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Old 5th February 2011, 01:06 PM   #1
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Default Heater wiring - Coaxial?

Hi there,

possibly stupid question that came to my mind last night:

Has anyone ever tried coaxial cable for tube heater wiring? With the outward and return current flowing coaxially, electromagnetic interference should cancel out perfectly.

Of course, the relatively small conductor thickness would require wiring each valve separately instead of daisy-chain.

Stupid idea?

Greetings,
Andreas
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Old 5th February 2011, 02:48 PM   #2
jjman is offline jjman  United States
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I'd be concerned with the current capacity of the outer conductor which is usually expected to be merely a shield. Is there ever a max-current spec for that on coax?
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Old 5th February 2011, 02:52 PM   #3
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I don't think there are max specs for the current given anywhere. One probably has to estimate from the cross section of the thin wires the shield consists of. For standard RG58, the outer conductor becomes a quite thick bundle if straightened and twisted together - at least 1-1.5mm≤ I would guess...

Greetings,
Andreas
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Old 5th February 2011, 04:03 PM   #4
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What if you used on cable for each side (assuming heater=6.3v) and grounded the shield?
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Old 5th February 2011, 04:11 PM   #5
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From an "inductive" point of view, this would mean large current loops, as forward and return currents would be spatially separated. This is just what the classical "twisted pair" approach tries to avoid...
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Old 5th February 2011, 05:25 PM   #6
el156 is offline el156  Portugal
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I had an old Telefunken Amp that used shielded wire for the heater supply in the microphone pre-amp. The heaters had AC supply ,and there was no hum....
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Old 5th February 2011, 05:33 PM   #7
Doz is offline Doz  United Kingdom
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A co-ax is only effective for un-balanced signals, so I suppose it would be good if one side of your heater referenced that way. The idea is (At RF anyway) that current doesn't flow in the screen.

My Mic amp has 70bB gain, AC heaters (referanced at 100 volts) and is silent. It just has a tight twisted pair.
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Old 5th February 2011, 05:58 PM   #8
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You would be better off using a shielded twisted pair. The twisted pair would cancel out the magnetic field (which a coax will not) and the screen connected to the chassis will provide electrostatic screening.

Cheers

Ian
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Old 5th February 2011, 06:42 PM   #9
Doz is offline Doz  United Kingdom
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Quote:
Originally Posted by ruffrecords View Post
You would be better off using a shielded twisted pair. The twisted pair would cancel out the magnetic field (which a coax will not) and the screen connected to the chassis will provide electrostatic screening.

Cheers

Ian
... just like a shielded mic lead does. Balanced , with electrostatic screen
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Old 5th February 2011, 07:27 PM   #10
jrenkin is offline jrenkin  United States
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Seems like overkill. I hear again and again that folks here at DIY have twisted pair heater wire and no hum. I suspect good twisting and well planned and placed wires is more than adequate and if you have hum, you either have a problem elsewhere, or you didn't twist the wire well enough.
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