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Old 27th October 2007, 06:51 PM   #1
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Question Buzzing Choke - Any Ideas?

Hi all.

I've got a KT88 equipped Williamson design amp, with a much uprated psu, as my main listening amp. Unfortunately, it has a buzzing choke in said psu, audible from a good 10-15 feet. Also unfortunately, I didn't build the beast and have very little experience with choke psu's.

The buzz is a physical sound, coming directly from said choke rather than through the speakers, and I have noted that if the h.t is applied to the valves before they've fully heated the buzz seems to 'fade in', getting louder as the valves start to conduct.

So far, I've managed to make the amp listenable by placing rubber grommets between the choke and chassis, building a cork & foam lined 'dampening box' over the choke (there's only a 2*c temprature rise after 4hrs use with the box fitted, so I'm pretty sure it's safe), and tightening the bolts holding the choke together.

Is there anything else I can try? It has been suggested (on another forum) that I should 'smack it with a hammer, to seat the windings better', but I'm kinda leery of that idea - a little too brute force and ignorance for my taste.

So, any ideas as to the cause of my buzz and how to cure it?

Cheers... G
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Old 27th October 2007, 08:46 PM   #2
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Although you don't say so explicitly, I suppose this is a choke-input filter? They are inclined to buzz. The buzzing may possibly be reduced by adding a small cap, typically 0.68uF, from the rectifier to ground (i.e. before the choke). With such a small value cap, it will still behave electrically as a choke-input filter but the noise should be diminished.

If that doesn't solve the problem, then the choke is probably under-rated for the DC current + AC current that it has to pass. The current rating of the first choke in a choke-input filter needs to be significantly higher than it would be in a cap-input filter.
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Old 27th October 2007, 08:56 PM   #3
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Back again.

Also, is there any chance that the choke buzz could be showing up something else wrong in my amp?

Cheers... G

(sorry for the double post- brain fade)
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Old 27th October 2007, 09:25 PM   #4
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You could always take out the choke and soak it in shellac or varnish.
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Old 27th October 2007, 09:36 PM   #5
Gluca is offline Gluca  Italy
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Quote:
Also, is there any chance that the choke buzz could be showing up something else wrong in my amp?
As Ray said, probably your choke is not rated for the current of the circuit.
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Old 27th October 2007, 10:44 PM   #6
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hey-Hey!!!,
The buzz coming up as the finals start conducting is a symptom of overcurrent. Depending on the service input or behind a C, will give some insight on how to replace it.
cheers,
Douglas
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Old 27th October 2007, 11:54 PM   #7
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With my PSU, I found that the addition of the 0.68uF cap before the choke makes a big difference. In fact, you can incrementally increase this cap, by adding other 0.1uF caps in parallel to further reduce hum, while ensuring that B+ voltage does not go beyond your desired B+ voltage.

However, a basic rule-of-thumb is that the choke size is determined by B+/current draw for a choke-input filter, so in theory, if you increase B+, then ripple should be larger. BUT, as you increase the value of the first cap from 0.68uF to a higher value, you gradually move away from a choke-input filter to a cap-input filter, which should reduce the buzzing anyway!

It is worth experimenting. You could use some aligator clips to add extra small value caps across your 0.68uF cap. Just make sure that they're firmly clipped as a short may be nasty.

As an example, I used a 0.68uF and got some 120Hz hum. A few days ago, I added 0.1uF and got reduced hum. I added 30K of current bleeder across my final cap in the hope that extra current would reduce ripple. I may not have added enough as the hum seemed a tad louder. Then I removed the 30K and replaced the 0.68uF with a 1.5uF. Things are better, although the voltage has increased from 312VDC with 0.68uF (no bleeder) to around 340VDC with the 1.5uF cap. I may even increase the 1.5uF with a 2uF.

Hope all goes well.

Charlie
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Old 28th October 2007, 12:19 PM   #8
Hi_Q is offline Hi_Q  England
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Is the choke mounted on a steel or aluminum chassis? If steel it may be the magnetic field and might be curable by mounting a rubber strip between the chassis and choke laminations. Other thing to try, as suggested in earlier response, to impregnate the laminations with varnish in case they are vibrating. Ideally vacuum impregnation if you know a local transformer manufacturer.
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Old 28th October 2007, 05:32 PM   #9
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Well, I've just spent the afternoon trying to work out which of the three complete psu designs the previous owner gave me the notes for, is actually the one he built. This is made even more complicated by the fact that he used most of the same components in the various layouts, and the inclusion of another four partial psu design sketches showing ongoing mods to various sections of the psu.

I'm getting the horrible feeling I'm going to have to sit down with a (very large) blank sheet of paper, and physically trace the circuit, just to work out what exactly I've got. Having already spent far too long staring into the chassis, that's going to be the winter project. Believe me, rats-nest is pretty accurate - tied up & taped off wires, random grounding, what looks like a 5A heater winding attached to nothing, 3x 25Amp bridge rectifiers, globs of solder spattered over fuses, etc.... I'm amazed that the amp it's powering sounds so good, with this unholy kludge on the end of the umbilicals.



Oh well, I'm off to take an aspirin

Cheers... G


p.s. If the choke current was being exceeded, would the choke heat up or not? Reason I ask, is that this one runs stone cold - well, room temperature. I always understood that an overloaded transformer tended to overheat?
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Old 28th October 2007, 08:17 PM   #10
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Rats' Nest - Eh? I sort of know how you feel. Chip away at it slowly and it'll all work out, I'm sure. Just remember to be cautious, especially with regard to the large valued caps as they can retained quite a bit of kick.

Having messed about with my Aikido, I can see how it is easy to get the rats' nest thing going. I recently replaced my main trafo with a better, higher voltage trafo with multiple primaries. While it allows for more choices for AC input voltages, it also means an excess of wiring. I have taped up the unused taps and primaries, but they really need to be neatly trimmed and then taped. All of this can wait until I get the PSU exactly as I want it. I paralleled a 1uF cap with my 1.5uF pre-choke cap. It works and reduced 120Hz hum, but has added yet another component, along with wires and hot-glue! When I re-house my Aikido, I want to make the interior wiring as neat and artistic as Jan Potgeiter's gear - he really makes amazing looking amp interiors!

You mention solder blobs. I try to avoid these and am very careful to remove them. I don't know how easy it is for blobs to get lose and cause shorts, but with 350V, I try to ensure that I remove all excess solder. I would suggest that you spend some time removing as many of these blobs as possible - IMHO.

Your amp will make a great indoor project. I remember the long, rainy winter evenings as a kid in northern England - what a great time to work on your amp. Having said that, the sound of windshield wipers always conjures-up memories of summer holidays in South Wales!

Charlie
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