Toasty autotransformer

[Craig405]

Hi,

I have been running my power amp from an auto transformer for a few hours using the 220v tap from mains voltage of 240v.

It is a kilowatt transformer and i estimate i have pulled no more than 2 amps from it during the listening session.
I put my hand on it after the session to check the temperature and it was very hot.
Even after leaving the amp off with the autotransformer still plugged in and on, it remained very hot.

Is this normal, my lower current variac ran the amp at the same levels without heating up?

Regards
Craig

[anatech]

Hi Craig,
Is it designed for 60 Hz or 50 Hz?

If it's designed for 60 Hz it will run hotter on 50 Hz.

-Chris

[Craig405]

anatech, i purchased it from a local radio shop secondhand with no markings on it although i suspect it may be this model as it looks exactly the same.. and the guy in the shop mentioned somthing about Farnell.

http://uk.farnell.com/jsp/endecaSear...=1166382&N=401

It says between 50 and 60hz so i guess its ok with either.

Can i connect the earth wire from the amp and the earthwire from the wall socket togeather through the autotransformers casing (screwed onder the bolts each side) ?
Or am i causing problems with this arrangement.

Thanks
Craig

[anatech]

Hi Craig,
If that is the one, you should be fine.

Connecting the earth grounds though the case should be okay. I would generally have used a common connection point for the transformer case and equipment ground from your wall outlet. They both work, make sure there is no paint to insulate the connection in either location.

Can you measure the current draw of the autoformer on it's own? It's possible there is a shorted turn. That will heat it up pretty good.

-Chris

[Craig405]

Hmm,
no load connected it measured spot on the 220v mark and consumed 130mA, this seems too low to indicate a shorted winding.

Its looking like it may not have a high enough current rating for the amp which is depressing because it is one big hunk of iron.
Even with the amp at idle the current draw from the mains through the autotransformer is 200mA, and with the music pumping at past what i was listening to before it only hits 0.35A average?

I have no idea why it was so hot?

Regards
Craig

[anatech]

Hi Craig,
Leave the ammeter in circuit. Run your amp for a while and monitor the ammeter. Once the autoformer is hot, take a reading and turn the amp off. Then take another reading.

Possibly the bias curent goes up in the amp, or you have a short once the autoformer heats up.

-Chris

[Stocker]

or,

it just gets hot. did you measure it? rule of thumb: if you can keep your hand on it for a minute or so, it's 45C or less. at around 55C you will pull your hand off right away.

leave it for a few hours and feel it again. Large bits of metal can retain heat for a good long time.


anecdotal evidence: I used to work with a particular type of heated valve that had a 5 to 7 pound chunk of stainless steel for the body of the valve, and under normal operating conditions it was heated to 150 deg. C.

It had to sit for MANY hours before cooling enough to handle after the heaters were switched off... I usually left it overnight or at least until the end of a shift.

[Craig405]

Hi,

Ok, i just listened to half an albums worth at uncomfortable listening levels.
The current into the auto transformer hit no more than 0.4 amps.

the amplifiers bias does creep up but seemed to reach a stable level at 0.3 amps. By this point the autotransformer was getting hot, not as hot as before but that was after 2 hours, but it was definatly getting hotter as time went by.

With the amp off and the autotraffo hot it still measured 130mA.

Stocker, when the transformer was hot going by your scale i would say it was approaching 55 degrees. I could hold my hand on it for about 5 seconds before getting a strong urge to pull away .


Regards
Craig

[jaycee]

Sounds like theres something wrong with that autotransformer to me.

Just a question - could you not replace the transformer in your amp with a 240V one?

[Craig405]

jaycee, no i cant its a custom transformer and is massive.

But i dont need to as it is designed for 220-240v operation, just wih the full 240v my supply rails are higher than i find comfortable with a 25 year old amp .

Regards
Craig