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Old 24th August 2009, 09:09 PM   #1
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Default Mounting filament transformers under chassis?

I am thinking about putting my filament transformers (4 in total per channel) under the top plate of the PS chassis, as these are open framed units, but I am concerned about stray fields under the chassis. Am I worrying about something that I shouldn't be, by placing them there? I have never mounted any transformers/chokes below decks like this before for fear of inducing hum in the wiring. Its not a matter of space on the top plate, just going on over everything one last time before I start drilling holes. What would you do in this case?
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Old 24th August 2009, 09:33 PM   #2
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Filament transformers do create strong fields around them. But these are magnetic (B) fields (rather than E fields), and unless the chassis is THICK steel, it will make little difference which side they're on.

Best plan is to pay attention to wiring: be sure that sensitive circuits (grid input-cathode return) are not in a large loop-area. keep these very short and closely parallel, twsit if more than an inch or so. keep input circuits furthest from the filament trafos. Avoid using the chassis for power/heat/signal current anywhere.

The orientation also matters. If at all possible, test the amp before drilling the fixing holes. try the trafos in different 90 degree orientations for lowest hum.
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Old 24th August 2009, 10:14 PM   #3
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I understand the 90 degree orientation of the transformers to each other, but I was concerned with putting any trannies/chokes under the chassis. I understand now that the magnetic fields will be there regardless now. Proper wiring layout will be a big hurdle.

I have attached a link to my proposed layout with all transformers on top of the power supply chassis. If you all would not mind reviewing this layout, I would appreciate it. I am about to breadboard the circuit as drawn to make sure that thr transformers will work in this orientation, but I have never embarked on a circuit this complex before, so I could use some sage advise.

http://themastins.com/PS_Chassis_layout.gif

T1: Main Power transformer
T2 and T3: Filament trhansformers for the two mercury vapor rectifier tubes (866A)
T4: Filament transformers for input/driver stage (on amplifier chassis)
T5: Voltage supply for bias circuitry (V6 (6V4), V7 (6U8A))
T6: Filament supply for bias circuitry V9 (6L6GC), V10 (12AX7))

The two squares in the lower right of the chassis are PS chokes.

Last edited by Curly Woods; 24th August 2009 at 10:24 PM.
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Old 24th August 2009, 10:29 PM   #4
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Depending on what function the 12AX7 has in the circuit, it may be upset by fields from the 5AR4 rectifier - the current through these peaks much higher than the dc, and has an unpleasant waveform. Placing the AX7 the far side of C6 would be an improvement - if it's a sensitve circuit anyway...
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Old 24th August 2009, 11:27 PM   #5
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Originally Posted by Rod Coleman View Post
Depending on what function the 12AX7 has in the circuit, it may be upset by fields from the 5AR4 rectifier - the current through these peaks much higher than the dc, and has an unpleasant waveform. Placing the AX7 the far side of C6 would be an improvement - if it's a sensitve circuit anyway...

Thanks Rod. It is used in my +440V bias supply circuit. It will not be a problem at all moving it farther away from the rectifier tube. Thank you for taking the time to review this for me. Here is a link to the actual bias supply circuit.

http://themastins.com/440V_bias_supply.gif
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Old 24th August 2009, 11:40 PM   #6
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Here is an updated version with the 12AX7 moved farther away from the rectifier.

http://themastins,com/PS_Chassis_layout2.gif
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Old 25th August 2009, 08:38 AM   #7
Alex M is offline Alex M  United Kingdom
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The 300B amp I am building will have two transformers under the hood, and Kevin Kennedy, the designer, says the amp is very quiet, as long as care is taken to separate signal and AC power wiring.

Here is Kevin's prototype: http://www.kta-hifi.net/projects/amp...mp/300bpp.html

Alex
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Old 25th August 2009, 08:41 AM   #8
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transfomers are traditionally put on top not only for the magnetic field but also for heat lost

the fields are also meant to affect the caps as well as tubes (as far as i understand they do)

i would put plate metal between the transformers and any thing else and make sure you have good heat lost.
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Old 25th August 2009, 08:56 AM   #9
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Originally Posted by Curly Woods View Post
Thanks Rod. It is used in my +440V bias supply circuit. It will not be a problem at all moving it farther away from the rectifier tube. Thank you for taking the time to review this for me. Here is a link to the actual bias supply circuit.

http://themastins.com/440V_bias_supply.gif
Must have been a long night. This circuit is for the regulation of the input and driver stages. The bias supplies are similar, but with negative voltages :-)
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Old 25th August 2009, 09:03 AM   #10
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Hey Michael,

what are you building? 845 SE?
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