Download Heater / Filament Supplies

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 centretapped 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 at least
2.5V above the 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. Higher-current regulators are available, however, at slightly higer
cost. The regulator must always be fixed to a suitable heat sink.
The simplest and most popular range of fixed-voltage regulators is the 78xx series. 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. 6V regulators do exist, but are not as
commonly available as the 5V versions.
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 must be positioned very close to the regulator.
It does not have to be tantalum, a ceramic will do at a pinch. A normal 6.3V
transformer winding will NOT provide sufficient voltage after rectification to power this
circuit.
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 (bellwire) 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 singleended 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.
A universal heater supply?
Everyone likes tube rolling, but it is somewhat dissapointing that the only valves
which are compatible with the ECC83/12AX7 pin-out are the ECC81/12AT7,
ECC82/12AU7 and 12AY7. But there are many other valves which conform to the
more standard pin-out such as the ECC88/6DJ8, ECC85/6AQ8, 6N1P, 6N2P and
other Russian types. Although we could provide a switch at every valve socket to
select between the two pin-outs, it would be nice if we could just plug in any type
without any changes. The following circuits are designed to allow this, but
automatically detecting which type is inserted. All these circuits operate the ECC83
types from 12.6V. If an ECC88 type is plugged in, however, a zener diode is placed
in series with the heater to limit the heater voltage to about 6.3V.
Each circuit uses pin 9 on the valve socket as a control port. If an ECC83 is plugged
in this pin goes positive, causing the SCR (U1 left-most circuit) or NPN transistor (Q1
middle circuit) to turn on, shorting out the zener. There is still a small drop across this
device though, which is why the supply voltage is shown a little higher than 12.6V (it
will probably be a little higher in this mode anyway, since an ECC83 only needs
150mA heater current).
When an ECC88 type is plugged in, pin 9 is connected to nothing, so U1 or Q1 turns
off, and current is steered into the zener diode, which drops roughly half the supply
voltage across itself.
For AC heater we simply use an NPN/PNP pair connected in parallel (Q1/Q2 in the
right-most circuit). However, zener diodes can't be used in the same way for AC, so a
resistor is used instead. Unfortunately this means that some of the Russian valve
types (notably the 6N1P) can't be used, since they require up to 600mA current
which causes too much drop across the resistor. Most European/American types and
the 6N2P should be ok though. Another disadvantage of this design is that it is not
balanced, so a humdinger pot will probably be needed to null any heater hum.