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Old 6th October 2005, 10:50 AM   #1
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Default Using transformers as choke?

Hi,

There are couple of shops in my place that sells small power trannies dirt cheap. I am wondering if they are useful to be used as chokes in tube amps. How do I check for core saturation and estimate the inductance?

Thanks
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Old 6th October 2005, 12:27 PM   #2
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Unfortunately I don't think that mains transformers are suitable for chokes for tube amp PSU's because they are not 'gapped'.

When a D.C. current flows through a transformer, then a D.C. flux is produced in the transformer laminations which suturates the material very easily. For this type of operation, manufacturers produce similar looking cores, but with a small gap in the magnetic path, which prevents D.C. flux from flowing. It's A.C. performance remains OK for smoothing.
Single ended output transformers are similar, for the same reason.

Sorry to be the bringer of bad news.
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Old 6th October 2005, 10:51 PM   #3
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Perhaps if they are EI core transofrmers,you could take the stack/laminations apart,and reassemble them in the gapped form.It's not too hard,usually.
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Old 7th October 2005, 01:18 AM   #4
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I guess a little work doesnt sound too much.. How to remove the varnish safely?

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Old 7th October 2005, 01:22 AM   #5
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skyraider,

If you are trying to save money, power transformers salvaged from microwave ovens make pretty good power supply chokes as-is. Use the HV winding and lift the inner end of it off the core or use the choke in the negative supply lead so that the core can be grounded.

Rob
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Old 7th October 2005, 01:35 AM   #6
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Hi Rob,

Not really want to save money to that extend, its just that many transformers are available cheaply and I am just wondering if it can be used without too much trouble.

For example, I just picked up some 28v-0-28v, 28VA at 4USD. If I would to buy a dedicated choke it would cost many times more.
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Old 7th October 2005, 01:45 AM   #7
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Quote:
Originally posted by skyraider
Hi Rob,

Not really want to save money to that extend, its just that many transformers are available cheaply and I am just wondering if it can be used without too much trouble.

For example, I just picked up some 28v-0-28v, 28VA at 4USD. If I would to buy a dedicated choke it would cost many times more.

Well there's nothing quite as rewarding as trying an experiment yourself to find answers to your own questions. If the outcome is clearly not likely to hurt anyone or damage property, experiment away I say!
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Old 7th October 2005, 07:45 AM   #8
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I use a wall wart transformer primary winding as a choke for the input tubes and it measured around 40henries, it was great in getting what hum was there to way below 1mv.

Cheers George
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Old 7th October 2005, 08:00 AM   #9
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Quote:
Originally posted by georgehifi
I use a wall wart transformer primary winding as a choke for the input tubes and it measured around 40henries, it was great in getting what hum was there to way below 1mv.

Cheers George
you can even try using the secondary with a capacitor for a tuned filter! anybody tried this?
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Old 7th October 2005, 10:52 AM   #10
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[quote]I use a wall wart transformer primary winding as a choke for the input tubes and it measured around 40henries, it was great in getting what hum was there to way below 1mv[/quote/

Hi, how do you measure the inductance? 40H is pretty high..?
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