Filament "polarity"?

[dai h.]

I have read somewhere (I cannot locate the reference at the moment) that heater wires should be up and away from the chassis and other wires (as you mention orthogonal to signal and other wires where possible) the goal being to reduce coupling AC to the chassis and thereby keep the amp quieter.

I pose this as a point of discussion only as I am interested. I have no opinion one way or the other and cannot detect a difference in the two recent chassis I have wired. It makes sense to me that it at least could make some difference.

I ultimately settled on wiring my 5E3 and 5F2a heaters with wires twisted together above everything else away from chasssis or other wires. My Champ project used the flat against the chassis approach and the wires are not twisted. There is no serious hum in any of them but the Champ probably has a tad more than either the 5E3 or nearly identical 5F2a.


Barry

keep in mind that when (as in Fenders) the heater wiring is done up and away, the heaters are still against the "chassis", that is the "chassis" in the form of the aluminum sheet on the inside of the cabinet cover.

(as you mention orthogonal to signal and other wires where possible) the goal being to reduce coupling AC to the chassis and thereby keep the amp quieter.

as for this point I think that is more about the relative angles/positions of the heater wires relative to the other ones. I think a good example of a clear rationalized approach (where you can see this in practice) is the Soldano SLO100 where the heaters are not twisted but paralleled strictly and are at 90 degrees relative to the other wires on the socket (which also seems to illustrate how twisting isn't required). The Reeves amps also but slightly different. re: AC coupling to the chassis, wouldn't that be favorable (as above) in reducing noise by a bit of capacitive coupling to reduce the buzz (so therefore they should be against the chassis)?

[DirtyGeorge]

IIRC there was a good thread on this on the old ampage (so prob'ly in the archives).

what comes to mind as re: heater hum, AFAIK, is that

a) tube heaters can be constructed differently, so there might be differences in hum from that resulting in hum differences between tubes

b) generally heater wiring should be routed at a right angle to the other wires (relative orientation of the fields as with orientation of transformers mattering and affecting hum) and down against the chassis (slight bit of shielding due to capacitive coupling--maybe a bit less buzz = higher harmonics of the 50/60Hz hum)

c) if there is one, the heater center tap should be in the right place in the grounding scheme (wired with the high current stuff since high return currents through ground ( = return path for the AC current), for ex. 12AX7 = 0.3A, EL34 =1.5A, etc.) for lowest hum (also assume the same w/fake fil.cnt. tap)

Why is c) necessary? It should not matter where you ground the center tap (assuming you are grounding and not connecting to a +Vdc bias). The center tap should have negligible current since all the filament current flows between the legs of the winding and none through the ground connection.

[dai h.]

it does matter where you put it, IME and the amount of currents (making the difference) was my rationale (I could be wrong). If it doesn't matter, you should be able to connect it to, say the input jack ground, but I think that would give you a major buzz (which means it does matter where it's placed). The fundamental reason is current flow in grounds.

[DirtyGeorge]

it does matter where you put it, IME and the amount of currents (making the difference) was my rationale (I could be wrong). If it doesn't matter, you should be able to connect it to, say the input jack ground, but I think that would give you a major buzz (which means it does matter where it's placed). The fundamental reason is current flow in grounds.

But the filament current flow does not pass through the center tap to ground. There should not be any appreciable current flow in the center tap ground.

DG

[dai h.]

wouldn't it be the same as a center tapped B+ winding (where it does matter where you put the center tap)?

[DirtyGeorge]

wouldn't it be the same as a center tapped B+ winding (where it does matter where you put the center tap)?

No, in the center tapped PT the current flows alternately through the center tap and each leg, steered by the diodes. In the center tapped AC filament circuit there are no diodes blocking current between the legs so no current flows through the center tap (it just flows from one leg to the other). Grounding the center tap only references the filament winding to ground such that one leg is +3v peak and the other is -3v peak. The polarity switching every half cycle.

DG

[dai h.]

so if my notion that it was high current returns making a difference is incorrect, what would explain a difference in getting less buzz when placing the heater center tap with the high current returns (B+ center tap, spk. jack return) and getting more when putting it towards the preamp? And why is the heater center tap a fat wire in my Marshall PTs? If there was no significant current flowing, couldn't they just use say 22AWG just for the fil. cnt. tap as with the B+ winding wires?

[DirtyGeorge]

dai h.
I can't explain why there would be a difference with grounding closer to the preamp. I always tie my filament center tap (usually virtual with two 100 ohm res) to a voltage divider for a DC offset. I have not experimented with grounding it.

As for the filament winding center tap being heavy gauge wire, I suppose the x-former manufacturers can't assume the user intention. You could use it for a full wave rectifier circuit and then there would be current flow through the center tap.

I guess one of us is going to have to measure the center tap and see if there is current flowing.

DG

[drewl]

I didn't measure, but when fixing an amp with a loose center tap, I remember a decent "snap" when I grounded it manually while it was on...seemed like a fair amount of current to me!

I know I've seen a few amps with dc preamp heaters that had the polarity's mixed....VHT comes to mind.

[gdgross]

Assuming a PP amp, I always wire my output tube heaters out of phase.

The thought being that any induced hum will be canceled in the OT, much like power supply ripple. I have no idea if it makes any difference in reality, but it's one of those things that is good practice and lets me sleep better at night.

[Enzo]

There are small currents from electrons flowing off the filament and to the cathode within the tube. Those 60Hz currents WILL share the ground path. That is the reason elevating the heaters to DC higher than the cathode reduces hum it stops that current.

Gerald's contention is this...

12AX7 have a centre tapped filament and are usually wired 6.3v humbucking. The statement was made in reference to 6EU7/12AX7 compatability, as 6EU7 cannot be wired humbucking.

Yes, without belaboring it, he says that, but the only way that would actually cancel hum would be if the two triodes are at the same signal level, in phase, and have the same gain structure after them until the signal paths are mixed.

Since the two heaters in a 12AX7 are joined at one end, and the two free ends are bonded to make this humbucker, can we really assume the two heaters in a 6EU7 are not configured exactly the same within the tube, other than the pins used? In the 12AX7 wired 6v, the heater current runs from pin 9 through the two filaments and out pins 4/5, and then reverses since it is AC. In the 6EU7, heater current enters pin 1, flows through the two filaments, then exits pin 2, and then reverse. Only difference is you can't split them into 12v on the 6EU7.

[dai h.]

There are small currents from electrons flowing off the filament and to the cathode within the tube. Those 60Hz currents WILL share the ground path. That is the reason elevating the heaters to DC higher than the cathode reduces hum it stops that current.

is that so called "heater cathode leakage"? So if the heater center tap needs to be connected to where the cathode is connected maybe that explains why the buzz was lower (and thus the correct place to put it)? (A return path for this small current loop.) Would that make sense as a correct explanation?

[dai h.]

Why is c) necessary? It should not matter where you ground the center tap (assuming you are grounding and not connecting to a +Vdc bias). The center tap should have negligible current since all the filament current flows between the legs of the winding and none through the ground connection.

okay I think I "see" this (mentally) better. So say you have no fil.cnt. tap, and a fake one w/two 100ohms, if the heater current for the tubes was going directly through the 100 ohm Rs, they would burn out. When operating normally, they do not, so they can't be a direct path for the heavy current. Say when you wanted to drop the voltage, and used an R to do it, you'd want it directly in the current path, and that would be in series with the outside htr. windings and not the cnt. tap.