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Old 14th April 2008, 07:23 AM   #1
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Default filament AC vs DC

I am curious as to why i don't see much in the way of turning AC to DC and then filtering it for a low hum heater voltage? What if i just used a battery? I am wanting to build an amp and am tossing the idea around in my head, but I don't want to waste my time designing if it's not a good idea. Yes, I am new to this forum, but not new to electronics. I have seen filaments in amps using DC, but really no filtering. I wish my amp to be different than the rest, the reason for thought. It is too easy to just copy an amp, there are so many diagrams tested and tried, but I find more fun in experimenting.
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Old 14th April 2008, 07:38 AM   #2
nitrate is offline nitrate  United Kingdom
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Yourownfree-

I'm with you on that one. I don't see any point in not using regulated DC on the filiment when it is sooo easy to do. Especially if it removes a potential problem of hum injection. I too am new to valves and have never actually fired one up in anger, so just like you i'm a bit sceptical of whether or not building an amp based around valves is a good idea or not. But what the heck? i've built the other types so i may as well have a go. Who knows... i might even be converted :-)

Leigh
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Old 14th April 2008, 07:42 AM   #3
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It just isn't all that necessary, except for very low level stages, something like a phono stage, or the up front stages of a SW XCVR, or something like that.

Secondly, clean AC on the heaters is much better than DC unless filtered very well since the harmonic content is liable to impair the sonic performance. I did a project with DC heating voltage, and another with AC, and it really didn't make any difference (not low level).

With DHT types, however, constant current DC does seem to work better than AC. For IHT types, clean, balanced AC works just as well.
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Old 14th April 2008, 07:51 AM   #4
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Quote:
Originally posted by nitrate
when it is sooo easy to do.
If you have a good design to get 6.3VDC from a 6.3VAC winding that is sooo easy, is inexpensive, is not super hot, and that can supply several amps I'm keen to see it.
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Old 14th April 2008, 08:33 AM   #5
nitrate is offline nitrate  United Kingdom
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seven amps?

What you building? a computer! LOL

Erm.. in that case AC may be the best action, your obviously not talking small signal here. But if you want a crack at it, easiest way i can think of is rectify your 6.3v to give you approx 9V with some rough filtering. take a 7805 and use 3 diodes to raise its ref up to give about 6.8v, stuff that into a large NPN hung off the positive rail and that should do it, you should end up approx 6.2v. if you find you dont have enough drop to keep the 7805 going then using a low drop version should do the trick like a 2905. raising the rail voltage is not the best idea as the dissipation accross your NPN is gonna get high.

Maybe not good for such high current but definatly workable for smaller currents in the ss stages.

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Old 14th April 2008, 08:33 AM   #6
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Hm, the problem with DC heating is you have semiconductors in the tube amp.

Kind regards,
Darius
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Old 14th April 2008, 08:36 AM   #7
nitrate is offline nitrate  United Kingdom
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But not in the signal path... It is after all 2008, you'd expect some silicon in there, it makes sense.

Leigh
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Old 14th April 2008, 09:14 AM   #8
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Default #7

Quote:
Originally posted by nitrate
But not in the signal path... It is after all 2008, you'd expect some silicon in there, it makes sense.

Leigh
Hello Leigh,
Yes, this is why I used a switched mode regulator in my RIAA preamp.
I am waiting for new manufactured ECC808. Than I'll make an AC
heated thing.

Kind regards,
Darius
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Old 14th April 2008, 09:19 AM   #9
nitrate is offline nitrate  United Kingdom
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LOL, nice. I had better shut now tho as i must confess i dont even have my hands on a single working valve yet. There in the post from Rapid ( they dont live up to their name me thinkX )

Leigh
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Old 14th April 2008, 01:15 PM   #10
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There are a number of extensive threads which address this very issue (use the search function) and will educate you in the technical reason why. There are very very few reasons to prefer DC over AC. I have many quiet projects with AC heaters (even a phono stage).

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