Running power to tubes so as not to effect the audio signal - diyAudio
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Old 21st April 2004, 05:38 PM   #1
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Default Running power to tubes so as not to effect the audio signal

I plan to run the 6.3V filament power right from the bridge rectifier to each tube in a "daisy chain" manner. In an effort to keep this power line from affecting the audio signal I was going to use microphone cable (twisted pair with a mesh shield) for the power and ground only one side of the mesh shield. The grounding for the tubes will be one of the twisted pairs and the hot will be the other, this way the grounding for the tubes will exist at the power supply's star ground and not the signal's star ground.

Is this sufficient? Am I going overboard or am I missing something? Thank you for your help.

Milo
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Old 21st April 2004, 06:07 PM   #2
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Mike cable is often too thin to avoid drops at these currents. You'd do better to use properly-sized single conductor wire, with the two sides of the run tightly twisted together and the wire run close to the chassis.
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Old 21st April 2004, 08:23 PM   #3
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Oh duh, that makes sense. Is it beneficial to shield the runs though?

Milo
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Old 21st April 2004, 09:12 PM   #4
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If you take reasonable care to keep the high impedance leads away from the hummy stuff, there's probably no benefit. My power amp uses filament runs just like this and it's dead quiet.

Since you're powering the filaments with AC, this must not be a circuit which handles microvolts, which makes life easier.
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Old 22nd April 2004, 12:09 AM   #5
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Quote:
Since you're powering the filaments with AC, this must not be a circuit which handles microvolts, which makes life easier.
I think he is using DC as he wrote:
Quote:
I plan to run the 6.3V filament power right from the bridge rectifier to each tube in a "daisy chain" manner.
Whatever the case I have never had to use shielded cable for heater wiring be it AC or DC.

I use the method recommended in all classic books, that is for AC wiring use twisted wires and keep them away from signal wiring and put it close to the chassis. I use this method in my OTL and achieve hum level that is below 0,4mV Pk-Pk or <-100dB relative to full power.

For preamps I use DC stabilised voltage for the heaters and therefore dont need to twist the wiring also in this case I have no problem to keep hum low and insignificant. (I use stabilised voltage due to 2 reasons, one is to reduce hum as much as possible and secondly to keep the voltage stable to minimise tube drift as I am using DC coupled gain stages)

Note! tubes from some manufacturers are much more sensitive to heater induced hum then others so if you have a hum problem it can be beneficial to change tube to something from another manufacturer.

Regards Hans
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Old 22nd April 2004, 12:31 AM   #6
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Hi,

Quote:
I use the method recommended in all classic books, that is for AC wiring use twisted wires and keep them away from signal wiring and put it close to the chassis. I use this method in my OTL and achieve hum level that is below 0,4mV Pk-Pk or <-100dB relative to full power.
Not to brag but in my OTLs the filamentary wiring for the ten 6080s isn't even twisted, just solid core pushed against the chassis and laid out carefully away from the signal carrying wiring (grids).

Twisting these very short runs of thick wire (2.5A per tube) is just a waste of time.

The amps are still absolutely dead quiet even with 96dB + speakers.

Note however that generally speaking one should twist the heater wires to avoid hum.
In some situations I even twist them when they themselves carry DC to avoid break in influence from magnetic fields outside the wires .

Cheers,
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Old 22nd April 2004, 02:55 AM   #7
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Thanks for the help folks.

Milo
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Old 22nd April 2004, 03:07 AM   #8
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It all depends on the signal voltage at the grid of the valve. Frank's 6080s have quite healthy signals, so he doesn't even need to twist his heater wires. I once made a phono stage where (due to poor layout) I had to screen the twisted pair carrying the AC heaters. It wasn't long before I changed those heaters to regulated DC. Twisting is usually sufficient, and if you need more than that, it's an indication that something is wrong. Most people use DC heaters as a matter of course on phono stages. Anywhere else, twisted pair AC is fine.
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