Heater / Filament Supplies
Valve heaters generally require much more current than the rest of the amplifier.
Valve heaters can be run in parallel or series, or with a little juggling a combination of the two. The common dual-triode pre-amp valves such as the ECC81, ECC82, ECC83, 12AY7, E88CC, all have three heater connections, so the heater for each triode can be wired in parallel, or in series at half the current twice the voltage. This is most useful for AC supplies because reducing the current in the heater wires reduces the electro magnetic radiation emitted from them, so it will be less likely to be picked up by other parts of the amplifier.
Most of the common valve types used in guitar amps are designed to have their heaters run in parallel from a constant voltage source. That is to say, they all operate at the same voltage (usually 6.3V) but may have different current demands.
Power transformers designed specially for use with valves will usually have a secondary winding solely for the heater supply. It is important not to exceed the maximum current rating of the transformer. If the valves are all being run in parallel then it is a case of adding up the current drawn by all the valves and checking it does not exceed the transformer’s rating. If it does, a separate low voltage transformer could be added so that some valves are run from the power transformer and some (or all) from the separate heater transformer.
A power indicator lamp can also be run from the heater supply, provided there is enough current available.
Audio valves used today have indirectly heated cathodes so can be run from an AC or DC heater supply, although AC is much easier to implement. Some valves rectifiers (e.g., the GZ34) have the heater internally connected to the cathode (to ensure the heater-cathode potential cannot be large) and will need their own separate heater supply. If possible, even indirectly heated cathode rectifiers should be run from a separate supply in the same way [right]- this will ensure long life [see the sections on full-wave and bridge rectifiers for more].
Voltage considerations: The heater voltage specified in the data sheet is the optimum value (usually specified as +/- 10%). Running them at higher voltages will considerably reduce valve lifespan and must be avoided. Running them at lower voltages will increase their lifespan but reduce their emission, although the grid curves and general performance remain much the same;- only the saturation current is reduced. Running heaters under-voltage is therefore perfectly acceptable, and provided the voltage isn’t too low there will be no noticeable difference in sound. Normal heaters rated at 6.3V can be run quite happily between 5V and 6.9V, maybe even lower, but not higher. The exception to this rule is rectifier valves, which should not be run below 10% of their rated voltage since they usually operate very close to saturation.
It is not uncommon for the mains voltage to be slightly high, resulting in a heater voltage that is also proportionately high. If this is the case where you live then the following may be useful: A pair of ordinary high-current silicon diodes can be added to the heater chain to drop the heater voltage by about 0.7V. (If using a DC heater supply, only one diode will be needed.) This method can also be used for a heater standby switch.
Series and parallel: Valves which are normally designed to be run in parallel can be run in series provided you ensure their current demands are met correctly, althopugh series heater chains are not recommeded for audio. For example, an EL84 and ECC83 could be run in series from a 12V supply. The EL84 is rated at 0.76A while the ECC83 is rated at 0.3A, therefore a resistor must be placed in parallel with the ECC83 to pass the additional current without damaging the valve. We want to pass 0.76 – 0.3 = 0.43A through the resistor, and we want the voltage across the resistor to be 6V. Use Ohm’s law to calculate its value:
6 / 0.43 = 14 ohms.
The power dissipated will be:
(0.43 * 0.43) * 14 = 2.6W
So we would probably use a 15R, 5W resistor.
All sorts of heater chain combinations can be created in this way.
Series AC heater chains will not benifit from a grounded centre tap [see below], but will benifit from an elevated centre tap, to reduce noise.
Because the heaters are in series it doesn’t matter in which order the valves are wired. However, if one end of the heater chain is to be grounded, then the most sensitive preamp valves should be closest to the grounded end of the chain.
Since the different heaters will often have different warm up times that could put stress on the other valves, a thermistor can be placed in series with the chain, or a resistor switched in and out by a standby switch [see the section on power and standby switches]. In fact, a thermistor makes an excellent addition to any heater chain, to reduce the current surge on switch on that leads to filament failure. This will considerably extend valve life span.
Reducing hum in AC heater supplies: AC valve heaters cause hum because the filament radiates an electro-magnetic field that can induce a hum voltage in the grid / anode, often via the valve pins (some HiFi audio valves such as the 6N3P have special pin arrangements to keep the heater pins away from the anode and cathode pins). With the ECC83/ 12AX7 and others, this cause of hum can be reduced by operating the heater from a 12.6V supply, since this requires less current meaning less radiated field.
Hum is also caused because the filament and cathode are conductors, separated by a small insulator (vacuum) and a semiconductor (aluminium oxide coating on the filament), and this is exactly how you make a solid-state diode. Electrons can pass from heater to cathode when the heater voltage is negative with respect to cathode voltage. When the heater voltage is below the cathode voltage, the diode is forward biased and a stray current will flow from heater to cathode causing an ugly 50Hz hum voltage to appear on the cathode, which will be mixed with our signal and amplified. If we keep the heater voltage above the cathode voltage at all times, the diode is reversed biased (‚off‘) and almost no leakage current will flow meaning reduced hum.
Hum is also worsened by having a large cathode-heater resistance, which is the case for cathode followers and long tailed pairs. Luckily, these stages usually come after sevral gain stages, so the signal-noise ratio is good by that point. Further hum is caused by stray capacitance between filament and grid / anode, and a humdinger was a popualar method of reduing this cause of hum [see below], though this problem is probably the lesser of the three mentioned here.
Single ended stages are most prone to hum, whereas a (correctly wired [see below]) push-pull stage or differential pair will tend to cancel any common mode noise like heater hum.
Power valves tend to be less prone to hum since they deal with high signal voltages, so the signal-to-noise ratio is higher. In fact, in most amps, most of the audible heater hum comes from the input stage.
The following tricks can be used to reduce heater hum:
Transformer centre tap: The traditional way to reduce hum is to use a heater supply with a centre tap and connect it to ground. In this way the valve heater will be at a positive voltage along half its length, and at an equal but opposite voltage along the other half, at any one time. The average stray current between filament and cathode is therefore reduced by a little more than half, and the frequency of the ripple voltage produced in the cathode is also doubled, so can be shunted to ground more easily by the cathode bypass capacitor. This method is usually enough to bring hum to a satisfactorily low level. Additionally, in a perfect world the out-of-phase radiated fields will cancel, and no hum will be induced in the cathode by that method either. The only problem with this system is that it is impossible for the centre tap on the transformer to be precisely centred to AC and DC, so perfect cancellation will not occur and some low level hum may be heard.
Artificial centre tap: A better way that can also be used with non centre-tapped heater supplies is to create an artificial centre tap with resistors. The resistors should have a low resistance so the maximum heater-to-cathode resistance of the valves is not exceeded, and so the reference to ground is as close to zero as possible. Values of 100R (1/2W min) and 220R (1/4W min) are usual. They will of course cause a small amount of extra current draw from the transformer (32mA when using 100R resistors at 6.3V) so bare this in mind.
The advantage of this system is that close tolerance or matched resistors can be used to create a perfectly centred ground reference, and the extra resistance to ground will reduce the chance of an arc occurring within the power transformer in the event of a speaker being unplugged.
Another traditional method uses a potentiometer with the wiper grounded- a so-called „hum dinger“. This allows minimum hum to be dialled in precisely by creating a Wheatstone bridge with the heater-to-cathode capacitance within each valve.
DC elevation: DC elevation is often used when a valve in the circuit has a high cathode voltage. The heater voltage is elevated to a higher level to avoid exceeding the maximum heater-cathode voltage rating of the valve. This is done simply by ‚adding‘ a DC voltage to the heater supply. The heaters still operate at 6.3V (or whatever you’re using), but the AC component ‚floats‘ on top of a DC voltage.
This method is also used to reduce audible heater hum by raising the heater voltage above the cathode voltage and ’switching off‘ the stray current between filament and cathode. This works providing the DC reference voltage is sufficient to raise the negative AC peaks above the cathode voltage of the valves- particularly the pre-amp valves. Reference voltages used are typically between 8V and 150V.
A typical way to apply the DC reference is by connecting the centre tap (real or artificial) to the cathode of a power valve, providing the power valve is cathode biased of course. The bias voltage of most power valves is usually more than 5V, and this will be ‚added‘ to the heater voltage.
The other common method is to take the DC reference from a potential divider from the HT (useful if the amp has fixed biased power valves). Typical voltage references are around 20V to 90V, placing the heater supply well above the potential of most cathodes in the amp.
The potential divider should have a fairly high resistance so there is no significant current drawn from the HT (it can also serve as the bleeder path for the HT smoothing capacitors).
The lower resistor in the divider (R2) should not be excessively high or the maximum heater-to-cathode resistance may be exceeded. Many data sheets do not quote this so it is advisable not to make it greater than 100k. A fairly large value capacitor (C1) can also be added to ensure a smooth DC reference and to prevent the 50Hz heater hum reaching the HT supply. It’s actual value is not critical, anything over 10uF should be fine.
DC heater supplies: Properly designed DC supplies do not cause hum since the stray current between filament and cathode is unchanging. However, DC supplies nearly always need to be voltage regulated or they can cause even more noise than an AC supply! (although simple rectification to DC does sometimes work).
Simple, three-pin voltage regulators usually require an input voltage that is a couple of volts above to output voltage in order to work. Most regulators cannot handle more than about 1A of current on their own, so it is quite common to operate the comparatively low current pre-amp valves from a simple regulator, and run the current-hungry power valves from an ordinary AC supply, since they are less prone to hum anyway. The regulator must always be fixed to a suitable heat sink.
The LM317 range is a popular choice of variable voltage regulators (up to 20V, 1.5A).
An even simpler option is a fixed voltage regulator that reqires no additional components to set the output voltage. A 5V regulator can have its output raised to 6.3V by elevating its ground terminal by 1.3V; the voltage drop across a pair of silicon diodes is almost perfect for this. Higher voltages could be obtained by using zeners instead, but the input voltage must always be at least 2.5V higher than the output.
In the circuit below, the 7805 regulator can provide up to 1A on its own. If more current is required, the transistor can be added to provide up to 5A max (it too will need a heat sink). The 1uF capacitor does not have to be tantalum, but it will improve performance. A normal 6.3V transformer winding will NOT provide sufficient voltage after rectification to power this circuit.
If you only have a 6.3V winding on your transformer, you can rectify this and obtain just enough voltage to run a 5V fixed regulator without elevation [below], and just run the valve heaters off 5Vdc, which is perfectly acceptable and will prolong valve life. This makes a super-simple regulator, and again the transistor can be added for up to 5A supply. The only problem with this method is that if the mains voltage falls, the secondary voltage (which will also fall) may not be sufficient to keep the regulator on.
Layout / lead dress: The lead dress of AC heater supplies is very important for noise reduction. The AC heater wires will have significant EM radiation and should therefore be routed well away from all signal wires, and are usually tucked into the corner of the chassis. The wires should either be made from twin cable (bell-wire) or better still, should be made by twisting the wires neatly and tightly together. In this way the wires are kept perfectly parallel and close to each other, which increases opposing field density and encourages the radiated fields to cancel out. Loosely twisted wires are no use at all.
When heaters are wired in parallel; power valves should be first in the heater chain, followed by driver valves, with the input stage being last in the chain. This keeps current, and therefore radiated fields, at a minimum around the most sensitive stages of the amp. Even better is to run the pre-amp and power-amp sections from separate heater chains. If signal wires must cross the heater wires, they should do so at right angles.
Valves in push-pull or in balanced stages (such as long tailed pairs using separate valves) should have their heaters wired in phase. Any noise induced will then be common mode and rejected by the stage (mostly). Valves in parallel single-ended stages should have their heaters wires out of phase for mutual cancellation. Using two different colours for the heater wires will make this easier.
The common pre-amp valves (ECC83 / 12AX7 etc.) when run from a 6.3V supply, should be wired from one side only [see right], not by looping one heater wire all round the valve socket, which would create a hum loop and cause excessive interference noise (though many amp makers DO make this mistake and get away with it). The wire twisting must be kept very tight right up to the socket, where it matters most. Their pin arrangement is also deliberate, so that the main heater pins (4 and 5) can be orientated towards the chassis wall, allowing heater wires to be run along the wall away from any other sensitive signal wiring.