Mains and Filament Wiring

All AC power wiring generates an external field that can induce audible hum into signal wiring. Filament wiring in the obvious problem, because it will unavoidably be close to sensitive signal wiring, but AC mains and the high voltage AC to rectifiers can also cause problems.

To save time on assembly and any subsequent maintenance, any fixing screw should be just long enough to use all of the thread in the nut and no longer, and this is particularly important when securing tube bases. Screws that are too extended on tube bases make heater wiring particularly difficult. Unfortunately, if you decouple your heaters to the chassis at the tube base with 10 nF capacitors, the necessary solder tags and star washers inevitably require a longer screw.

Electromagnetic Fields and Filament Wiring

The electromagnetic field is due to the current flowing in the power wires, which induces in any nearby signal wiring. This means that not only are tubes with 12.6V filaments cheaper, but they are better in terms of noise too. Halved filament current means 6dB less heater induced hum.

Filament wiring is usually taken from a winding on the mains transformer to the nearest tube, and looped through, from one tube to the next, until each tube has filament power. The input tube is the most sensitive stage, and this should be the last in the filament chain, in order that the wiring leading to this tube carries the least current.

To minimize the external electromagnetic field, the filament wire should be tightly twisted. This means that although any given twist induces a current of one polarity, the twists either side of it induce currents of opposite polarity, and so the fields tend to cancel. This twist should be maintained as close up to the pins of the tube as possible, and when one phase of the filament wire has to go to the opposite side of the base and return, as is the case when wiring past 12AX7 to the next tube, the wiring should go across the base and be twisted as it passes across. The worst way to wire filaments would take the incoming pair connecting to the two heaters pins from one side, then loop around the opposite side to form a circle of filament wiring around the tube base.

Filament wiring leading to tubes using B9A sockets such as EL84 is best twisted from 0.6 mm (conductor diameter) insulated solid core wire, which is rated at 1.5 A. Octal tubes generally require more filament current, so the larger tags on their sockets can accommodate thicker wire, which could not have been connected to the pins of B9A socket. When wiring to tubes other than rectifiers, it is useful to use a different colour for each phase such as black and blue. When wiring to a push-pull output stage, if the same colour goes to the same pin on each tube, then the hum induced within each tube will be the same phase, and will be cancelled in the output transformer. This assumes that both tubes were made by the same manufacturer under the same specifications.

Tube rectifiers such as GZ34 and GZ37 not only have a dedicated 5 V filament supply, but they also have the incoming high voltage AC, so the two should be clearly distinguished. We use a red twisted pair for B+, and a blue twisted pair for the filament.

Twisting wire is not difficult. Cut equal lengths of wire to be twisted, pair the wires together at one end and clamp them in a vise. Gently tension both wires equally at the far end, and grip them in the chuck of the drill. Hold the wires reasonably taut by pulling on the drill and start twisting. When the wire begins to accelerate you towards the vise it will have about ten twists per inch. Switch the drill off, and while maintaining tension by holding the wire with your fingers, undo the chuck. The wire will now try to untwist, and if allowed to do so suddenly, it will tie itself in knots. Gently release the tension in the wire and release from the vise. You now have perfectly twisted wire.

You will find that it is easier to achieve a perfect twist on longer lengths of wire than short ones, because it is easier to equalize the tension between the wires. Equal tension is important because if one wire is slack compared to the other, it tends to wrap itself around the tighter wire (which remains strait). For this reason, it is worth twisting 4 or 6 meters of wire at a time, but these longer lengths are more easily twisted with a power drill, whereas shorter lengths can be twisted with a hand drill.

Electromagnetic fields decay with the square of distance, so filament wiring runs should be as far away as possible from signal circuitry, and only come up to the tube at the last possible moment and in the most direct manner possible. Tube sockets should be oriented so that the pins receiving filament wiring are as close to the chassis wall as possible, and filament wire should never loop round a tube except for rectifier tubes, where hum is not an issue.

Electrostatic Fields and Filament Wiring

The electrostatic field is due to the voltage on the wiring. Filament wiring should be pushed firmly into the corners of the conductive chassis, since the electrostatic mirror at the corner tends to null some of the electrostatic field. Filament wiring must not run exposed from one tube to the next, but should return to the corner of the chassis to re-emerge at the next tube. These strictures mean that good filament wiring requires considerable time/cost, so modern commercial amplifiers sometimes skimp on the quality of their filament wiring.

AC filament wiring should be connected to the transformer in a balanced fashion. Unfortunately, filament wiring must have a DC path to B+ 0 volt in order to define the filament to cathode voltage, and this can be achieved in various ways. The worst way to define the DC path is simply to connect one side of a transformer winding to 0 Volt. This ensures that one phase of the wire induces no hum, while the other phase induces maximum hum.


The ideal way of defining the DC path is to use a transformer with a centre tap on the heater winding, but if this is not available, fixed or variable resistors can be used to derive a midpoint. Accurately matched resistors used to be rare, so a variable resistor known as humdinger control used to be fitted, and adjusted for minimum hum. Once a filament midpoint has been derived, and connected to B+ 0 Volt, each wire has equal voltage with opposite phase hum, adn electrostatic fields tend to cancel.

This hum cancellation technique is not perfect, and for ultimate reduction of filament induced hum, we should screen the filament wiring with braid or thin walled aluminum tubing (available from modeling shops), and/or use DC filament supplies. Even when using DC filament supplies, it is worth treating the filament wiring as if it were carrying AC, as this will ensure that the finished project has no filament induced hum. the output of a DC filament regulator has virtually no AC present, so the filament 0 Volt can be connected directly to the B+ 0 Volt.

Filament wiring is the first piece of wiring to go to a project, thereafter, it is obscured by signal wiring. Once all the other wiring is in place, it is impossible to replace the heater wiring, so it must be installed correctly. Because it is so difficult to make changes to filament wiring, it is a good idea to immediately do the mains wiring and test it all by plugging all the valves in and making sure that they glow.

Mains Wiring

Mains wiring should be as short and direct as possible because the wire's insulation is often quite thick, so it cannot be twisted well. Mains wiring inevitably generates considerable electrostatic interference fields, so put the mains switch near the back panel, and if necessary, add a mechanical linkage to bring its control to the front.

Modern semiconductor equipment sleeves all exposed mains wiring with rubber PVC sleeving such that it is moderately safe to rummage inside a piece of powered equipment. Valve amplifiers operate on such high voltages that it is never safe to rummage inside of powered equipment, and even unpowered equipment should be approached with caution. Safety is therefore not greatly improved by sleeving mains wiring, but it is still good practice to sleeve mains wiring with heatshrink sleeving or purpose-made rubber boots to fit over mains sockets and fuse holders. The mains connectors used on classic tube amplifiers are, without exception, outrageously dangerous.

Mains Switching

To switch a piece of equipment off, all we need to do is to break the circuit from the source power. A mains switch could therefore equally well be inserted in the live or neutral wire, and still perform the job, and this is known as single pole switching. However, a switch in the neutral leaves all internal mains wiring live and continues a shock hazard within the equipment. Single pole switching should therefore always switch off the live circuit to minimize shock hazard.

Double pole switching switches both the live and the neutral, and ensures safety even if the live and neutral wires are reversed. Where there is no possibility of live/neutral reversal, single pole switching is safer, and more reliable, because failure of the switch ensures a break in the live connection to circuitry. A double-pole switch has twise as many contacts to fail, and if the neutral contact fails, the equipment could appear to be safe, even though the mains wiring is connected to live mains, and still continues a shock hazard. 


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