heater wiring - the Good the Bad and the Ugly

[DF96]

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.

[Goodlagger]

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

[DF96]

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.

[DF96]

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.

[Sheldon]

Originally Posted by DF96

Miswiring heaters is unlikely to blow 450V caps.

They might have been connected to AC?

[DF96]

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.

[12E1]

Originally Posted by DF96

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.

[DF96]

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.

[wiredwithtubes]

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.

[DF96]

Could be a sign of spurious RF oscillation getting propagated along the heater wiring, rather than normal hum.