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PT failure dangers
PT failure dangers
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Old 18th February 2009, 11:24 AM   #1
overdrajv is offline overdrajv  Slovenia
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Default PT failure dangers


I recently got made a toroidal PT with 6,3V, 175V c.t. and 320V c.t. on secondaries. However the manufacturer said he doesnt guarantee anything and he thinks that the wires could overflash, becouse he never made high voltage trafos.

I want to use it in my guitar amp and I know Im touching (holding) the signal ground which is actuali the center taps of the 175 and 320 windings.

Can I be electrified if the house or club doesnt have proper grounding. How about if it does?


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Old 18th February 2009, 11:44 AM   #2
Geek is offline Geek
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This is likely to happen only if the chassis is unearthed (use a 3-wire plug, ALWAYS!) and there is sufficient design deficciency in the primary-secondary isolation.

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Old 18th February 2009, 01:09 PM   #3
SY is offline SY  United States
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PT failure dangers
Can I be electrified if the house or club doesnt have proper grounding.
Yes, you can. At a minimum, follow the rules that Geek mentioned (chassis ground = safety ground) and use a three-wire power cord with the ground lead FIRMLY connected to the chassis. If you think that the club's ground is dodgy and you are using a transformer which the manufacturer will not guarantee, I'd run a separate ground wire to a known-good ground, just in case you happen to step in a puddle of beer.

Read about Stone the Crows.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 18th February 2009, 04:02 PM   #4
jrevillug is offline jrevillug  United Kingdom
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In a club with a dodgy ground, might it be worth using a separate 1:1 ratio safety isolation transformer? That should prevent most issues, I would've thought.
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Old 18th February 2009, 04:44 PM   #5
richwalters is offline richwalters
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H.V Toroid transformers have to be carefully wound with multiple insulation tape (not cost effective) and I've never seen one which has an approved safety and temp rise label. The limit volts is 1:1 mains and lucky if that. The only way is complete vacuum varnish impregnation and many toroid manufacturers are skipping this.
Unfortunately, the way the industry is going downhill, transformer expertise is failing from the days when engineers knew how to wind a transformer and the correct varnish grade.
The most important safety bit is the interwinding screen, which to be effective has to be from Cu foil. For an E&I type this is easy.
Ce la vie.
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Old 18th February 2009, 05:48 PM   #6
Johan Potgieter is offline Johan Potgieter  South Africa
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Not to discourage Overdrajv (since you already have the transformers) ....

but I have never been trusty of toroids, per the above statements. I think they are an overrated investment, supporetd by a sizable dollop of popular opinion.

I am not a frequent user of clubs, but from what local experience I had, somewhere someone IS going to wire the red (live) lead to the earth pin - in a manner of speaking. (Cannot believe it will be much different elsewhere).

The advice of Jervillug is prudent though cumbersome. What do other "band players' do?

(PS: ... or use batteries! )
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Old 18th February 2009, 09:26 PM   #7
richwalters is offline richwalters
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By the late 1970's, mains toroids were in trend as mosfet amps filling the market and I got this custom 500VA HT tranny made up by an expert winder for a modified GEC 100W tube amp. The construction is very good with even winding around the doughnut which was left in a vacuum varnish impregnation oven for 2 days. However, rated max 80C is still high but with the iron core totally enclosed, the real drawback is heat dissipation which has to go through the windings to the outside world and with far higher HT turns (carefully layer wound takes time) creates more hotspot temp/heat insulation, . Bmax is reduced to 1T instead of 1.6T. The internal expansion of a toroid with so much varnish is higher, and the design cannot have creepage in the E&I conventional sense. So the winding construction is important with empire cloth used instead of tape. Total weight 10kg, probably 1/3 of which is solid shellac varnish.
Unfortunately the maker and the business has past gone. Finding a good toroid designer, someone who understands the issues is hard to find now and a custom one-off isn't viable.

However it can be done.
Is it for sale ?..sorry... nup

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Old 19th February 2009, 12:30 AM   #8
Tweeker is offline Tweeker  United States
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So whats going on with all the Antek toroids then?
Be sure your foil hat has a good low impedance ground.
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Old 19th February 2009, 02:30 AM   #9
Tubelab_com is online now Tubelab_com  United States
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So whats going on with all the Antek toroids then?
Someone posted here that they are Chinese made. I don't know that for sure. I have several including one of the new ones with the 70 volt tap. I have beat on them pretty hard with no issues. I always connect the circuit ground to the power line ground. That way if there is a short in the transformer, the fuse will blow.

As for live gigs, the opportunity to be toasted increases. Sweaty performers in intimate contact with circuit ground, plenty of opportunity to contact other metal objects that "should be grounded". A shorted transformer in a properly grounded amp, blown fuse. A shorted transformer in an ungrounded amp, guitar in one hand, other hand on a grounded microphone, youre toast!

A good toroid should have a layer of insulation between the primary and secondary. I have cut the secondary off of a few toroids so that I could wind a new one. I have always found a layer of tape over the primary. There is a practice of using half of a 230 volt primary for the line input, and the other half of the same primary as the secondary. This eliminates this extra layer of insulation, and is a recipie for disaster.
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Old 19th February 2009, 02:56 AM   #10
astouffer is offline astouffer  United States
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Magnet wire has better insulation than you think. At work a few years ago we bought a new HIPOT tester that went to 5kv ac/dc. I took two pieces of 32awg wire and twisted them into an X shape to keep the ends far apart. The enamel coating on that wire took the full 5kv without arcing or failing an insulation test. Our custom mini coax with teflon insulation failed at around 900v and RG174 also had no issues.
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