quiescent current.

Once you know the current and voltage requirements for your inductor, just pick the largest inductance you can afford or fit on your chassis. Keep an eye on the DC resistance (or ESR) of the inductor as you'll lose some voltage across this resistance. Note that plate chokes are usually fairly resistive, hence, not suitable as supply inductors. The Electra-Print 70 H plate chokes I have, for example, have about 1300 ohm DCR.

A less expensive solution with significantly better ripple rejection performance would be an actual voltage regulator for the B+ voltage. But that's a different topic...

~Tom
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Last edited by tomchr; 15th March 2013 at 08:37 PM.
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Old 15th March 2013, 08:41 PM   #9
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Here is the schematic. I have no idea how to read it. Like I said. In over my head. Most of what I have read above is like reading Greek.

Full disclosure:

1) I have disconnected U5 (6.3v 1.2a) tap from the 4 pre-amp tubes and installed a separate 4a 6.3v transformer to power those 4 tubes, which allows me enough current to run 12BH7 in place of 12AU7.

2) I have replaced the stock .22uF coupling caps with .22uF Sprague 716P.

3) I have replaced the stock .47uF grid return caps with paralleled .22uF (.44uF total) Russian K-40-Y PIO.

Click the image to open in full size.
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Old 15th March 2013, 10:36 PM   #10
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Perhaps you could post a picture of the wiring in the amp?

Schematic doesn't make sense.

I don't know how well-built these amps are (evidently not so well-built that they don't hum), but the actual wiring, particularly in a tube amp, can easily contribute to unwanted hum.

It could be that rearranging the wiring could improve matters without resorting to a choke.
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