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Mains transformer orientation

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Hoi. Sorry for boggeling you guys with such a basic question but I am building a very small spud amplifier, and the layout is so compact I worry it might hum easily.

I have attached a model of the amp with the main's trafo bells pointing towards a shielded OPT, it is then pointing sideways to the rectifier tube. Should I leave it like this, or should I turn it around with the bells facing the rectifier (perhaps a better idea since the OPT is shielded?)?

Thought I'd better ask advice before drilling instead of drilling twice.
 

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This way they will have 90deg angle but they get scary close to the signal tubes... I will also have to fit two more irons but they are small mumetal-potted lundahl inputs so I think this is the last orientation concern... thoughts?
 

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I think this will be it. I pulled the rectifier a little back, the signal tubes a little forward. I'll check the orientation on the Tangos first so I can decide orientation of mains tranny, and then I'm off drilling chassi. Chassi will be 1mm copper - nickel plated at the school lab, so if nickel plating is to happen before christmas, I will have to make the chassi within a week or so. Iron is on it's way from all over the world, so it looks like christmas holydays will be rewarding this year. I plan to go for european rectifier (some majestic looking Telefunken mesh) and the active device will be 6S45P-e delivering just below a watt. Lundahl input iron, and battery bias.

My next weeks is booked with exams so not much building progress is going to happen before holydays starts in mid december, but I'll post some pictures later on as the project evolves. If any wizeguys has something clever to say about the layout, please speak now ;)

This is the CAD-drawing of how the semi-final result looks:
 

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Ray,

I am unsure about the orientation of the Tango trannies. For some reason I assumed they are oriented sideways, can't really say why. Anywayz, I came up with a solution; I slightly adjusted the hole spacing so that the Hammonds can be mounted either way. This way I can just scope the hum and choose the favorable orientation. I will take notice of your advice though, I'll try it sideways first (I will be building during christmas, and I only have access to scopes and signal generators on the electrolab at school, so I will not be able to measure until January)
 
Rocky,

One other thing if it isn't too late. I would also mount the OPTs at 180 degrees from each other. If for instance the tab for the B+ on the OPT closest to the PTX is on the same side as the PTX then turn the other OPT so that its B+ tab is closest to the edge of the board.
 
dhaen said:
Apply mains to the mains trans primary (carefully insulating secondary leads).
'Scope an OPT primary: tweak orientation for minimum signal.

The chokes should be mounted in a 3rd plane. If they don't have suitable mountings for this, make a couple of 90 deg brackets.


See the attached picture.. looks good? Thanks!

Bryan said:
Rocky,

What software package did you use to model the chassis layouts? It seems like a great way to pre-plan the specifics of layout in th final form (though some real world tweaking is always necessary).

Thanks,

Bryan

I am using SolidWorks 2005. We actually have lessons in it on school. Very nice to visualize many things before building.. After christmas, we're even supposed to get a part printer capable of printing out actual parts made from some powder stuff... Very cool. :cool:

Sherman said:
Rocky,

One other thing if it isn't too late. I would also mount the OPTs at 180 degrees from each other. If for instance the tab for the B+ on the OPT closest to the PTX is on the same side as the PTX then turn the other OPT so that its B+ tab is closest to the edge of the board.

I will try with uniform orientation at first. For asthetic reasons. The tangos looks much better that way me thinks.
 

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Somewhat more.

I presume you are using a CLC type PS filter; chokes look a little small for L-input. In such a case I never really had any effect ever from choke field interference, however mounted.

Power transformer-wise I would go with dhaen's suggestion, if at all practical; only hope you do not get stuck with a funny orientation. Much will depend on the construction of the transformer. Also perhaps note if you do not know, that stray field induction in an unloaded output transformer is far worse than in a loaded (i.e. with circuit powered up) one, because of the damping effect of the loudpseaker and valves. This is especially true if you use UL or triodes. But you should be OK with a copper chassis. (Not to be alarming, but I recently had a whopper of a problem trying to "cure" power transformer induction on a steel chassis. Best position was at a 50 degree angle - I had to abandon in the end and go to alu.)

Keep us posted of your findings.
 
dhaen,

No, OK, point taken, although there are often questions regarding the orientation of the choke simply because it is seen as a possible source of magnetic field.

But interesting point: At first consideration I would imagine that especially in a CLC filter, the choke would be so "shunted" by the capacity across it as to be rather insensitive to external fields. Something to investigate next time I hook up such a circuit. Have you experienced something in the line of what you mentioned?
 
The filter is indeed CLC. 22uF BG VK to split channels - 30H - 150uF BG VK. To simplify initial construction, I'll probably install the chokes without the brackets at first. When I do make the brackets, I'll compare the measurements.

I had not thought of the chokes picking up the field from the mains tranny. But in this sense, as I get it, the 90deg angle between the chokes is of (much) less importance than the 90deg angle vs the mains tranny, given a CLC filter topology, as the magnetic interference between the chokes would be minimal?
 
"Also perhaps note if you do not know, that stray field induction in an unloaded output transformer is far worse than in a loaded (i.e. with circuit powered up) one, because of the damping effect of the loudpseaker and valves. "

I found this out the hard way. The amplifier shown on my home page had this funny hum that appeared on power up, and disappeared as the rectifier tube warmed up, and plate current began to flow. The hum was even present with the output tubes removed, and did not go away (no plate current).

When I brought the amp over to a friends house who has speakers that are 10 db more efficient, it became obvious that the hum didn't go away, it just got a lot quieter. Rotating the power transformer 90 degrees solved the problem.

http://www.tubelab.com/
 
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