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heater wiring - the Good the Bad and the Ugly

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28th April 2012, 10:04 PM

#1

nazaroo

Sin Bin

Join Date: Feb 2012

Location: http://goldentubes.blogspot.ca/

heater wiring - the Good the Bad and the Ugly

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:

-----------------------------------------------------
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.

(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.

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:

Quote:
"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)."

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.

nazaroo

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28th April 2012, 10:14 PM

#2

nazaroo

Sin Bin

Join Date: Feb 2012

Location: http://goldentubes.blogspot.ca/

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

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:

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...

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

nazaroo

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28th April 2012, 10:42 PM

#3

nazaroo

Sin Bin

Join Date: Feb 2012

Location: http://goldentubes.blogspot.ca/

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

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

nazaroo

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28th April 2012, 10:46 PM

#4

nazaroo

Sin Bin

Join Date: Feb 2012

Location: http://goldentubes.blogspot.ca/

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.

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28th April 2012, 10:50 PM

#5

nazaroo

Sin Bin

Join Date: Feb 2012

Location: http://goldentubes.blogspot.ca/

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.

nazaroo

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28th April 2012, 11:07 PM

#6

nazaroo

Sin Bin

Join Date: Feb 2012

Location: http://goldentubes.blogspot.ca/

The Electrical Layout can be as important as
the physical one:

Here is a typical hookup:

Wire
is rated by current-carrying capability. A lower gauge wire means a
thicker one, and it is able to handle higher current.

The power
tubes are placed first in the chain. This is not as most suppose, that the
signal-tubes with their lower current needs are protected from the large
current draw. That is not relevant. The average current and voltage will
not fluctuate significantly, regardless of the order in the
line.

What is being controlled here is current through the feed
lines.

The more current in a wire, the more hum it can induce
in other components. The high-current lines must be kept as short as
possible, while the lower-current lines (for the signal tubes) can
be longer and live more amiably with other components, since they produce
less hum.

This usually means that the power-tubes are located
closest to the power transformer, which in turn is kept away from the
input section generally, along with its current-carrying lines.

nazaroo

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28th April 2012, 11:14 PM

#7

nazaroo

Sin Bin

Join Date: Feb 2012

Location: http://goldentubes.blogspot.ca/

Final note on heater lines:

The
insulation restraints can generally be lower on heater-lines, because of
the low voltages (5, 6.3, 12.6). However, some runs may be held at some
D.C. (direct current) voltage above ground, to prevent leakage of current
to/from the cathode.

Thus a whole heater line might be floated at
+100 volts or higher, to prevent both leakage and arcing on large signal
swings.

Also, an amp might have two different isolated
heater-lines, floating at different voltages for different tubes in
different parts of a circuit (such as a SSRP or Mu-follower).

Thus
one should never assume that a heater line is "only 6 or 12 volts".

It might be carrying 200 or even 500 volts D.C., and could give a
lethal shock!

Thus 600volt insulation should be used for
heater-lines if needed.
Other than that, the thinnest possible
insulation should be used,
so that twisting can be tight as
possible.

Nor should they be directly grounded to the chassis,
unless it is known that they can rest at 0-volts D.C. without degradation
of the sound quality or damage to tubes and/or power supplies.

If
one suspects that hum on a line is generated by grounding problems from
the heater-circuit, one should first try grounding it through a High
Voltage small value Capacitor such as .1 uF 1Kv rating.

nazaroo

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28th April 2012, 11:35 PM

#8

Alvis

Banned

Join Date: Apr 2012

Quote:

Originally Posted by nazaroo

"The insulation restraints can
generally be lower on heater-lines, because of the low voltages (5,
6.3, 12.6)."

"Thus one should never assume that a heater line
is "only 6 or 12 volts".

"It might be carrying 200 or even
500 volts D.C., and could give a lethal shock!"

"Thus 600volt
insulation should be used for heater-lines if
needed."

Well done. I learned a
little bit with this refresher course.

I should add that high
voltage insulation NEEDS to be used regardless of the voltage of the
heater. (5v,6.3v etc.)

You need to stop the voltage from getting
into the wire, as well as out.
Often heater wires may contact with
bare high voltage parts. (Regardless of neat layout or a bowl of
spaghetti).

I look forward to more of your educational ramblings.

Alvis

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29th April 2012, 12:49 AM

#9

nazaroo

Sin Bin

Join Date: Feb 2012

Location: http://goldentubes.blogspot.ca/

The infamous RCA / Taylor
Filament theory FAIL!

In the early days, when hum was
not so pressing an issue,
RCA recommended the following layout,
to
alleviate current and voltage imbalances when many power tubes were hooked
in series.

The idea was to run two hypothetical 'rails',
connecting power to opposing ends of each.

The
voltage drop caused by current imbalances in the various sections of
hookup wire was balanced by the voltage drop at the other end on the other
'rail' (wire), ensuring that all tubes got the same average voltage, even
if they floated up and down a little in their absolute voltage position
(relative to each other) during each cycle.

Presumably the runs
between each tube would be twisted as usual.

This however resulted
in NO HUM Cancellation whatever!

Lets see why:

Even
though the wires are twisted together, they cannot cancel,
because the
current is flowing in the same direction in both wires,
at any one
time.
Instead, doubling up the current just doubles the
hum!

The only way this circuit could work as intended,
would be
to switch to constant current D.C. voltages:
Now since there IS no hum,
no cancellation is needed.
But of course no twist is needed either,

and one might as well run two physical rails.

nazaroo

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29th April 2012, 07:02 AM

#10

nazaroo

Sin Bin

Join Date: Feb 2012

Location: http://goldentubes.blogspot.ca/

oops! The following line from the last post
should read:

"...to alleviate current and voltage
imbalances when many power tubes were hooked in
parallel."

Wire twisting will also work when tubes
are connected in series, however,
care must always be taken to
ensure that current not only flows in opposite directions,
but also
that it is balanced in strength.
This is actually easier with
series circuits, since the current is the same in every part of the
circuit.

All this effort will only eliminate some of the
heater-hum:

Other leaks will occur inside the tubes in the form of
leakage currents between heater and cathode, unless steps are taken to
float the D.C. offset voltage appropriately in relation to the D.C. on the
cathode.

One way to minimize hum from heater-leakage is to use a
12.6v center-tapped winding for heaters and connect the center-tap to
ground. This in theory balances the hum signal.

In the
example below, we can see this technique:

It
may be more difficult to do this with 6.3 windings that don't have a
center-tap. In that case, you can make your own Center-Tap (c.t.) by using
a high resistance resistor divider:

Typically, a power-amp heater
circuit draws over 2 amps (.9 to 1.6 amps per tube). Ohm's Law says the
heater circuit must be less than 3 ohms! We can add a divider with 1k ohm
resistors (or higher) with no effect on the load at all, and connect the
center to ground.

The idea here is to use as low a resistor as
practical to lower random resistor-noise too.

Particularly with
large Directly Heated Cathode tubes (where cathode and heater are
the same object), the idea is to 'balance' the hum coming from each end of
the large heater/cathode (the resistance across the heater causes the
voltage to differ at each end during maximum voltage swing of the heater
transformer).

Below, an adjustment has been built into the circuit
to minimize hum. The extra resistors ensure an approximate balance is
maintained even if the pot wiper opens its circuit,
and the caps take
out any transient spikes and HF noise as well.

The circuit is
grounded through a resistor to prevent any current flow and/or
interference with other grounded circuits.

This
arrangement would work equally well with a circuit that
is floated at
some D.C. value by connection to a voltage rather than ground, provided
that this high-current source
cannot interact with the D.C. source in
an undesirable manner.

Some designers refuse to convert A.C. to
D.C. for these tubes,
because they fear uneven wear on the cathodes
from chronic voltage differences
at each end of the heater inside.

But that worry can simply be alleviated with a polarity
switch,
which is periodically toggled to even up the strain on the
tubes.
Why have hum when you don't have to?

Last edited by nazaroo; 29th April 2012 at 07:11
AM.

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