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Old 11th December 2012, 12:13 PM   #61
DF96 is offline DF96  England
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Solid cores are better for heaters as the wires stay where you put them. Screened twisted pair cables almost certainly don't have a tight enough twist to deal with the magnetic field. Stick to the tried and tested method: solid core, tightly twisted, no need for a screen if done properly.
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Old 12th December 2012, 03:25 AM   #62
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Default Shielded cable

Thanks DF I thought the shield would suppress the field almost to zero if grounded but if that's how it's always been done who am I to break tradition
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Old 12th December 2012, 10:25 AM   #63
DF96 is offline DF96  England
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The shield will suppress the electric field but not stop the magnetic field. A heater winding CT or humdinger pot will also deal with the electric field. Tight twisting reduces both fields for anything which is further away than the twist length, hence a tight twist is needed.
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Old 13th December 2012, 02:02 PM   #64
DF96 is offline DF96  England
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Miswiring heaters is unlikely to blow 450V caps.

12AX7 and some other twin triodes (but not all) have heaters which involve pins 4, 5 and 9.
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Old 16th December 2012, 06:55 PM   #65
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by DF96 View Post
Miswiring heaters is unlikely to blow 450V caps.
They might have been connected to AC?
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Old 16th December 2012, 07:47 PM   #66
DF96 is offline DF96  England
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I suppose anything is possible! Would 6.3V AC blow a 450V electrolytic?

Remote debugging when we don't know what he did, and probably he doesn't know what he did, is not easy.
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Old 17th December 2012, 12:52 AM   #67
12E1 is offline 12E1  United Kingdom
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Quote:
Originally Posted by DF96 View Post
I suppose anything is possible! Would 6.3V AC blow a 450V electrolytic?

Remote debugging when we don't know what he did, and probably he doesn't know what he did, is not easy.
I don't know the specifics of the amp in question, but if mis-wiring the 6.3V AC also took out the rectifier for the HT, either by a direct short or through some other component breakdown, then the caps might see high voltage AC... for a short while...

Low voltage AC could do the damage but I'd expect it to take longer.
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Old 17th December 2012, 10:09 AM   #68
DF96 is offline DF96  England
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Quote:
Originally Posted by gkenne
connecting pins 4 & 5 of the EL84 to pins 4 & 5 of the 12AX7s,
That won't work. Most B9A valves have the heater on pins 4 and 5, but some double triodes have a different arrangement involving pin 9 too. For these, such as 12AX7, you connect 4 and 5 together, then wire the heater supply to 4/5 and 9. Or you can use 12.6V on 4 and 5, ignoring pin 9.
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Old 4th January 2013, 02:49 AM   #69
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I've found that on filament windings with no center tap bypassing both sides of the filament line with caps works better to eliminate hum than a balance pot. I used .47uf @ 100V ceramic caps.
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Old 4th January 2013, 09:30 AM   #70
DF96 is offline DF96  England
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Could be a sign of spurious RF oscillation getting propagated along the heater wiring, rather than normal hum.
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