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Wednesday, June 13, 2012

Heater Wiring: The Good the Bad and the Ugly (1)

looking over photos of DIY projects on the net, and hearing the many stories about hum, makes me want to offer some observations that might help amateur builders.

Understanding the problem of tube heaters and hum is the first step toward ensuring your wiring job will be good enough to give happy results (no hum).

(1) The heater supply is usually A.C. (alternating current), a 50 or 60 cycle voltage taken from a winding off of the power-supply transformer. As such, it is usually more or less a sine-curve (but often with additional components, harmonics), but can actually appear as gross as a sawtooth depending upon the quality of your line voltage.

(2) For hi-fi and musical applications, the heater lines unfortunately provide a way in for noise from the mains (random hash, transients), and also radio frequency noise (RF), because the wires can also act as antennae, picking up noise from nearby motors and other equipment.

(3) So even if you don't convert your Alternating Current (A.C.) power to Direct Current (D.C.), you will still want to have some filtering and protection from line and radio noise.

(4) Also, if you don't convert to D.C., the 60 cycle A.C. hum will leak into other nearby circuits through the electromagnetic field coming off the heater wires, unless you take some precautions.

(5) As well, noise from other parts of the amp (the High Voltage HV supply, the screen supply and bias supplies) can also pick up and inject noise back into the heater lines.

(6) Many people don't bother to convert A.C. to D.C. for the heaters, and so other methods must be applied to minimize hum and noise. This is sometimes a cheaper solution.

In any case, hum and noise remains a problem with amplifier circuits, so basic strategies should be used.

Regardless of whether or not you convert A.C. to D.C. (removing most of the hum), the Mains coming into the amp should be filtered and/or isolated from line noise. This can be done with a basic capacitor network on the lines.
It can be built into the line plug:

Click the image to open in full size.
Next, comes the layout of the heater wiring itself.

(1) The A.C. supply wires are twisted together, to collapse and minimize the field around each. At the same time as current flows in one direction on one line, it is returning in the opposite direction on the other line, and the two fields interact and cancel each other out.

Click the image to open in full size.

(2) But there is ANOTHER reason
why the wires are twisted together, which is often forgotten or neglected: It also stiffens the delivery system and minimizes movement of the wires. What movement? Any motion in a wire carrying current, or in a magnetic field generates a signal, or creates a coupling field. The motion can be very small, almost invisible, as in a guitar or violin string. It will still be very significant here.

Click the image to open in full size.

The practice of twisting wires is also done to greatly stiffen them, minimizing motion and coupling fields.

(3) This is also the reason why SOLID wire (such as 18 to 14 gauge) should be used for heater leads. In fact, any and all electrical activity has an impact on all metal in the area, and in case you haven't guessed it, will also cause physical forces of motion on the wires. Here we DON'T want flexibility, but rather stiffness.

If you doubt this, or think it is insignificant,
try loosening the bracing bolts on your power transformer,
and listening to the horrible vibrating rattles as the E-I sections bounce around.

(4) Getting back to the theory behind twisting the wires, We want to recognise also that the number of twists is also strongly related to the DISTANCE to the next object that might pick up the signal, by the Inverse Square Law:

"The circuit is essentially a coreless electromagnet with only a single turn of wire. By twisting the wires, we produce many small electromagnets, instead of one large one. Furthermore, these many small magnets are pointing in alternating directions. If we were very close to one of these magnets, we could still detect it, but once we are a small distance away, the adjacent opposing magnets will cancel it out (figure 3)."

Click the image to open in full size.

Thus we want as many twists as possible (per inch), to minimize the proximity effect to other nearby objects.

On the other hand, if you twist the wires too tight, you can also short them or break them or weaken them to the point where they generate resistance and heat unevenly, so there is a limit to how much you can coil / twist the wires before you risk damaging them.

Now we are ready to understand why many seemingly good efforts at heater-wire dressing fail.

The hum is directly related to distance:

(1) The most sensitive components are the ones closest to the heater wire. That would be the area right around the socket! This is typically where the twisting-dressing technique is done the most poorly and sloppily!

(2) Wires at a right-angle to the heater-wire are the least sensitive to hum. Often wires and components are laid out badly in regard to this obvious issue: orientation.

(3) Other components may or may not be more or less sensitive to heater hum depending on orientation.
THERE ARE NO general rules for this. For instance, its a bit tricky to orient a coil (or a cap!) for minimum hum, because it has a 3-dimensional field! Some components will be sensitive in ANY orientation, and must be shielded.

 Almost Perfect: Good even twisting, right up to the sockets practically.

Click the image to open in full size.

Secondly, the wires have been laid right into the corners to attempt to exploit the maximized shielding provided by the chassis.

This however is a secondary strategy, and must be tempered by the overall length increase created by this layout, which significantly offsets the gains from shielding.

However, this method can increase the distance of the heater wiring from sensitive components, and may payoff there if care is taken.


(2) Almost right. However, there may be some confusion in how this method works, because of not color-coding the wires:

Click the image to open in full size.

Here the builder has wound all the way down to the connectors. This works because the direction of flow down one wire MAY be the opposite of the flow going up to the next tube. But see conditions on the question of electrical hookup to see how this may fail!


(3) Very Nice:
However, it must be remembered that its not about neatness, but strictly about control of heater-field interference. Doubling a twisted pair can help smooth the field, but it can also amplify field imbalances...

Click the image to open in full size.

Color-coding helps to keep track of the phase of the heater-lines and also the effectiveness of the twisting.

Surprising Failure: Here is a tricky example, which disguises itself as a success, but in fact is a failure in technique:

Click the image to open in full size.

What is wrong with this picture?

The color-coding actually helps us to nail the problem.
The twisting together of two black wires for instance in the first power tube socket is a fail. Its over two inches long.

The twisting however, does NOT cancel out the current flowing into the tube heater here, which enters on the upper right and exits on the lower left.

The twist ONLY cancels out the current flowing up to the terminal and back down to the next tube's heater.
The current flowing into this power tube is NOT cancelled out,
and will produce maximum hum at the most critical area,
closest to the heater harness, the tube-socket (assuming all other parts are laid out intelligently.)

Stripped of the additional currents and circuits,
we have here an unshielded 1-inch red lead, and a 2-inch black lead, sprawled all over the socket.

The builder has not properly understood the nature of the problem.
He has only taken care of 2 out of 3 items on his layout list:(1) Twisting the leads properly : PASS

(2) Tucking the twisted lines away: PASS

(3) Twisting/shielding the leads in the critical socket area: FAIL

A Second Look:
Here one ought to question the strategy of adding almost a foot of extra heater-dressing, just to run along the edge: Especially when this results in the heater running right under the input jacks!

A 4-inch run directly from socket to socket might have been better,
with components mounted an appropriate distance,
so to minimize the overall size and coverage of the heater-field.

Click the image to open in full size.

 Click the image to open in full size.

This guitar amp looks in better form than many a hi-fi layout.
Here the heater lines are isolated from chassis and all other components by distance, and the orientation of all signal lines
are at right angles to the heater.
Also, the heater is at the back of the chassis,
while the sensitive input circuits are at the front.
Given mass production assembly constraints, this is a pretty good layout.

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