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Old 18th November 2012, 05:11 AM   #21
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Originally Posted by vitalstates View Post
I've bumped this up because I'm still scratching my head over this one....

I have taken both transformers off the chassis and run them on the bench with their secondaries in a screew block with nothing attached. i.e absolutely no load. I have a maplin wall meter which measures, among other things, current taken from the wall.

both transformers(ht: 0-600v 200ma spec & heater: 2x9v 12amp spec) consume 250ma at the wall and both get hot after 1 hour running no load. The ht transformer gets very hot.

I have very limited experience with transformers, to date all my Hammonds have worked as per spec, so I really don't know if this is acceptable or not. Certainly I don't feel happy running an amp with transformers that get this hot.

I'd be grateful for any further input. I'm still waiting for delivery of the shoulder washers to see if they have any effect.

Ed
the 250mA no load current seems typical for that size....shoulder washers on the traffo bolts is a good practice.....you may also try to dip your traffo in a vat of air drying insulating varnish without the bolts on so as to allow the varnish to penetrate in between laminations, this will help lower eddy current somemore...
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Old 18th November 2012, 09:25 AM   #22
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250mA?

I did not notice that at first time of reading.... that's about 60W of idle power. If that trafo is working to specification, it's junk!

I looked up the specification (actually, an emailed answer to my question) for iron losses in the JMS 250VA trafo. The value is 11W. Even the 400VA is 17W.

I imagine that the primary is rated for 230V and Ed has 240V at the wall socket, for the losses to be so heavy, and that the trafos were built to a price, without concern for quality.

If they were supplied in the UK, I would return them, on the basis of not fit for purpose. The specification for UK mains is 207 to 253V, and at 253 these things will be on fire.
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Old 18th November 2012, 12:21 PM   #23
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thanks for the input guys

Hi Rod
also, thanks for the attention....A couple of points

1. JMS was the first company I contacted. The chap on the phone said I should send an email to Sean Harris?..After 2 un-answered emails I looked elsewhere..well you do, don't you. It's 200 too late now, unless I get my money back.

2. The transformers were wound to a spec(no names yet) and they were requested at 240v primary. 'Not fit for purpose' was exactly what I was thinking but there are a couple of things that are delaying me before I get back to the manufacturer.

a. I wanted to see what effect putting the shoulder washers in would do.
b. I've painted them and cut the wires to suit, so I'm not sure how that affects my rights.
c. I was waiting for some more informed opinions as my transformer knowledge is close to zero. I'm not even sure if transformers are likely to get hot after 1 hour with no load.
d. My mains is 245-253, and I wasn't sure whether it would affect the heat on a transformer specced for 240v.

rgds Ed
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Old 18th November 2012, 12:37 PM   #24
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Ed, you could test the trafos one at a time, without mounting hardware, and nowhere near any conducting materials.

If you still get 250mA, report the input voltage and current to the supplier, and ask for explanation.

A 240V primary should be able to handle UK mains at 253V without overheating. With 60W of unloaded dissipation, you would need a fan just to mount it in an enclosure, and the efficiency would be less than 50% for moderate loads - this is not acceptable or reasonable.

Sean is usually responsive, but if you need their trafos, just use the online shop. You can specify primary=240V and just give the rms secondary voltage(s) you need on checkout. You can specify any voltage you like, up to 1000V, and they arrive in a few days.
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Old 18th November 2012, 09:32 PM   #25
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250mA is not all unity power factor, some if it is just energy stored, you can check that using a power factor meter...if you have a variac you can try 220volt input and see what the idle current is....if the traffo was designed to work off 220v mains at high flux density, then that would explain the high idle current....245 to 253 is too high for comfort...

insulating the bolts using shoulder washers prevents eddy currents, it will have nowhere to go so it helps...
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Old 18th November 2012, 09:36 PM   #26
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Sounds like a lot of it is iron losses, if the core gets to 70 deg. C in 45 minutes!
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Old 18th November 2012, 09:50 PM   #27
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i suspect the quality of the iron used is poor........what i do in such cases is add more primary turns to lower the idle current and lower the heating effects...if i want the heat to be minimal, i use a bigger core than what the design called for...
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Old 19th November 2012, 01:48 AM   #28
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Hammond transformers are pretty decent. They shouldn't get that hot without a load attached. They all can't be bad so most likely something is not hooked up right.
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Old 19th November 2012, 02:40 AM   #29
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as i recall, tube amps have traffos that run hot, so that may not be an issue with those in the states where ambient temps are really low....

in my side of the pond, ambient temps of 30*C are not uncommon, so designing traffos that does not run hot is a must....
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Old 22nd November 2012, 07:56 PM   #30
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I think I may have solved this finally....

The shoulder washers arrived this morning and I fitted them and ran the transformers on the bench.

Both transformers consumed 130ma with no load. I ran them both for an hour and they didn't even get warm.

I put them back in the chassis, tested everything without valves and got the same result, i.e no heat. I did mount the transformers on rubber tap washers which lifted them off the chassis by 1/8 inch ot so.

I ran the amp for 1 hour with all valves in and both transformers got warm. I think thats a result, fairly certain that without the shoulder washers both transformers were acting like they had a shorted turn.

Many thanks to all for the suggestions and encouragement.
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