On Vacuum Tube Heaters
In this Wurlitzer 700 amplifier, the heater wires are black and brown. In this photo, the black wire starts at pins 4 and 5 of the 12AX7, while the brown wire starts at pin 9. This is not an optimal way to wire heaters, because the circuit is not center-tapped and the wires are not twisted.
The heaters of your vacuum tubes are one of the most critical parts of your amplifier. A well-designed heater circuit is quiet and unobtrusive: truly an unsung hero of any quality amp. On the other hand, flaws in a heater circuit can produce noise.
What is a tube heater? All vacuum tubes follow the same basic principles: electrons are released at the cathode and attracted to the anode. This happens because of a phenomenon known as thermionic emission. Basically, as you heat a piece of metal, electrons become more likely to fly off it towards any positively-charged surface nearby. Different classes of tubes, like pentodes and triodes, have various layers of metal mesh inside to manage the flow of electrons, but every tube requires a hot cathode to get the flow started in the first place.[1]
There are two ways to heat the cathode: directly, or indirectly. Most tubes have indirectly heated cathodes. In this case, heater current is applied to a filament which is mounted near the cathode of the tube. The cathode becomes hot due to the radiated heat of the filament. The majority of tubes are indirectly heated.
On the flipside, in a tube with directly-heated cathodes, the filament and cathode are one and the same. In this case, heater supply voltage is connected directly to the cathode. The advantage is that less heat is required to release electrons from the cathode. However, applying A.C. heater voltage directly to the cathode will introduce hum into the circuit. Tubes with directly-heated cathodes are typically rectifier tubes, such as the 5Y3.
Heater circuit voltage requirements. Compared to the rest of an amplifier circuit, heaters require a small supply voltage and a substantial amount of current. To achieve this, the heaters are connected to their own winding on the power transformer. This supplies either 5v or 6.3v and has a current rating somewhere in the ballpark of 1A and 10A.[2]
In this image of the Wurlitzer 700 amplifier, you can see that the black heater wire is connected to ground. A center-tap would be a quieter alternative option.
Tube heaters are usually supplied with A.C. voltage. For this reason, the physical layout of the heater wires is very important to prevent A.C. currents from introducing noise into the rest of the circuit. Noise can also enter the cathode inside the tube by breaching the insulation between the cathode and filament.
It is possible to rectify the heater circuit and convert the A.C. supply to D.C. The downside is that the D.C. supply requires filtering, because otherwise the post-rectifier ripple current will also induce hum. It's also tricky to get the full required 6.3v from a standard transformer when using a rectifier, due to the rectifier diode’s voltage drop. A D.C. heater circuit also draws more current, which means that the amplifier will require a larger transformer to power the same number of tubes.
Heater requirements for common tubes.
This section is provided as an illustration. Always check the manufacturer datasheet for specific information on your particular tubes, because current requirements can vary between tubes of different manufacturers.
Preamp tubes. Most preamp tubes in the 12A-7 family have the same heater specs. However, it is still worth checking the datasheet before using them in a circuit. Note the 12BH7, which is very similar to a 12AU7 but has twice the current requirements.
12AX7: 6.3v, 300mA OR 12.6v, 150mA
12AU7: 6.3v, 300mA OR 12.6v, 150mA
12DW7: 6.3v, 300mA OR 12.6v, 150mA
12BH7: 6.3v, 600mA OR 12.6v, 300mA
The above tubes have an optional center-tapped heater circuit. Wire them in series for 12.6v, using pins 4 and 5. Or, wire them in parallel for 6.3v by connecting one wire to both pins 4 and 5 and the other to pin 9. Parallel wiring is more common because the lower voltage results in less hum.
Rectifier tubes. There are two classes of rectifier tubes: directly- and indirectly-heated. Directly-heated rectifiers warm up faster, but require a separate 5v transformer winding. The 5v heater supply is connected directly to the cathode and floats on the regular B+ voltage. Because a directly-heated rectifier heats up faster (and therefore starts conducting faster) than the other tubes in the circuit, it is particularly important that all components in the circuit can handle the full B+ voltage.
5U4: 5v, 3A
5Y3: 5v, 2A
5AR4: 5v, 1.9A
6CA4: 6.3v, 1A
The 6CA4 is an indirectly-heated rectifier alternatively known as the EZ81 (because it’s e-z to wire it straight to the 6.3v heater tap). The 5AR4 is also indirectly-heated, but it requires a 5v heater winding. Because the filament is internally connected to the cathode - and one of the filament pins therefore doubles as the cathode pin - it superficially resembles a directly-heated rectifier. However, like the 6CA4, it has the slower start-up time associated with indirectly heated tubes.
Power tubes. Power tubes typically connect to the same 6.3v heater winding as the amp’s preamp tubes. They have a wide range of current requirements - compare the 6V6 to the EL34, for instance - so it’s particularly worthwhile to double-check the datasheet when choosing a power tube.
• 6L6: 6.3v, 900 mA
• 6V6: 6.3v, 500 mA
• 7868: 6.3v, 800 mA
• EL34: 6.3v, 1.5A
On heater center taps. For the least hum, the heater circuit should have a ground reference. There are two ways to accomplish this.
First, you could ground one leg of the heaters. In this case, one leg would be at 6.3v, while the other would be at 0v. Vintage amp manufacturers used this strategy until the 1960s. It results in less noise than floating the heaters with no ground reference, but isn’t as quiet as the next option: installing a center tap.
In a center-tapped heater circuit, each string sees half of the 6.3v supply, because the center tap is grounded (or elevated, which is discussed later in the article). Many transformers have a center tap wire. If not, it is possible to add an artificial center tap by connecting each leg of the heaters to ground through a small resistor, around 100-200 ohms.
On elevated heaters. Sometimes, it is advantageous to connect the center tap to a ground reference that is greater than 0v. On all tubes, there is a maximum heater-to-cathode voltage that should not be exceeded. Otherwise, the insulation between heater and cathode will break down, introducing hum into the circuit and eventually causing the tube to fail.
Some people achieve elevated heaters by connecting the center tap to the cathode of the power tubes, which is usually around 30v. Alternatively, you can create a voltage divider in the power supply. This allows you to fine-tune the elevation voltage, because you can set the voltage divider to give you a specific number whereas the cathode voltage is dependent on the bias of the power tubes.
How to Wire a Heater Circuit
First, choose your wire.
Select the correct gauge for your application. Thicker wire can handle more current. For many heater circuits, 18 AWG wire is sufficient, but this of course depends on the particular current requirements of your amplifier.
At some point, somebody else had re-wired the heaters in this Wurlitzer 112 amplifier. (They’re the twisted green and black wires.) It’s not perfect because there are still large loops around the tube sockets. This wiring could be improved if the twist were to continue all the way up to the sockets.
Stranded or solid core? Stranded wire is more flexible, but solid core wire fits more easily into tiny preamp pins. Wiring heaters requires twisting them, shoving them into corners, and maneuvering them around the chassis, so when choosing a wire you should take the relative fragility of solid-core into account.
What kind of insulation? The most important difference between different wire insulation is its heat resistance. Certain types of wire will start to melt as soon as the soldering iron touches them. This is a problem because your heater wires will be tightly twisted together, so if the insulation melts they will certainly short. Best case scenario, this results in an amp that smokes when you turn it on. Also, melted plastic insulation also isn’t great for the air quality in your shop. Choose a wire with insulation that is highly heat-resistant, such as PTFE (aka Teflon).
What color should I select? Green for tone, obviously. But seriously, choose two different colors so you can easily differentiate between the two sides of the heater winding. If you only have one color and you are desperate to wire some heaters asap, use a sharpie to mark up one of the wires so you can tell the difference at a glance.
Determine the path your heater wire will take.
Heater wires should be positioned closely to the chassis and tucked into the corners where possible. This will minimize noise. However, the length of the heater string is also an issue. The longer it is, the more likely it will introduce noise into the circuit. If you have a choice between a short, direct path between two tube sockets, or a significantly longer path that routes the wires into a corner, choose the short path.
It is also important to properly position the wires at the tube socket itself. Keep the wires tightly twisted all the way up to the pins. Do not loop wire around the tube socket. On many tubes, the heaters occupy adjacent pins, so this is easy advice to follow. But in other cases, the tube heaters are on pins that are not adjacent. Here, you can often mount the wire across the socket, instead of looping it around. If you must loop the wire around the socket, continue the twist around the loop.
When planning, consider the layout of the circuit as a whole. Preamp components in particular should be mounted at a distance from the heater wires. Consider how you can rotate the tube socket to achieve this. Tube sockets should be mounted so that the heater pins are closest to your intended heater path, while preamp grids and other sensitive areas occupy the opposite side of the tube socket.
On twisting the wires.
Twisting the heater wires minimizes noise by reducing the radiated electromagnetic interference. This is because the noise in each side of the heater string is identical but out of phase with the other. When the two wires are in close proximity to each other, the noise voltages cancel.
Running the wires parallel to each other has a similar effect. However, twisting is often preferred because it ensures that the wires stay close together and don’t shift in position over time. For best results, the twists should be uniform across the length of the wire.
Tips for mounting heater wire.
Mount the heater wire first, before any other components. It’s a lot easier to wire the heaters when you have unobstructed access to the pins. In fact, we often wire the heaters even before we mount the tube To accomplish this, we first mount the sockets to a wooden board, spaced apart exactly as they are in the amplifier. Then, we draw in the perimeter of the chassis so we know exactly how long each piece of wire should be in order for it to be tucked into the corners. This gives us the ability to freely rotate the tube sockets, so we’re always working from the most convenient angle. When we’re done wiring the sockets, we unscrew them from the board and mount them into the amp chassis.
Do not solder the heaters in place until you have finished mounting all the wires. Until you’ve had a lot of practice, it’s easy to make a mistake when wiring heaters. You can cut a piece of wire too short, nick the insulation, or accidentally mount a wire to the wrong pin. It’s a lot easier to redo a length of wire if it hasn’t already been soldered into place.
The transformer winding should be the last part you connect. Using a multimeter, you can perform a continuity test to ensure that the two sides of the heater aren’t shorted - until the transformer is in circuit. At that point, the heater string will show continuity whether it is shorted or not. So, wire the tube sockets first, double-check that there is no continuity between opposite heater pins, and then attach the transformer.
Mount the heaters of preamp and push-pull power tubes in phase. In other words, each leg of the heaters should be connected to the same number pin on the power and preamp tubes. (This is where color-coded wires come in handy: same color on the same pin.) This helps minimize hum.
[1] There is such a thing called a cold cathode tube, but you won't find one in an instrument amplifier. Cold cathode devices include Nixie tubes, neon and other gas-discharge lights, and Geissler tubes.
[2] There are exceptions (such as widowmaker amplifiers) but this description covers most amplifier heater requirements.
Further Reading
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