Why does my oversized preamp trafo gets much too hot?!?!? - diyAudio
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Old 19th October 2005, 02:14 PM   #1
Klimon is offline Klimon  Belgium
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Default Why does my oversized preamp trafo gets much too hot?!?!?

Hello,

I just plugged in my newly built double mono preamp and after a few minutes one of the trafo's started smoking... (B+ measured as expected however)

I unwired it and tested it without a load; everything fine (all the voltages measured fine); seems it had been the plastic covering the windings that was smoking and not the internal isolation. (lucky!!!)

Rewired it again and after a few minutes it began to smoke again...

Now: my problem is that I really don't understand why the windings get so hot (which is the real illness, smoking plastic is just the symptom). Both trafo's are identical besides the plastic covering the windings, that probably explains why one trafo smokes and the other doesn't (they both seem to get quite warm instantaniously). I used one of the trafo's before in a el84se pentode integrated with silicon diodes that drawed a lot more current + the filament winding isn't being stressed either. A el84se draws around 100mA; this preamp (12b4) draws 30mA = 15mA for each trafo (= 15% of the el84 current draw but the trafo gets much hotter)!!!


I'll give a short description of the setup as I'm not an electrical expert and maybe the problem is clear to someone reading:

HT winding is 125-0-125V; each anode of the rectifier tube is connected to one of the 125V taps (because I need 250V), two diodes are used to form a bridge together with the tube and supply the - . The rectifiers are AZ1 (electrically similar to 5y3: big voltage drop + not too high current rating).

Don't think the mistake could be located further in the circuit - will be checking everything out again (although I'm rather sure all is wired as should be...) / The diodes & rectifier tubes are wired correctly; tubes are tested etc.

If this rings a bell, any help would be extremely welcome
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Old 19th October 2005, 02:27 PM   #2
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First you must identify the windings! which you obviously haven't done yet.

Have you got any data of the transformer? If yes, have checked the output voltage before you connect any load?

Do you have any sketch of your wiring?
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Old 19th October 2005, 02:46 PM   #3
Klimon is offline Klimon  Belgium
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Hello,

The voltages are marked on the trafo. Without a load, they all measure as should.

What do you mean by 'First you have to identify the windings'?

I have no sketch of the wiring; the schematic comes from diyparadise.com & I carefully designed the ps myself...
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Old 19th October 2005, 02:54 PM   #4
Yvesm is offline Yvesm  France
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Mmmh, you tell about TWO trafos ?

How are they connected together (if they are) ?

The center tap of the HT winding is floating ... of course.

Yves.
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Old 19th October 2005, 05:15 PM   #5
Klimon is offline Klimon  Belgium
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Hello Yvesm,

Anode a of rectifier tube one is connected to anode b of rectifier two, and vice versa (I know I did this right because I have another dual mono amp with identical wiring that runs perfect / That one didn't have a center tap in the HT winding though).

The center taps of both trafo's aren't connected to anything (or to each other)// is this the magical solution? Should I keep the other connections as described above when also connecting the center taps with each other?

MANY MANY THANKS!!!
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Old 19th October 2005, 06:19 PM   #6
Yvesm is offline Yvesm  France
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Hi !

HV center taps must be left floating.

I still don't understand how you connetc the TWO tranfos.
Are they completly independent or have something in common ?
Certainly a schemo would be helpfull !

Just a though: have you checked if center taps wasn't internally tied to something else ?




Yves.
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Old 19th October 2005, 06:43 PM   #7
Klimon is offline Klimon  Belgium
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Hello,

I'll explain a little better:

I use a bridge rectifier (one for each channel) that consists of two silicon diodes followed by a rectifier tube. The anodes of each tube are connected to the opposite anodes of the other tube (think this is for balancing, found it on audiodesignguide.com but can't seem to find it right now).

The center taps of the two transformers are 'floating' (not connected to anything) - I measured without load and they measure as should; no problem here. (125volts to each of the two 125v taps)

The two power supplies are dual mono, but after the last ps capacitor I connected the two grounds with each other + the circuit ground.

Hope this helps and you can help me!
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Old 19th October 2005, 07:04 PM   #8
Yvesm is offline Yvesm  France
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Quote:
Originally posted by Klimon
Hello,

I'll explain a little better:

I use a bridge rectifier (one for each channel) that consists of two silicon diodes followed by a rectifier tube. The anodes of each tube are connected to the opposite anodes of the other tube
Do you mean you use 2 tubes in // ? ? ? in each channel ? ? ?

The silicon diodes must have their anodes tied together and to the ground. Right ?

Yves.
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Old 19th October 2005, 08:08 PM   #9
Klimon is offline Klimon  Belgium
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I use one tube in each channel + anodes of the silicons tied together

But: the anode 1 of tube 1 (left channel) is connected to anode 1 of tube 2 (right channel) & anode 2 of tube 1 is tied to anode 2 of tube 2 // quite sure the mistake isn't to be found here... (seems like I've put the power trafo secondaries in parallel - don't know if that's good or bad however)

What startles me is that B+ voltages (measured at the last cap of each channel) are very near what I expected & simulated (around 270V) --- If the voltage is right and the trafo's get too hot (= VAC too high), the current draw must be too high - but where is that current going???

Thanks for thinking along
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Old 19th October 2005, 08:17 PM   #10
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... please don't do any more "smoke" tests on the circuit and post a schematic, i think it will be much easier to answer your question then...

Greetings,
Andreas
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