B+ fluctuation during transients

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B+ fluctuation during transients

Postby Blair » Sun Oct 07, 2007 2:12 pm

Hey guys,

I notice that my amp distorts a bit when being driven hard. This is not my typical listening levels, but something I just noticed. Since I noticed this, and since my amp is a bit of a one off version of the poseidons, I decided to try several things. My amp shares a common PS for both channels.

I pulled my tubes on one channel out and cooked a cheap meter banana clipped to the b+ Yellow_Light_Colorz_PDT_02

This did not assist the distortion issue at all. I'm driving it with a 6SN7 preamp and the gain is a bit high for the amp.

My B+ sits around 496v at idle, and fluctuates down to as little as 475-470v during higher volume applications. Does this sound right? I have 130uf on each of the first two caps before separating the B+ for the driver tubes, so there is plenty of capacitance. It is also SS rectified so overdoing the capacitance is not as big of an issue.

Any ideas or tests besides lugging my 125lb towers :o into my tube area?

Thanks,

Blair
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Postby audiowize » Sun Oct 07, 2007 2:17 pm

Tell us more about the amp. What comes to mind for me is that you may be drawing more current than the transformer can supply. Also, you might just be clipping... it helps to have more detail. Amplifier tube complement, speakers, pictures... etc.
-Paul
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Postby TomMcNally » Sun Oct 07, 2007 2:25 pm

Don't turn it up so loud ! Yellow_Light_Colorz_PDT_07
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Postby Blair » Sun Oct 07, 2007 2:34 pm

I can do that, but you will have to give me a few minutes. This beast weighs 65lbs.

I doubt I am saturating the power transformers output. It is a large toroid by anetek rated as a 800vct with dual 400v windings rated at 500ma each. I am using the standard configuration for the poseidon PS, but beefed up significantly. Use this as a guide, but it looks very different:

[img]http://img.villagephotos.com/p/2005-12/1115293/BCapbank[1].JPG[/img]

The first two caps after rectification are 130uf at 600v, and the other four caps are dual can caps rated at 50uf at 500v.

The choke is a Hammond HC193D now. I chose it because if its low DCR. I am consdiering moving the B+ over to before the choke to put 525v on the plates to see if this helps, but I see the same voltage fluctuation there. I'll get those pics up soon if it will help. Pardon my craftsmanship on the preamp. She ain't done Yellow_Light_Colorz_PDT_02

Blair
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Postby Blair » Sun Oct 07, 2007 3:20 pm

Here you go, and sorry about the sloppy wiring:

Image

Image

Image

Image

Image

Hope this helps.

Blair
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Postby erichayes » Sun Oct 07, 2007 6:20 pm

Hi Blair,

If you're running at 496 volts idling, and you're only dropping 20 volts on peaks, you have a power supply that many old farts would have killed for back in the last century. It wasn't uncommon to lose 60 to 70 volts from idle to max due to rectifier losses and the lack of reasonably priced high capacitance/high voltage caps.
Eric in the Jefferson State
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Postby Blair » Sun Oct 07, 2007 9:24 pm

I'm even going a little high there Eric. It really does not even sag +/-5v during loud listening levels. I really think it is a combo of room size and my preamp drives my amp a bit too hard. This power supply is pretty rock solid IMO. However, running with a load, my B+ is about 486-488.

Thanks,

Blair
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Postby audiowize » Sat Jan 12, 2008 2:47 am

I don't see any issues with your power supply. A little bit of sag isn't the end of the world, you're probably just running out of power. What're you running for output tubes? I'd bet you could put some big old 6550's in there and crank the current up a little bit. You could run upwards of 70-80ma per 6550 on some reasonably well made tubes.... if your output iron is ok with that.
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