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Valve Technology

A
series of articles from 1993 by Graham Dixey C.Eng.,
MIEE republished by kind permission of Maplin
Magazine.

Extras
▼

No
extras

In the final part of this series, we conclude last months
discussions on practical audio amplifier systems with the
final link in the chain between pick-up and loudspeaker - the
power output stage.

The previous amplifier stages have been concerned with
producing a faithful replica of the input signal, but at a
larger voltage amplitude. The function of the output stage is
rather different and certainly more demanding. Because it
requires work to drive the speech coil of a loudspeaker, and
because work equates to power when we introduce the time
element, the output stage must produce large excursions of
current as well as voltage - and it must do so with the
minimum amount of distortion being introduced. This means that
the output valves must be capable of passing large current
flows, which implies a heavy emission of electrons from the
cathode surface in the first place. There are two factors that
limit the amount of current that a valve can carry They
are:

the surface area of the cathode capable of emitting
electrons, coupled with the power input to the heater

the amount of heat that can be dissipated at the anode
without causing degradation of the valves performance, eg,
the emission of gases from the surface, which would
seriously impair the operation of the valve by causing
collisions between the gas molecules and the electrons in
transit; excessive heat could also cause failure of the
anode structure.

It is obvious from the above points that valves intended
for high power applications (using the word high in a relative
sense, since one mans watt is anothers kilowatt!) must have
larger electrodes than those intended just for voltage
amplification.

Triodes versus Pentodes

These two types are obvious contenders for the role of
output valve; for the moment we will lump beam tetrodes in
with pentodes. What we need to consider is how well each type
of valve will be able to handle the task, given the rather
stringent requirements that apply Let us consider triodes
first.

lf the characteristics were absolutely linear and parallel,
no amplitude distortion at all would result. Both half-cycles
of the signal input would produce equal swings at the output,
each being a faithful copy of the input.

The mutual (grid) characteristics for triodes tend to be
both linear and parallel, which means that swings of the
signal voltage along these characteristics should not cause
too much distortion in the output (we wont, at the moment,
quantify how much distortion is tolerable), so this is a plus
point for the triode. On the other side of the coin, however
is the low gain of the triode. This means that, for a given
output signal amplitude, the input signal has to be fairly
large if the stage is to be driven fully At the same time the
triode also takes a great deal of current from the HT supply,
so is not very economical in terms of power consumption (when
compared with its amplifying capability).

The type of amplitude distortion that the triode does
produce is largely second harmonic. Thus, an input signal at,
say 400Hz would have, after amplification, a significant
amount of an 800Hz component also. This fact is, as we shall
see later not as serious as it might at first appear, since it
is relatively easy to reduce this type of distortion to
acceptable proportions.

Looking at the pentode now, and bearing in mind some of the
deficits listed above for the triode, one advantage that we
can call to mind immediately is going to be its higher voltage
gain, allowing the use of a smaller input signal in order to
obtain a given output. Also, it takes less HT current, and so
is more economical with supply power than the triode. However;
its mutual characteristics are not as linear or as parallel as
those of the triode, though it is possible to compensate for
this deficiency.

As far as harmonic distortion is concerned, the dominant
harmonic produced here is the third harmonic. Eliminating this
is not as easy as getting rid of even order harmonics: 2nd,
4th, etc. One point of comparison that can be made with
triodes, that we shall see is significant, is the value of the
anode slope resistance, ra. In triodes this is quite low,
whereas in pentodes it is by comparison generally extremely
high.

The Triode Output Stage

Any valve amplifier, being an active device that is
delivering a signal to a load, can be considered in a general
case to be a generator. Any generator cannot avoid having an
internal resistance, and the value of this internal resistance
will determine the optimum value of load impedance into which
the generator can deliver its maximum power. For valves, the
internal resistance is the parameter ra, this being the
reciprocal of the slope of the output characteristics, as we
have seen in Part
One and Part Two, and so is quite clearly the output
resistance of the valve when feeding a load.

The maximum power transfer theorem states that a
generator delivers maximum power to a load when the source
resistance of the generator equals the load resistance. Before
anyone reaches for pen and paper to tell me that I am wrong,
let me just add that this theorem as stated applies when the
load is resistive. If the load is reactive then maximum power
is transferred to the load when the load impedance has a value
that is equal to the conjugate of the generator impedance. If
the word conjugate isnt understood, dont worry about it; it
comes from complex algebra, which I have no intention of going
into here!

The valve as a generator; (a) actual triode circuit
(without bias components) and (b) the AC equivalent
circuit.

The figure above shows the valve represented as a generator
Figure (a) shows the triode (it could equally well be a
pentode) with an input signal Vg, a load RL in the anode
circuit and the internal resistance ra. The grid bias voltage
is represented by a battery and since this voltage is DC
Figure (b) shows the equivalent circuit which can be used to
analyse the performance of the amplifier This comprises three
components: a voltage generator μVg, which represents the
valve action and two resistances: ra, the generator internal
resistance; and RL, the load resistance. In the specific case
of the valve, whether triode or pentode (in theory at least),
the maximum power will be transferred to the load when the
latter is equal to the ra of the valve. This raises a problem
immediately.

The load on an audio output stage is the speech coil of the
loudspeaker it is totally impracticable to wind such a speech
coil so that it has an impedance anything like that of the ra
of even a triode valve, which will be of the order of
thousands of ohms! A practical value for the speech coil
impedance is unlikely to be more than a few tens of ohms
(eight ohms (8Ω) is a common nominal value). From this it is
quite obvious that it is impracticable, in the case of valves,
to connect the speech coil directly into the anode circuit of
the output stage as the load. There are other considerations
that make this undesirable anyway, but the above argument, on
the grounds of load and source impedances, should make it
clear why this course of action cannot be undertaken.

The Output Transformer

The answer is to use an output transformer with a step-down
turns ratio. The required ratio n can be calculated from a
simple formula, which is as follows:

n = √(load required by valve) / (resistance of speech
coil)

Notice that the above formula says load required by
valve and not actually the valves ra. This is because the
load presented to the valve by the loudspeaker through the
transformer is not resistive and it has been found in practice
that the load that the valve needs to see is approximately
equal to twice the ra of the valve.

As an example, if the triode in question had an ra of
8,000Ω and the speech coil resistance was 16Ω (using
convenient figures for ease), then twice ra is 16,000Ω and the
tums ratio needed is equal to √1,000, which is approximately
33:1.

The penalty that one has to pay by making the effective
triode load equal to twice the valves ra is an increase in the
drive voltage required for full output.

Design of output transformers for quality audio
reproduction requires great care. In particular the primary
inductance must be as high as possible (necessitating a bulky
transformer) if it is to offer a constant impedance to the
valve at all frequencies of interest.

Modern cores are not made of 'ordinary electrical steel as
may be found in a mains transformer but of a higher quality
purer stuff with superior magnetic performance; this also
makes it possible to keep the bulk down to reasonable
proportions, certainly modern examples are a good deal smaller
than their early equivalents.

Other factors such as self-capacitance and leakage
inductance must also be optimised if reproduction at the high
frequency end of the spectrum is not to suffer. This is
usually achieved by not winding the former in the normal way
like you would with a mains transformer Instead, both primary
and secondary windings are split up into sections and
interleaved with each other: some of these arrangements can be
quite complex and, to go really over the top, one layout
incorporates a split bobbin, where the order of the layers on
one half are reversed on the other half. Add to this a choice
of secondary taps for different speaker impedances, and it can
be appreciated that manufacture can be extremely labour
intensive, and, hence, valve output transformers can cost a
small fortune if you want genuine Hi-Fi quality. For more
information on the subject, see the small book Coil Design
and Construction Manual by B B Babani (available from
Maplin, Code RH53H, Price #2.50) which came out as a first
edition in 1960, and gives detailed guidelines on how to make
your own. We wouldnt recommend designing and making your own
output transformer however: there are, fortunately, still a
few competent manufacturers around.

Tetrodes and Pentodes

In contrast to the triode, both of these types of valve
have extremely high values of ra that require careful matching
to the loudspeaker impedance. The rule regarding triodes does
not work in these cases, and it has been found that these
valves work best when they see an effective anode load that is
between one-third and one-sixth of their ra value. Care is
required in the design of output stages using such valves
since, otherwise, they can generate an excessive amount of
distortion. In general, the safest technique is to choose the
load value which is recommended by the maker of the valve.

Although the beam tetrode may often be considered as the
equivalent to a pentode (especially since they are alter-
native answers to the same problem of the inter-electrode
capacitance Cag), there are significant differences due to
their different modes of operation. One of these differences
is the type of amplitude distortion that each introduces into
the amplified signal.

In the case of pentodes, the distortion is principally
third harmonic with only a little second harmonic; this is the
opposite to the beam tetrode, where mostly second harmonic
distortion is produced. Thus, in terms of harmonic distortion,
beam tetrodes behave more like triodes.

Push-Pull Amplification

There are several ways of approaching the design of valve
power output stages:

Single-ended, where the output transformer is in the
anode circuit.

Parallel operation, where two or more valves are
connected in parallel in order to boost the power output,
the output transformer being again in the common anode
circuit.

Push-pull operation where two valves (or sets of
paralleled valves) are driven alternately by the input
signal, each valve having half of the primary winding of the
output transformer in series with its anode, the centre tap
of this transformer being connected to the HT supply
positive line.

We can dismiss both (a) and (b) immediately on the grounds
that, since the primary winding carries the full anode current
in one direction only its core would have to be excessively
large or specially constructed in order to avoid saturation
arising from the quiescent DC. Again the Coil Design and
Construction Manual shows how this is done; the common
arrangement is where the magnetic circuit is effectively
broken by introducing a thin layer of waxed paper or similar
to separate the core into independent stacks of E and I
sections. The gap reduces the cores sensitivity to DC, yet
allows alternating magnetic lines of force to pass through. LF
response is critically linked to the choice of gap spacing, so
tolerance must be tight. Many consumer quality radio-grams and
record players et al used this type of output stage,
but it is not worthy of serious consideration, especially as
the type of harmonic distortion introduced by the output
valve(s) cannot be reduced by this type of connection.

However in the case of (c), push-pull operation, the DC
anode currents flow in equal and opposite directions in
the half-primary windings, and so their fluxes cancel out.
Saturation is thus avoided, even with relatively small cores,
the core size now being dictated by the consideration given
above, of providing a constant load at all frequencies. The
second advantage that arises from the push-pull connection is
the cancellation of all even order harmonics. The image below
shows the arrangement of one possible type of push-pull output
stage.

A Push-Pull output stage using triodes.

Valve V1 is the driver stage and valves V2 and V3 form the
push-pull output stage. The phase-splitting action is
performed here by the use of a driver transformer T1, in which
the secondary winding is centre-tapped to 0V and so provides a
pair of equal, anti-phase voltages to the grids of the output
valves as well as DC bias. With no signal input to the driver
stage, both output valves draw only their quiescent current,
and this flows in opposite directions in the half-primaries of
the output transformer T2.

When a signal is being amplified by V1, both ends of the
driver transformer T1 are at opposite potentials with respect
to the centre-tap, the latter being connected to the cathodes
of both output valves via the cathode bias components R1 and
C1. Thus, as the grid of one output valve is being driven in
one direction, say positively, the grid of the other output
valve will be driven in the opposite direction, in this case
negatively. The terms positively and negatively
are here used in a relative sense, since whether the signal
voltages are actually positive or negative with respect to 0V
will depend upon the way in which the output valves are
biased. The classes of bias possible with output valves are
illustrated by the mutual characteristic shown below.

Classes of bias.

Classes of Bias

These are more commonly used to describe 'classes of
amplifiers'. Taking Class A first of all, this is the bias
method commonly employed with most single-ended amplifiers, ie
not wired in push-pull. The valve is biased to the mid-point
of the characteristic such that the quiescent anode current is
large, and the signal swings cause equal changes in this
current for both half-cycles. Distortion is minimised, but
efficiency is low because of the high DC power input required
to obtain a given AC power output. The available AC power
output is also reduced, because the valve is contributing to
both positive and negative half-cycles of the output signal,
and having to do it within its available total signal
excursions.

If, instead, the valve is biased to the point of projected
cut-off, the valve is then operating in Class B. The quiescent
anode current is extremely low giving very high efficiency but
the amount of distortion introduced is high, making this mode
unsuitable for quality reproduction. Its main application is
in public address systems where quality is less important than
cost.

A compromise class of bias then is Class AB, where the bias
point lies between those for A and B. This gives an
improvement in efficiency and possible power output over Class
A, with better quality than can be obtained with Class B.
Class AB actually divides into two sub-classes, depending upon
how hard the grids are driven. In Class AB1, drive is
restrained so as not to cause grid current to flow; in Class
AB2, grid drive is increased and grid current flows at the
peaks of the positive half-cycles.

Finally, in Class C operation, the grids are biased well
beyond cutoff and are driven very hard in order to make anode
current flow in short pulses, just at the peaks of the
positive signal half-cycles. This type of bias is restricted
to radio-frequency operation, where the pulses of current
merely excite a resonant circuit in order to produce a full
sinusoidal output. So now you know!

Harmonic Cancellation

Because of the fundamental way in which push-pull output
stages work, the signal currents in the output valves flow in
opposite directions in the output transformers
secondary winding. The magnetic flux that links with the
secondary winding of this transformer induces voltages
that are additive in this secondary Thus, a positive
half-cycle developed in valve V2 at one instant causes a
corresponding voltage to be developed in the secondary
winding; this is followed by a negative half-cycle produced by
valve V3, which also induces a similar voltage in the
secondary winding. As a result, the secondary voltage appears
as a continuous voltage, compounded from the successive
efforts of the two push-pull output valves working separately
but in co-operation.

However any even order harmonics, 2nd, 4th, 6th, etc.,
generated by the output valves cause opposing magnetic
fluxes in the primary winding which, consequently cancel out.
No even order harmonics appear in the secondary winding. This
is a major advantage of push-pull operation. It is obvious
that this is of more significance when triodes or beam
tetrodes are used, since this is the type of harmonic
distortion that these valves generate. This somewhat dampens
the popular idea that the main characteristic of such
amplifiers is that they generate lots of 2nd order harmonic
distortion!

Value of Anode Load for the Output Stage

The actual value of the anode load in push-pull operation
is not the same as that calculated for single-ended operation.
Again, the best bet will be the figure provided by the valve
makers since this will have been computed to give the minimum
amount of odd order harmonic distortion. In pentodes, it is
third harmonic distortion that is the major type, and it is
possible to minimise this by a suitable choice of load. In
fact, it is essential to do this, since push-pull operation
does not result in any reduction in the odd order harmonic
content. It has been found that reducing the value of anode
load substantially below the nominal value reduces third
harmonic distortion and increases second harmonic distortion,
the latter then of course being cancelled by the push-pull
connection.

Negative Feedback

It is possible to write a book on this subject; many have
already done so and I have no intention of adding to the
published material. Suffice it to say that it is a matter of
great importance in the design of audio amplifiers, of any
type. Thus, the theory is not specific to valve amplifiers.
Its benefits are well-known in reducing distortion of all
types, in giving gain stability and in coping with the
resonance problems of loudspeakers. That is all that need be
said about it here.

Examples of Hi-Fi Audio Amplifiers

We shall now have a look at a few audio amplifier circuits
which will illustrate the various approaches that designers
have adopted in the past.

The Garner 10W Main Amplifier

The Garner 10W Main Amplifier

The first of these is shown above and was designed by Major
H H Garner in the early 1950s. It doesnt have any pretensions
to exceptional performance but is, nonetheless, a creditable
performer according to G A Briggs, who tested it exhaustively
at the time. It was paired with a preamplifier; which had the
obligatory bass and treble controls as well as input switching
for pick-up and radio, and compensation for LP and 78rpm
records.

The circuit is fairly simple and the line-up is quite
conventional for those days. (The line-up was the collective
expression used for all the valves.) No miniature valves were
used; these were still a little way off in time for general
use. The output valves were the popular 6L6s
and these were somewhat under-run. The distortion figures may
not seem exciting but were seen as being adequate for the
purpose. At an output power of 5W the THD was 0.8%, which rose
to 1.5% at 10W output.

The pentode first stage, V3, included overall negative
feedback from the output transformer secondary winding. This
could be varied between zero and 22dB of NFB (Negative
FeedBack). Apart from the fact that this varies the
sensitivity of the amplifier no justification was offered for
this arrangement. The points that were made were as
follows.

Since the 6L6 output valves made no great demands upon
their input drive, a concertina phase-splitter was
considered adequate, with balancing of the bi-phase outputs
being achieved simply by matching the values of load
resistors R24 and R26.

Because of the high input impedance of the
phase-splitter; it was possible to use a pentode first stage
to give both high gain and good top response.

A 10W Amplilier for Mobile Public Address

A 10W Mobile Public Address Amplifier

The circuit above shows an amplifier that appeared in the
American magazine Radio-Electronics in March 1957. This was
published in response to a letter from a reader who wanted a
power amplifier both for his FM tuner and for mobile PA
requirements. Since it had to be mobile, it couldnt use a
mains supply, so a feature of the design was a vibrator pack,
to convert the low level vehicle DC (6V no less) into the
higher level HT required by the amplifier.

The first stage, V1, was a 6SJ7
pentode, used purely as a microphone preamplifier and giving
enough gain to allow any high output, high impedance crystal
or dynamic microphone to be used.

A separate tuner input was provided that tapped directly
into the grid of one half of the double triode V2a. Here we
see a choice offered between the older and larger 6SL7GT
valve and one of the newer all-glass the 12AX7
(ECC83).
The output valve is another popular choice of the day the 6V6;
or a 6AQ5 could be used instead. The phase splitter is of the
paraphase type with V2a as the first stage and V2b as the
second stage. Anode loads are 150k and these feed the purely
conventional output stage comprising pentodes V3 and V4.
Overall NFB was taken from the 8Ω tapping on the output
transformer secondary winding to the cathode of V2a. A grid
signal for V2b is contrived by tapping the grid leak resistor
chain for V3.

The Craftsman C-500

The Craftsman C-500 Williamson Amplifier

In the May 1956 issue of Radio-Electronics there
appeared a circuit for a typical Williamson amplifier, the
Craftsman C-500, shown above. For those not in the know,
Williamson was a famous name at the time in the field of Hi-Fi
amplifiers and belonged to D T N Williamson who was employed
as an engineer with the British firm of Ferranti. His
Craftsman C-500 amplifier uses a pair of KT66s,
which were very popular high power valves in those days.
Although they are actually beam pentodes, they were connected
as triodes by wiring the screen grid to the anode via a 100Ω
resistor (The use of the KT66 in triode mode is validated by
applications data; not all pentodes and tetrodes may
automatically be connected up this way).

Note also the circuit symbols used for them. Do you
remember in Part Four mentioned that often beam tetrodes are
given pentode symbols in circuits, whereas the beam tetrode
version should really be used for clarity? Although the KT66
is classified as a pentode, it has beam forming plates in
place of a suppressor grid, and is practically
indistinguishable from a beam tetrode in construction. Some
would say the KT66 is a classic beam tetrode-ed.

In this circuit the first stage, V1a, is a pre-amplifier
whose output drives a concertina phase splitter V1b, thus
neatly putting both stages into one double triode. Note also
that, as was mentioned in Part Six on the subject of phase
splitter configurations, that here is an example of the signal
grid of the splitter stage, V1b, being DC coupled to the anode
of V1a, which then DC biases V1b. This makes a local bias
resistor network and a coupling capacitor unnecessary, a
saving in component count.

The third stage driver is a push7pull type whose antiphase
outputs directly drive the push-pull output stage grids, and
is included to develop a healthy driving voltage swing and
increase the open loop gain of the whole system, bearing in
mind the modest gain of the output valves, operating as they
are in triode mode (see discussion about output triodes
above). A balance preset is provided to equalise the anode
currents of the output pain while the bias pre- set sets the
net bias current in the output stage. In the Williamson design
this is typically 125mA (62 to 63mA for each valve). These
adjustments have to be made with the aid of a multimeter after
temporarily removing the link shown. Overall NFB is taken from
the 16Ω tapping on the output transformer back to the cathode
of V1a.

It was a feature of the Williamson designs that the output
transformer was of a superlative specification. The bandwidth
was enormous, sometimes in excess of 100kHz, with extremely
low distortion and of massive size. It is said that Williamson
amplifiers were designed around the output transformers. The
wide bandwidth introduced in stability problems, however in
the presence of NFB. It was possible to alleviate this by
including a bypass network within the loop that was only
effective at frequencies at or above 100kHz. This is the RC
network R1 / C1 between anode and ground of V1a. Its function
is to load V1a at high frequencies, reducing its gain and
ensuring a top end roll-off. As a further anti-instability
measure, a small value capacitor C2 was also added in parallel
with the feedback resistor R2.

The Ultra Linear Amplifier

The Williamson Ultra-Linear Amplifier

In 1952 or thereabouts, an unusual output circuit, which
had been patented some years previously was resurrected by
David Hafler and Herb Keroes. This circuit combined the best
features of triode and tetrode/pentode operation and was known
as the Ultra-Linear configuration. The important change
to the normal push-pull connection was minimal and consisted
of connecting the screen grids to taps on the primary of the
output transformer Oddly enough, opinions are divided on how
the circuit actually works, but the secret is believed to rest
on the introduction of NFB. Whatever the modus operandi, there
is no doubt about the benefits. The operating curve is more
linear than for triode operation even, and the output power
capabilities are about one-half those of pentodes and double
those of triodes. 'l"his means that, for the same HT supply
and signal drive, the output of an ultra-linear amplifier is
double that of the same amplifier when triode connected. In
fact, distortion at low levels is reduced as is the
phase-shift at high frequencies. This makes it possible to
increase the degnee of NFB without incurring any further
instability penalties. The Ultra-Linear Williamson ampliiier
is shown above.

Another aspect of the design of amplifiers such as the
Williamson types is interesting. At the time, valves might be
classified loosely as being either transmitting types or
receiving types. The former type of valve was obviously
designed for handling high powers, a prime requirement for
audio output stages. It was natural, therefore, to adopt
transmitting valves for Hi-Fi purposes, with consequent
advantages. They didnt make too stringent a demand upon the
driver stages as far as the amplitude of the driving voltage
was concerned, and they were operating on the linear parts of
their characteristics when delivering the high powers
required.

A Low-distortion 12W Amplifier

Design for an Ultra-Linear 12W Amplifier

Finally another ultra-linear amplifier circuit, but rather
simpler than the Williamson version. The power output from
this is a modest 12W but even this is often considered to be
adequate for the average living room. I reproduce here the
designers original criteria, taken from the August, 1958 issue
of Radio-Electronics.

lnaudible distortion at all feasible levels (in a 10 x
15ft. room).

Low source impedance.

High efficiency.

Best possible stability characteristics.

Hum and noise below audibility (under specified
conditions).

The designer concentrated most of his efforts on
consideration of the degree of NFB needed to meet his criteria
1 to 4 above, and opted for the ultra-linear mode of
operation, using a pair of 6V6s.
To obtain this type of operation with these valves requires
the output transformer primary winding to be tapped at 25% on
each side of the centre7tap. The choice of output transformer
from what was on the market at the time, fell on the Acrosound
TO-310, which had a rating of 10W over the bandwidth 20Hz to
30kHz, and 20W over the reduced bandwidth of 30Hz to 20kHz,
the latter being adequate for good reproduction.

The circuit itself uses a 12AX7 (ECC83) double triode, with
one section, V1a, being a voltage amplifier directly coupled
to a concertina phase-splitter This gave a number of
advantages.

The DC coupling eliminates one stage of capacitive coupling
which gives greater frequency stability Placing the phase
splitter directly before the output stage (which is possible
due to the low drive requirements of 6V6s) eliminated the hum
frequently resulting from the large potential difference
between the heater and the un-bypassed cathode of V1b, being
amplified by a further push-pull driver stage. The anode load
of the voltage amplifier acts as the grid leak of the phase
splitter thus allowing the voltage amplifier to work into a
very high impedance, so that high gain with low distortion are
obtained from the first stage of amplification.

Efforts were made to improve the performance by matching
pairs of resistors and some capacitors (if possible) to within
1% of each other These are indicated on the circuit diagram.
It seems, from the comments made upon completion and testing,
that the amplifiers performance lived up to expectations.

That just about concludes this series on Valve Technology,
which has been restricted to audio applications only, and this
is now one of the few fields left where valves still survive -
we havent even mentioned radio, TV, Instrumentation and
industrial applications, which of course at one time valves
had to cater for also! The only other area where valves are
still in common use is for radio transmitters (a very large,
high voltage, high power triode operating in class C).

We hope you have found the series both interesting and
informative, and that it has answered a lot of questions you
may have had about these devices. You may now be sufficiently
better informed to try some experiments of your own. Elsewhere
in this issue you will find details for Maplins own valve
power amplifier project, which closely resembles Mullards 520
ultra-linear design of the late 1950s and early 60s. Happy
metal-bashing!

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