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Old 17th June 2010, 06:43 AM   #1
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Default Have 7v ac need 6.3v ac, need simple ideer


i am building a tube amp, where i need around 15A for the heaters, which run at 6.3V..

but.. the power trafo i have, is abit old, so it was made for 220V, but i have 230-240V in my outlet, so all the outputs are just abit higher then what is printed on the trafo.

so now i have a 7Vac where i need a 6.3Vac.

i could use a resistor in series.. but that would have to disipate 10W....

could maybe do something with some diodes.. but is there diodes that can take the current, and in a package that is suitable for some cooling?

the power supply will be powering 2 seperate monobloks, so would be nice if the voltage wasnt load dependent which it will be with the resistor..

i am thinking the resistor is the best ideer... but there could very well be a super simple ideer i just havent thought of, so i though i would ask in here :-)


Last edited by heinrichs; 17th June 2010 at 06:50 AM.
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Old 17th June 2010, 06:53 AM   #2
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Have you tested the voltage with a load that draws 15A?
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Old 17th June 2010, 07:27 AM   #3
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that is correct ...voltage will drop no matter what with 15A load .... even possible that it drops more than needed ....
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Old 17th June 2010, 07:28 AM   #4
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Put two antiparallel diodes in series. This will reduce 0.6 volts

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Old 17th June 2010, 07:29 AM   #5
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offcourse... just didnt think about that... normally i use switchmodes for everything....

gonna do some measurements when i get home from work today, thanks :-)

just have to be some pretty big diodes... but should work though..
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Old 17th June 2010, 07:43 AM   #6
Elvee is offline Elvee  Belgium
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The diodes will work, but they will dissipate exactly the same power as the resistor.
In addition, they will generate severe harmonics and switching noise.

Of course, if your wiring is absolutely perfect, this doesnt matter, but.....

If you want a solution that is both non-dissipative and "quiet", you could use a series inductor: in this case, 150H would do the trick (it obviously has to be rated for 15A).
It will not only adjust the voltage to the correct level, but also somewhat soften the cold start, and improve the noise filtering.
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Old 17th June 2010, 09:04 AM   #7
AndrewT is offline AndrewT  Scotland
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connect up your 15A worth of heaters and measure the voltage across the heater pins, before you do anything else.

If you want to draw a continuous 15Aac from your transformer, the secondary winding must be rated >=15Aac and preferably >=30Aac, (200VA). What size is your transformer?
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 17th June 2010, 01:46 PM   #8
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I ended up puttig my heaters in paralel 2 by 2, since i have multiple 6.3v outputs on my trafo, so i could connect them making 14Vac.

and then i found some 160H 6A inducters, and 2 of them in series for each mono blok does the trick.

thank you all for your help.
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