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Old 19th November 2012, 11:53 AM   #71
pancaQe is offline pancaQe  United States
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Quote:
Originally Posted by Jaap View Post
Just use the 6SN7 where you had 12au7 in your original RCA-design. Take other component values, for instance from post 56. If it works you can try to use a (v)ccs on top of the first tube and a led for the cathode resistor. You can find a lot of info on this forum.
I like that idea. Sorta build of an old design and experiment with it.
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Old 19th November 2012, 12:19 PM   #72
Jaap is offline Jaap  Netherlands
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That is what I think. You could also make the 6V6 outputstage differential by putting a LM317 under it.
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Old 25th November 2012, 08:20 AM   #73
pancaQe is offline pancaQe  United States
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When wiring AC heaters or any of the high voltage lines in the chassis, it's recommended to twist those wires. I usually twist both legs of the heaters together, but since there is only one B+ conductor, do I run two and twist them together? Or twist the B1 with the B2.
thanks
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Old 25th November 2012, 02:06 PM   #74
DF96 is offline DF96  England
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How much AC current does your B+ line have?
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Old 25th November 2012, 03:08 PM   #75
TheGimp is offline TheGimp  United States
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With heater AC wires twisting nulifies radiated fields.

The only reason I can see to twist B+ would be to prevent susceptance (EMI/RFI pickup). In this case one would twist with a ground return wire.

If there is that much RFI in the area, you need to address that instead of wiring.
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Old 25th November 2012, 03:43 PM   #76
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I have never noticed that +Ub wires need to be twisted and they should not, since there is "pure" DC, which do not radiate. Also the impedance at this line is low.
Basically it is AC-grounded.
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Old 25th November 2012, 05:52 PM   #77
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The reason I asked is that I'm troubleshooting a buzz that only occurs at about half volume. As I turn the volume up or down the buzz fades in about half way and then fades out . The chassis is a little on the small side and things are cramped so I'm not surprised, I'm actually surprised it's not worse. But I've been doing a lot of reading on proper wiring practices to prevent hum and other undesirable noise and read about twisting the HV DC lines
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Old 25th November 2012, 07:54 PM   #78
DF96 is offline DF96  England
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A buzz which disappears at max and min volume could be due to capacitive coupling to the volume slider and associated circuitry.

Twisting wires works when you have two wires carrying equal and opposite AC currents. Their magnetic fields then approximately cancel. It could also work, but perhaps not quite so well, if two wires carry equal but opposite AC voltages - then you can get some cancellation of the electric field (but screening is usually better).
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Old 26th November 2012, 03:37 PM   #79
pancaQe is offline pancaQe  United States
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thanks DF96, I took a closer look and realized I had the wrong value coupling cap. Decimal points are so small.
One final issue with this build. I was changing the cathode to ground resistors on the 6V6's for biasing and noticed that a slight hum and mechanical vibration coming from my power trany. If I got the biasing too hot it was worse, too cold and it was less so it's related to current load. I don't know the mA rating of the transfomer, they're harvested from old equipment I had laying around. Unloaded it's secondary HV is 715VAC. Could my trany be toast or toasting, or just underrated. Is there any way to determine the amps rating for an unknown transformer?
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Old 26th November 2012, 03:44 PM   #80
pancaQe is offline pancaQe  United States
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Quote:
Originally Posted by DF96 View Post
Twisting wires works when you have two wires carrying equal and opposite AC currents. Their magnetic fields then approximately cancel. It could also work, but perhaps not quite so well, if two wires carry equal but opposite AC voltages - then you can get some cancellation of the electric field (but screening is usually better).
what do you mean by screening?
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