Using transformers as choke?

[skyraider]

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

 

[bobhayes]

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.

 

[DigitalJunkie]

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.

 

[skyraider]

I guess a little work doesnt sound too much.. How to remove the varnish safely?

thanks

 

[rcavictim]

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

 

[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.

 

[rcavictim]

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!

 

[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

 

[TonyTecson]

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?

 

[skyraider]

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..?

 

[Alastair E]

Multimeter

Some of the gimmicky multimeters have an inductance range, up to around 20H

Trouble is, they are a bit delicate, Ive blown up two already, while my old Beckman has lasted 20 years without mis-hap, and some really distressing service.--Like everything, you pays your money and takes your chances

The Beckman was 300 GBP nearly 20 years ago, while the cheapie 'quality' meter was only 60GBP around a year ago...

Ive been thinking about the chokes from large flourecent fittings, the ones for 8 foot tubes are pretty chunky--Ill have to try some out, as Ive got access to a couple.......

 

[georgehifi]

My multimeter measures up to 20henries it was average price at Jaycar or Dick Smith cost $130aus, but I borrowed the shops one which was more expensive at $260aus, Just went through all the plugpaks they had, and got the one with the highest henries on the primary with resonable dcr, for memory it was about 80ohms and the primary winding is rated at 240v 1amp.

Cheers George

 

[rcavictim]

Vertical sweep output transformers or autotransformers from vintage B&W TV sets from the 40's and 50's can sometimes serve as plate chokes in tube amplifiers. It may be necessary to place an R in series or parallel to de-Q the stage as a midband peaky bandpass filter sometimes results.

If the set is a round screen 40's vintage unit and is in really nice shape, please offer it to TV collectors on ebay rather than destroy it for a transformer. These aren't being made any more and are getting scarce. If the set is junk, by all means strip it for parts. You will find useful tubes in there too. 5U4's, 5V4's, 6SN7's, etc Grab the power transformer. This can power a nice tube amp project!

Small 120 to 12/24 vac'ish xfmers from microwave oven control panels can also be used as plate chokes, as I imagine wall wort xfmers. Only bench testing will reveal the suitability of a given xfmer.