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Old 5th September 2007, 10:57 PM   #1
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Default New PSU for Aikido?

My 6SN7 Aikido has a slightly buzzing transformer. It is a Hammond 200 series that puts out 125-0-125. I believe that some of this rattle is making its way into the tubes. Also, this transformer is rated for 115VAC, and my mains typically puts out more like 120VAC, sometimes as high as 124VAC. The transformer tends to run hot.

There are a couple of options:

1. I could easily change the transformer out to the 300 series equivalent, which is built larger and has universal primaries so that I can set it at 120VAC. Leave the rest of the PSU as is: 1N4007 (paralleled with caps); 0.68uF "tuning" cap; 10H (103R) choke; 60uF cap; 10H (103R) choke; 60uF cap. 20K bleeder resistors across the final cap to bring total current draw from the PSU up to around 32mA.

However, I am struck by the idea of tube rectification - the larger tubes just look, well awesome! So....

2. Choke input PSU using 350-0-350 into 5V4GA into 10H (103R) choke: 60uF cap; 10H (103R) choke; 60uF cap. With 32mA current, PSUD gives me 300VDC with 3.61mA ripple.

3. "Cap" input PSU using 275-0-275 into 5V4GA into 1.5uF "tuning" cap; 10H choke; 60uF cap; 10H choke; 60uF cap. With 34mA current, PSUD estimates 308VDC with 1.04mV ripple.

Empirically, I get around 290VDC from my current set-up. Of course, 120VAC into the transformer likely yields more than 125-0-125 out. However, PSUD does not give me this voltage, more like 240VDC.

So, what would folk recommend? Should I accept a little (miniscule) hum and a hot transformer)? I like the idea of option 3, and I could change out the cap to tweak the VDC.

I am sticking with only changing out the transformer because it only involves replacing one part.

Bruce Anderson believes that the "flywheel" PSU would be better, but unless and make a deeper chassis, I cannot house the large sized choke that is required! I trust in Bruce's judgement of the efficacy of the "flywheel" (I heard it working in his Aikido) and it sounded really, really good.

Also, how do I estimate transformer DCR, etc? Hammond do not give this information for transformers.

Thanks,

Charlie
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Old 5th September 2007, 11:11 PM   #2
SY is offline SY  United States
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I think your biggest bang-for-buck is going to a separate heater transformer. That way, switching noise from the high voltage supply will not be efficiently coupled back into the circuit via the heaters. Yes, tube rectifiers have switching noise, too.

What looks ultra-cool is a potted transformer. They're also mechanically quiet. Surplus is your friend- you're not going into production, this is a one-off.
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Old 6th September 2007, 12:42 AM   #3
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Sy,

I already am using a separate transformer for my filaments. It is a 6.3VAC 15A and huge in comparison with the 125-0-125 transformer.

My ignorance is pretty huge - What is a potted transformer? Anything like potted plants?????

Charlie
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Old 6th September 2007, 12:45 AM   #4
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Sorry....another reply

The hum, as far as I can tell, is 60Hz. It is very quiet. Over the weekend, I experimented with the grounding position for the PSU - moving it from PCB directly to chassis GND, etc. Anyway, with the PSU connected directly to chassis ground, I got a more audible 120Hz hum quite different from the 60Hz very quiet hum.

Charlie
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Old 6th September 2007, 12:48 AM   #5
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Sorry (what a doofus I am) another reply.

One source of noise in my current set-up could be the Omron relay that is used to switch on the B+. This relay is itself tripped by an Amperex Octal delay relay.

By going with indirectly heated tube rectification, I can remove the Omron and Amperex. The rectifier would take 8 to 15 seconds to get up to voltage giving the tubes a little preheating prior to B+.

Charlie
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Old 6th September 2007, 01:03 AM   #6
SY is offline SY  United States
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A potted transformer is one put into a can and filled with an insulating material (often tar or epoxy).

Here's a somewhat overpriced example:

http://www.electrojunk.com/products/milpotted.htm
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