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Preamplifiers ln General
Make no mistake, a good preamplifier is hard to build, and much, much harder
to design. This is mainly due to the fact that one is dealing with miniscule
signal voltages hindered by the 40 dB loss of the RIAA curve. Furthermore,
the preamplifier is required to be "all things to all men" in the sense that it is
expected to have sufflcient gain to amplify almost nothing up to and beyond line-
level without adding noise, distortion or its own sonic signature. And having
met those demands, the next person requires that the same preamplifier accept
a moving-magnet cafiridge with perhaps fifty times the output voltage and still
not overload the little point one millivolt input ["and why is the volume control
not pointing to 'midnight', you incompetent dolt???"1 Some tough assignment,
we think you'll agree. It's made somewhat tougher yet by the fact that the
preamplifier has another role to Iì11, which is that of a central routing switchboard.
In fact, some of the older literature calls them "control centres". In essence, it
cannot be done. Or rather, the examples available from people who think they've
done it do not impress us.
The only way, as we see it and do it, is to divide the problem into a pre-
preamp for the MC followed by a phono and line preamplifier. (In the same
cabinet, we mean, because surely nobody really rrants another couple of little
boxes plus more interconnects to pick up more noise with more losses on addi-
tional plugs/sockets plus the extra mains cord to induce more hum to these and
the original MC cable?) And, yes, you can use an MC step-up transformer, but
they are very susceptible to EMI (electro-magnetic-interference) and the reviewers
we respect don't seem to like their sound either.
Our preamps all have the moving-coil stage built in (excluding the Maxi'
mal) and, considering that they're all-tube units, the noise-level of the MC stage
is very acceptable. (Funny, we always think, that here is one place where the tran-
sistor should be in its element: a low-impedance 'coil, say around 100/200 ohms?
44 THE VTL BOOK
An ideal match for the little beastie. . .low noise-level requirement? Just what the
doctor ordered (only he was a deaf doctor, remember).
Yet another duty that some folk expect of a preamplifier is that of "dubbing-
console switcher", and here we draw the line: so sorry; we honourably refuse to
oblige. May we tell you why, please? Primarily it concems contact resistance
or loss factors, affecting the higher frequencies; and remember, we're talking
pre-amplifier language here with the signal in its smallest (and therefore most
fragile) state. Let's count the number of 'pressure' or touch-contacts that could
in a worst case arise in the chain before the amplifier:
1) Cartridge to clips in arm-shell
2) Head-shell to arm (Horrors!)
3) Under-arm DIN-plug
4) ncR-females on tumtable-back
5) RCA into phono input
6) Input selector-switch
7) Output selector-switch (recording)
t Tape-loop swirch. moniror
8
9) Ba lance-cr-rntrol wiper
10) Volume-controÌ wiper
I l) Bass-control wiper
1 2) Treble-control wiper
13) Filter or doo-dar switch
l4) Preamplifier ourpur RCA's
l5) Amplifier input RCA's
Phew! Poor little signal! We obviously, as do High End manufacturers, leave out
the totally unnecessary tone-controls; but we leave out more still in senseless-
switching, and in our design of balance-control. Have you any idea of the losses
caused by the above chain? We have both listened and measurement-quantifled
them to commence onslaught at 8 kHz. (Gadzooks! This is thirty-buck portable-
radio territory!). True: this is half a dB down ar 8 kHz, one dB at 12 kHz, one
and three-quarters at 16 kHz, three at 20 kHz. . .
Now, if you want to add ten or twelve inches of PCB track to make some of
these switches look a little neater in assembly (and neater still in balance-sheets),
why, just deduct some more upper-frequency dBs. By using Rhodium connectors,
good wire and solder, omitting 'toys' plus eschewing signal-on-tracks, we are able
to go'flat'to 100 kHz and above. Ourpreamplifiers are designed with thefrsr
playback foremost in mind; we are not prepared to compromise this standard of
our critical design criteria to the level of a lo-fi mass-produced (by the mafia of
mediocrity) cassette deck. Tapes for your car? Well, if you must, you must.
CARTRIDGES, LOADING, & RIAA 45
Which is why our preamps do hav e an excellent record-feed output some of
them are even buffered to reduce the risk of contamination.
-
What they don't have is a multi-deck rotary switch marked "lnput deck # l,
2,3" etc., routing to outputs '3' and '2' but not '1', etc. Also, they don't have
(never had, never will) the switch marked "source/monitor" (sometimes called a
"tape-loop" switch). This is exactly the doo-dat which forces the critical Iistener,
who may have no desire whatsoever to re-record a record, to accept a sound
degraded by the signal cables being brought all the way to the front panel via
an extra switch so that a less discerning listener can make copies. Interestingly,
switching over to "monitor", which is to the playback or third head, only ascertains
that: a) sufficient finger-pressure was exerted on the Recono button; b) the ruined-
looking tape left on the dashboard sounds even more ruined than it looks; and
c) Stone me! Overmodulating much into the red does catse distortion, like the
instructions said.
Besides, standard operating procedure in the professional recording game
advises that one should use good tape of known quality, avoiding the need to
monitor on "third head", because the possibitity of a tiny leak in the change-over
switch (even 50 dB down) from the playback head to the record-head circuitry
will print a very faint echo-effect, thereby marring the recording anyway. (Please
ignore this last paragraph if your tape-recorder happens to be a tubed Studer,
EMI BTR2 or Telefunken M10).
Cartridges, Loading and RIAA equalization
Proper attention to cartridge-loading is one of the most worthwhiÌe improvements
that can be made to a system, especially with regard to moving-magnet cartridges.
No, don't write them off: a good MM correctly mounted in a compatible arm can
produce wonderful music when facing into the optimum load in resistance and
capacitance. The MM cartridge manufacturers know this, and their recommended-
load suggestions should be followed at least as a starting point. The superb Decca
(as modifled by the Garrott Brothers of Australia) can easily take an 8 k O to
12 kO load to optimise it. Loads for MC cartridges, however, seem to be a very
contentious matter.
One school of thought says that the MC, any MC, should and must perform
properly into a standard 47 kC) load. There is support for this from studio-
microphone experience: most microphones (and a dynamic moving-coil mic is
very akin to a moving-coil cartridge) perform best when looking into a load
of ten to twenty times their nominal impedance (if you can spare the gain).
Another school says that a 100 ohm cartridge should "see" a 100 ohm load. The
third school says it all depends on the individual cartridge specimen, its number
of hours run, and how it sounds with various loads. We take that viewpoint.
46 THE VTL BOOK
and therefore offer many switchable options in our preamplifiers (except in the
Ultimate and Manley units with Floating Symmetrical Mode; with the reduced
'tracing' distortion offered by FSM, an optimum average load is fltted).
A cartridge, like a loud speaker, is a motor of an electro-magnetic nature
and, to some extent, the load serves as its gearbox, clutch and brakes, affecting
its "speed" as well as optimising its linearity and distortion characteri stics. Since
the designer of the preamplifier endeavours to keep deviations in the RIAA equal-
ization to under one quarter of one decibel, it is only fitting that effort should be
made to see that the cartridge itself is optimally loaded to maintain its accuracy.
And what about the RIAA (Record lndustry Association of America) curve?
Ahh, the RIAA curve. . .we'll tell you a little about it if you promise faithfully not
to abandon records completely in these troubled times. Well, the actual master-
disc from which the record is eventually pressed (we'll leave out the sordid details
of mother-matrix-stamper) cannot in f act accept the full frequency . spectrum at
equal levels of amplitude. So, for two major reasons, the curve has to be altered
very drastically to accommodate this problem. (You promisecll)
If one attempted to cut a microgroove LP with the full frequency range
absolutely 'flat', or unequalised. (as we have indeed done many times for test-
purposes, at 78 rpm on grave-silent lacquer with a groove-pitch of 80 lines-per-
inch which yields all of 2 minutes on a l2-inch side) two horrible phenomena
would occur: one, all the lower frequency sounds would cut literally over and into
each other's groove (and you thought you had mistrack problems, buddy?) and
two, you'd fìnd that you could not hear much of the upper frequencies clearly
either, because of the rushing-faucet sounds produced by the playback stylus'
friction in the grooves, locked or otherwise. Whence cometh the need to attenuate
(reduce) the bass frequencies under 1000 cycles (Hz) and boost (increase) the
treble frequencies above the same turnover point in the master-cutting process.
Then when you play the final record back, you have to reverse (in the preamplifier)
the order of events by lifting the bass and also by cutting the treble and, by
courtesy ol the latter efTort, the surface noise goes down with it.
One of the not-so-elegant by-products of the scheme is that in lifting the
bass in ptayback, you also accentuate any mechanical vagaries of the turntable in
the fbrm of rumble. This is why a really good High End turntable needs to be
built at or near the same standard of quality as the mastering-lathe and costs
accordingly. The amount of cut-and-lift of the RIAA curve is 19.6
- dB. Praise
be that thq world at least saw the sense of adopting it as a standard; before that
one had "Columbia t-P", "British t.P'ì "NARTBLP" and others [see Appendices:
RIAA Tablel. Also included is an easily constructed int,erse RIAA network of
resistors and capacitors; this is a very useful little test item used for checking the
accuracy of a preamplifier's RIAA curve.
PHASE INTEGR]TY 47
We come now to the the actual circuit design we use for reversing the RIAA
recording curve in the VTL preamplifiers, and why we choose this type of circuitry
[see relevant schematics in the Appendices]. We accomplish the RIAA EQ with
active techniques the use of frequency-selective negative feedback. We have
-
listened, measured, and Iistened while measuring exhaustively to our circuitry.
and remain convinced that this system is musically far superior to the àlternative
of passive circuitry (resistor and capacitor networks which cause an insertion-
loss of 40 dB to the circuit). We have found passive correction networks to be
grainy, slow and lifeless: and disproportionately sensitive to type and design of
capacitors used in circuit" Moreover, the elegance with which the RIAA curve is
reversed in an active circuit is almost breathtaking.
Consider: you need to lift the bass by 20 dB at the lowest frequency so
you simply have 20 dB /ess feedback at the lowest frequency and appropriately
-
less for the other intermediate frequencies (also thereby minimizing phase-shift
to boot). Now you need to cut the treble, so with active cicuitry you apply
feedback (which is gain-reducing) most strongly down to the frequency you wish
to attenuate most, in this case 20 kHz, and proportionally less on down to the
tumover point of 1000 cycles (1 kHz). And just look, while you were having
maximum feedback at the highest frequencies, you were simultaneously reducing
the tube rush, or hiss! Just like the surface noise mentioned in the recording
process. Elegant.
Let us now examine thc passive method: fìrst you need to arnplify the
information from the cartridgc with its excessive treble and dramatically reduced
bass by at least 20 dB (becausc you're going to have to lift the bass by that much)
and then you need to amplify it another 20 dB to make up the insertion-loss of
the filter circuit needed to cut the treble by that much. . .and then you are still
going to be without the gain-equalising effect offered by the feedback that arises
from minute discrepancies inherent in each individual tube (or transistor) stage.
(It is not technically feasible to have feedback over two or three stages if that
chain includes a volume control or other filter network between stages.)
Preservation of Phase lntegrity
There are a great many folk who aver they can hear 'Absolute Phase': they can
clearly hear a difference when the phase (at the speaker terminals, say) is flipped:
provided, of course, that the recording itself is 'clean' and fully phase-correct,
there is no doubt that one way will sound more natural than the other. We call this
the 'preferred phase'. This phenomenon was researched by a gentleman named
Wood, and our friend Clark Johnsen has written an interesting book entitled Tfte
Wood Effect.
48 THE VTL BOOK
The problem of phase-inversion is rife in the recording process, and very
hard to controll Every time a transformer or an odd number (one, three, etc.) of
amplifying devices enter the chain, the phase is inverted. The reason is supposed
to be that designers of all equipment (recording and playback) had a Golden
Rule that everybody would strictly adhere to (v'e do)'. "Thou shalt not invert the
phase."
Notwithstanding history and geography, we play the recordings that contain
the music we like, right? And many of them are out-of-phase N'illrirl themselves,
more commonly with 'pop' multi-track material: baked and bumed music, we
calt it! With this type of music, neither phase-flip will be more correct [see also
the section on FlonttNc Svltvstntcnl Moon].
One of the benefits oi our recent breakthrough (we sincerely believe it
is a genuine breakthrough) cartridge-connection and two-phase circuitry is the
massive restoration of phase coherency, manifested in clearer acouslic din'rension
and location of lower frequency instruments. It seems "kinder" to muddled-
phase recordings, too. We find this easy to understand whcn compaled to the
standard ly-practised mode ol' cartridge connection with onc side contrected to
ground or 0 Volts. Wc now see this common practice in near-human terms ol
trying to perfbrm with one leg nailed to the ground. When onc has two excellent
recordings, both acousicalll' recorded with simplistic phase-corect technique but
each of clill'erent phase. this difference is even more palpable; but àotli sound
marvellous !
There are only two.qoozl ways to flip phase: at the cartridge wires or at the
amplifier/speaker teminals. Circuit manoevers of an an-balanced nature, where
one 'cuts' a stage to create an 'invert' position are therefore not, by definition,
exactiy identical in both modes. Most folk (ourselves included) are not too keen
on fitting a FLIP switch at the cartridge connection because of the miniscule
voltage, contact integrity, etc. However, we will fit such a switch by request for a
nominal charge to our Manley Reference and Ultimate preamps which feature
Floating Symmetrical Mode.
The other good way to flip phase, at the speaker/amplifier terminals, has
the opposite problem of a large amount of volts and amperes. Enthusiasts are
rightly very keen on maintaining contact integrity there, too. We have, on many
occasions (mainly fbr cornparative tests) run speakers through very heavy-duty
switches or relays using 100 Amp contacts with absolutely no uutlible 1o.is. After
all, there are not a f'cw speakers whose midranges and/or tweeters go through
pooky little 5 Amp or less fise-wire tbr protection!
We have often - wondered whether, properly conceived and executed, a
-
remote-controlled loudspeaker 'phase-flipper' would gain any support in the mar-
PHASE INTEGRITY
ket place. It would not be an inexpensive item to buitd well, but what an extremely
useful facility it would provide.
Tube Power Amplifiers A General Overview
-
VTL amplifiers, like all current tube power amplifiers, are loosely described
as 'resistance-coupled' amplifiers the distinction that
- between tubes) nteaningand/orthe ampÌifier
contains no 'inter-stage' (or coupling step-up impedance-
converting transfomers, as did some of the very early examples of high quality
amplifiers. The main reasons fbr the very old equipment relyin-q on step-up
transfbrmers within the circuit were the unavailability of sufficiently'high-gain
input triodes as well as the fact that they were often driving triodes in the output
stage: inefficient and notoriously hard to drive, requiring huge drive-signal volt-
ages. D.T.N. Williamson, the famous British tube-designer, demonstrated with his
equally-famous 'williamson' amplifier in 1947 that a resistance-coupled amplifier
using the more efficient tetrode (but triode-connected) could easily outperform the
older design concepts, and the truty high-fidelity amplifier was bom. The circuit
topology of the williamson had really only three main features worth mentioning
here: a) an unusual and new method of balancing the output grids and cathodes
used in "self-bias" mode, b) a tasteful and essential amount of negative feedback
and c) a brilliantly-conceived and executed output-transformer, without which
one had a very good, but not outstanding, amplifier of modest output 15 to
18 watts. vrl amplifiers pay the same kind of heed to transformer design - and
execution that Mr. williamson believed in (please see Tnrrrusnonumn section) but
our circuitry and other design concepts bear no other relation to his work.
Let us describe a typical VTL amplifier's general topology, starring with the
all-important power-supply/s, for it is a fact that an amplifier (tube or solid-state)
can only be as good as its power-supply (since the amplifier, after all, modulates its
power-supply). we use diode rectifiers in a full-wave bridge configuration because
they are totally reliable when built with the best components. we go to the trouble
and expense of importing, lrom Cermany, the Siemens type By255,s or By229,s
for our larger models because these are the the only types specifically designed for
TUBE AMPLIFIERS 51
our application. We have never had one break down yet; besides their inherent
reliability, they're grossly over-capable with their rating of 1500 peak-inverse-
volts (VrrM) and a current-handling capability of 2.5 ampercs. A 1000 volt diode
able to handle I ampere suffices in models of 100 Watts and under.
The second reason for choosing diodes is because our design calls for a
very "stiff" supply, meaning inordinately high capacity at a very low impedance.
The alternative to diode rectifiers is, as you knew or guessed, (a reverent pause,
please) the vacuum-tube rectifier. But, and it's a big but, tube rectifiers were
never contemplated to push capacity banks much over say, 100 microfarads,
whereas we require ten or even twenty times this capacity. Even when driving
up to 100 microfarads or thereabouts, tube rectifiers were and are notoriously
unreliable, really having been designed for 4 to 32 microfarads. It would be
true to say that the older tube equipment (radios, TV sets, amplifiers) usually
pooped out because of power-supply problems: the "cap" started to die and
slowly murdered the rectifier; the rectifier collapsed (by a short) suddenly and
took the cap with it or variations on this theme. Mark you, the tube-type rectifier
had one superb attribute: the rectified DC voltage increases gently to full value
as the tube warrns up, but one can live without that f one has the right bridge
and computer-grade capacitors. We do.
The benefits of a "stiff, fat" capacity bank in our amplifìers' powcr-supply
are palpably obvious when you first hear a VTL unit: the bass and low-bass is
at once clearly deep and powerful, fast and 'tight' attributes that are wrongly
- amplifier. The following
believed to be the exclusive domain of the solid-state
formula shows the relationship between capacitance and voltage in determining
audible energy:
yz
E-Cx ,
Joules.'
VTL amplifiers (300's, lZO's, 225's and 500's) operate with 1650 mFarad at
550 volts, which is equal to
5502
,2
o:00165 x = 250 Joules.'
A typical solid-state amplifier operates with 10,000 mF at 50 volts (12.5 Joules);
for a higher-priced amplifier, the specs might be 20,000 mF at 60 volts (36 Joules).
This is the reason why our tube amplifiers sound "louder" than their solid-state
counterparts one can really hear the effect of the high volts and high "instanf'
energy. The - output transformer has a great deal to do with this too, but no
output circuitry can perform properly without this type of energy storage in the
power-supply. It is possible, but very costly in terms of space, weight, heat and
I
52 THE VTL BOOK
actual money to have a fully-regulated "B" or high-voltage rail operating at 500
or 600 volts at one half to two amperes of DC as it would need to.
Using the size, type and capacity rhat we do (typically 1700 microfarads
rated at 700 to 800 volts-capable at eight or ten amperes of ripple current) we
derive almost the same benefits as we would tiom full-regulation circuitry in
terms of peak, instantaneous voltage, plus the low-frequency benefits that would
not come from regulation by itself. It is the main power supply strongly aided
and abetted by the independent supply for the input tube that gives our amplifiers
their impressive sle\À,-rate and hair-trigger rise times which reflect not only in the
low-frequencies, but right across the full power bandwidth. This also enables
the amplifier to handle transients well and to keep down the intermodulation
distortion. (lntermodulation is described by, say, a strong pattern of string bass
accompanying a solo flute many octaves apart.)
We might mention here that our lìlter capacitors are built to our order and
specification to what is known as "Special Test Requirement." They are tested
at ten percent over their marked working voltage at 10 amps of ripple current at
950 Centigrade simultaneously in an oven. When they come to us, we "form"
them first, slowly building up their voltage to accustom thcm to their brave new
world by 'pushing' them for a short period to 15 percent over their limit to ensure
that they understand our demands.
Coing on fiom the main high or "B" rail (and leaving out the DC heater an<Ì
bias supplies which we feel are self-explanatory from the circuits) we come now to
the separate and regulated "B" rail for the input tube as found in all our deLUXE
monoblocks: we'd like to explain this only to the extent that we believe the input-
circuitry should be treated like a preamplifìer in terms of its voltage-rails, since it
is handling signal voltages of close-to-preamplifier proportions. Furthermore, we
like that input tube to "see" its operating voltage clear and clean, and in no way
related or dependent upon the "B" rail used for the driver and output sections:
thìs is called "purism", and it is where we want to be.
The input-stage in all models is a pair of Class A triodes (both sections
of a l2AT7) connected in parallel. This configuration performs some interesring
duties: it increases the gain by doubling the transconductance, thereby lowering
the noise and impedance, and offers double the "headroom" (overload potential)
of a single tube. Neat. 'fhe l2Nl'7 is an extraordinary tube in itself, having
almost the amplification factor (80) of a l2AXl (100) but yer having an anode
or plate dissipation of two and a half watts; it could drive a small loudspeaker to
quite a respectable output !
Now on to our driver/combined phase-splitter stage: it is necessary to divicle
the signal into opposing phase-halves for all push-pull output stages, and the tube does this while adding some more amplifìcation to tèecl the output tubes,
NEGAT]VE BIAS 53
grids. Here again we chose the hairy-chested l2AT7 or its even harier-chested
big brother, the l28H7, and configured it in a combined "long-tailed pair/floating
paraphase" circuit. In the Manley series, we add yet another dedicated power
supply for the driver stagc/s, to also keep it spearate fiom the output stage's
supply. As with our input circuitry, every possible variation was constructed,
measured and listened-to extensively in real music terms, both in playback and
recording. The ones we chose to go with won hands down.
A fèw words on our fairly high input-sensitivity are worthy of mention
here: this is on the order of 0 dB or 775 millivolts; we have expìained more
about this elsewhere. The main reason for choosing this is that by requiring less
output from the preamplifier, the preamplifier itself gains more head-room (and
yes, also noise; so the preamp needs to be up to the job). Another rcason was
that we like our amplifiers to be at "studio standard" so that they can be (and
are) used as monitor amplifiers. The third reason was not pre-planned but arose
out of the last-mentioned studio aspect: most tape-recorders and CD players can
drive our amplifìers to full output and we do mean full otrtput which is
-
why we fit volume-controls to our stereo amplifiers if so required. -
We'd like to mention here that a competent technician (with our approval,
please) can easily alter both the input-sensitivity and impedance if our input
standard is too sensitive fbr your requirements.
Negative Bias: Why and How
Let us begin by accepting and understanding that tube amplificrs require a neg-
ative, or polarising, bias voltage at the grids of the output stages, which is the
portion of the amplifier we will now discuss. Let us also hasten to point out that
the whole topic of "bias and biasing" need neither be a constant source of worry
nor a bothersome continuously-required adjustment. In fact, we designed our am-
plifiers to specifìcally minimise and almost exclude this chore. (Having read and
accepted the last statement, you would be forgiven for nany further because we warn you! it gets pretty involved.)
- a/1 VTL power amplifiers is loosely describedThebeing of
general
topology and design of - as
the "fixed bias" type. This follows the best traditions of American tube-design
technology, which historically has been the leading-edge of the art, and which
has predominantly favoured the fixed-bias topology for very sound reasons that
have always interested us greatly. We'd like to share these reasons with you.
First, 'fixed bias' produces much more power (as much as 7 07o) at lower
distortion than 'self-bias' at the 'design centre'-nominated value of the cathode
resistor. Second, the tube will run cooler (and therefore longer) in the fixed
bias mode. Interestingly, a good many outstanding early American designs from
RCA, Westem Electric, General Electric, and others employed fixed bias of a
54 THE VTL BOOK
totally fixed and non-adjustable type. A calculated amount of negative voltage
(often comfortably and harmlessly excessive) would be injected at the output
grids forever, and that was that! The output tubes ran much cooler, though with
minutely more crossover distortion, therefore lasting much longer. Years longer,
we mean.
It is this philosophy that we chose to follow in our designs: we run the tubes
with less standing-current but with proportionally higher rail voltages which, in
turn, requires higher bias voltages which produces the desirable combination of
highest power with lowest distorti on at power while still running cool and ensur-
ing long life. We create our bias supply by having a separate dedicated transformèr
winding interleaved with the "B" rail winding, which achieves the magnificent
objective of obtaining a gently self-adjusting negative bias that smoothly tracks
the voltage of the "8" rail according to possible variations of the line or mains
voltage.
If you're wondering whether this problematic conundrum of parameters is
a cinch to solve and can be looked up in a tube manual (aided by a slide rule or
calculator and a ballpoint-pen) we would like to assure you that it definitely is not.
Importantly, it involves the turns-ratio of the output transformer and the anode-
to-anode impedance rating one elects to award a given set of output tube types.
This, along with how one treats the screen grids, in tum affects the comfortable
power the tubes can produce [.ree TneNsnonuens elsewhere in this chapter]. We
have evolved our own set of tube "knee" curves and tables as they relate to the
type and style of tubes we use, treating the information contained in the old tube
literature as approximate guide-information only almost no two tube-manuals
give the same answers and, in any case, the tubes - are being made so differently
from those covered by out-of-print manuals.
VTL amplifiers, with two exceptions, operate in Ultralinear Class A1 for
mrtst of their power output. The first exception in the VTL range is the lchiban,
which is pure Class A triode all the way and, at 200 warrs, is not very efficient
for its weight, size and cost, because one is selecting from the "linear portion" of
the tube's knee-curves, only a 'B'-rail voltage of the lower order can be used, to
keep the required bias-versus-grid voltage swing in the correct proportion. (This
is why, say, a 6L6 pair would only yield 7 to l0 warts in pure'Class A'.)
These last-mentioned factors are inherently a limitation of pure Class A
and cerlainly the main reason why very few Class A tube amplifiers have ever
been in commercial production: the notable American engineer, Ulrich Childs,
produced one in limited quantities, Morikawa-san of Japan still builds a couple
and the Lux rra,,u'r Corporation of Japan have built somc smalìer designs in the past,
including that S-watt single-ended version we mentioned elsewhere. The French
NEGATIVE BIAS 55
firm Jadis builds a 160 watt unit using four pairs of KT88's; four pairs of this
tube{ype can yield 400 watts but not in Class A!.
The VTL exception is the - deLUXE 120 monoblock amplifiers which are
able to switch.over to a half-power 50 watt Class A mode from their regular AB1
tetrode mode. Lest you think that 50 or 100 watts is possibly a little puny for
your aspirations, please allow us to explain that Class A triode-watts are a whole
different matter when it comes to loudness or apparent power! In a class of their
own, one might say. Though fixed bias has also been used by other nations, they
seemed more in favour of the alternative method ol biasing, loosely described as
"cathode bias" or sometimes called "self bias"; American manufacturers, wisely
in our view, virtually disregarded this approach.
In clear-cut terms, "fixed bias" indicates not the'Class' of operation, but
that there is a fixed amount of negative voltage, by way of a separate rectifier
circuit, available and intended for the output tubes'bias requirements. With the
"cathode-bias" system, the output tubes get their bias from a resistor (or group
of resistors in a balancing-circuit) connected between cathode/s and ground. This
component was commonly referred to in tube manuals as "the cathode-resistor".
Quite often the manual would quote a viable "design centre" value of resistor for
the cathode under or near the heading of Class A. However, the mere presence
of a cathode resistor (and the absence of a negative-bias power-supply) does nor
necessarily a Class A amplifier make...Many is the number of times this mistake
is made, probably in all innocence, arising out of a mis-interpreted notation in the
tube-manual. If you wish to study and understand correct "Class" nomenclature
and descriptions, we have authoritative definitions in the appendices. We include
them because we feel this is lfte single most-often confused (and sometimes
downright misleading) issue ever mentioned about tube equipment.
In the high power genre of cathode-bias amplifiers our experience has been
that the tubes themselves run hotter and age faster; it becomes a race (unto death!)
as to how soon the tube will demand that the cathode resistor provide a greater
current-flow than its wattage-rating can handle. This discrepancy will cause either
the tube or the cathode resistor to bum out: the single most common failure in
older tube amplifiers of this cathode-bias conflguration. Installing a monster
cathode resistor of, say, ten times over its calculated current rating will solve
its burning-out problem, but won't address the highly undesirable under-chassis
heat; nor will it change the varying current-drain by the tube.
So why did the Europeans go more strongly for this method? Well, one
reason is cost, though we feel that the saving in components is so small as to be
insignificant (especially when measured against later costs in tube-life and other
repairs). Another probable reason is the power factor. In Europe, a 12 watt tube
amplifler is not uncommon, and 50 watts is deemed to be pretty high power.
56 THE VTL BOOK
Hence on a ten to twenty-five watt unit, the cathode-bias system would seem to
be acceptable. We would still elect to use fixed bias, even if the amplifìer vr'ere to
be of a five watt size. The American market has always favoured higher-power
amplifiers which, of course, has to do with the prevailing sizes of rooms as well
as loudspeaker elfic iency.
When, Why and How To Set The Bias In A VTL Amplifier
The aforegoing pages on bias highlighted our design objective, which specifies
that you should be listening to the amplifier in prelerence to measuring or worrying
about the bias setting! And truly this is so we have many amplifiers in
continuous daily use over three and four years -that have never had the bias re-
set. It is the individual tube itself that dictates ihe amount of bias required, and
therefore we go to a great deal of trouble and expense to fit tubes of known
high and stable quality; we match them closely, culling out those that indicate
excessive current draw.
The absence or lar-ele shortfall of negative bias in output tubes makes itself
very apparent when the anodcs (the large dark-gray portion inside the tube) start to
run red-hot: cherry-red is the approved term. Not many seconds in this condition
will terminate the tube. If all the tubes' anodes suddenly glowed cherry-red, the
quick diagnosis would indicate total faiÌure of the bias supply. Because of the
design and construction of our negative supply, we can tell you honestly that this
type of failure has never occurred and is very unlikely ever to occur. A much
more probable cause for all the anodes running cherry-red would be serious low-
frequency oscillation coming from outside the ampìifier, which could "swamp"
(negate) the bias [see Oruen Axcrllenv Equleuar].
We "stand" our output tubes on l0 O resistors (5 A in the older 300 and
500 watt units) which allows easy measurement of the bias current without "open-
ing" the circuit and thereby having to rely on some jack-socket's switch-contact
to re-close the circuit, and which also acts as a final fuse in the rare event of a
tube going full-short intemally. We run the output tubes at around 30 milliamps
quiescent current, which is a reading of 300 mV across the 10 O resistor by
Ohm's law.
There are a great many tube amplifiers around whose design calls for a
much higher "standing current" as measured in the quiescent (i.e., no programme)
state of up to double that of ours: 60, 65, even 70 milliamps. This approach is
taken to get more output power from a lower "8" rail (ofien 450 Volts) and
to combat distortion of the "crossover" or "notch" type. We do not applaud
this choice, believing that the distortion could, and should, have bccn taken care
of elsewhere. This high-current thinking is thc very thing our designs seek to
eliminate; it leads to dramatically shortened tube life, hotter-running amplifiers
TRANSFORMERS AND FEEDBACK 57
and a bad name fbr tubed equipment among audiophiles and public alike. True,
one can "lean down" any amplifier's bias (to a point where the tubes are just on
the edge of starvation and failure) with a tiny measurahle (not audible) decrease
in distortion, particularly at lower power, but we regard this as wanton waste of
tube lif'e and totally unnecessary in well-conceived designs.
When the fitting of an entire set of new output tubes is contemplated, it
would be desirable to have the bias re-set, and then only if one suspects that the
new tubes were of unknown quality.
We stress and continually repeat that our equipment is only to be
opened and serviced by experienced and trained personnel. If you feel you
have a problem, we earnestly recommend that the amplifier be brought to a
dealer or service engineer. Dangerously high voltages are present even when
the unit is switched off and/or not plugged in to the wall-socket.
Output Transformers and Feedback in Power Amplifiers
"To be or not to be." Will's Audio
-
"To have, to hold and to love." Marriage Vows
-(direct
A power amplifìer requires DC current) to operate whether it's tube or
solid-state. Loudspeakers, on the other hand, require the amplifìcation power but
with a total absence of DC; and therc's the rub.
We have to harness that AC voltage (the music) and present it to the loud-
speaker at a workable matching impedance, and simultaneously get rid of the
DC, without impairing the music. There are a few ways of achieving this: by a
coupling capacitor, which can colour the music; by having a sensing circuit which
identifies and cancels the DC known as a'servo' system (but these can and do go
out of adjustment and let some of the DC sneak through); or by utilising a trans-
former which neatly 'transforms' the AC programme volts to a suitable amount of
current and makes the impedance conversion and positively will not,indeed can
not, pass DC.
If a transformer is not of premium quality (and therefore very costly to
make) it can also colour and detract from the programme greatly. Hence the
obvious statement "If you can't have a good transformer, better to have no trans-
former" is very true. But f you can have a good transfomer, there simply is no
better coupling device. It is also true that to omit the transformer is easier yet with
solid-state than with tubes because the output devices are at or near the common
speaker impedances. Interestingly, the revered Mclntosh Corporation (who set
standards of build and finish in tube amplifiers of old that we unashamedly en-
deavour to emulate) chose to couple with a transformer when they changed over to
solid-state. (Oh, rue the day! We still hold regular mouming sessions with friends
58 THE VTL BOOK
who get us to up-grade their treasured antiques). Also, it is possible to choose
and configure tubes to work in the mode that New York Audio Labs/Futterman
chose to register as a trade-mark, O.T.L. (Output Transformer-Less); using meth-
ods of DC-exclusion commonly found in solid-state and configuring the output
tubes nominally as "totem pole" cathode-followers, or cascode followers, to bring
down the impedance.
This technology has been tried by all of us on the test-bench. Others have
elected to market it; we have not and will not. One of our colleagues in another
tube-amplifier company (we do not see our opposition as rivals; to us they're
friends and brothers) who, when asked by us why he chose to build OTLs, looked
us straight in the eye and said "quite honestly, because I do not know as much
as you do about transformers and nor how to get my hands on good ones." That
man has our respect; if he ever asked for our help on output transformers, he'd
get it right swiftly. And some of the OTLs produce some pretty good sound,
often outstanding sound, on their favourite 'demo' Quad electrostatic speakers.
A point of interest here is that it is easily possible to tailor-connect ESL speakers
very directly with tubed OTL amplifiers, rhereby omitting the ESL's own inrernal
transtbrmer/s, often with surprising results.
The reason, we fèel, that OTLs are often demonstrated with ESLs is that
the 'sound'being highlighted is of the kind that one wouldn'r be seeking anyway
from ESLs that were un-augmented by cones in the bottom: a sweet, smooth
presentation very suited to baroque music, not noted for deep, sonorous bass
content. The why of this is not hard to find either: the ESL does not go all that
low, so the ultra low impedance that a dynamic speaker reaches on extremely
low-frequency information which could embarrass the OTL does not arise (and
the demonstration will not include a thundering pipe organ with 26 cycle notes).
OTL amplifiers suffer when the impedance drops to very low values. Pub-
lished specs state "100 watts into 8 ohms, 35 watts into 4 ohms" and the (un-
published) information would continue "20 watts into 2 ohms" et cetera. But the
matter of the loudspeaker's propensity to decrease in impedance as frequency is
lowered does not end with OTLs; it affects transformer-coupled tube amplifiers
too, but to a much lesser degree. It affects solid-state amplifiers too, but incremen-
tally as we all know from reading the ads and specs. Such as in the hypothetical
(but true) promise: "100 watts into 8 ohms, 200 watts into 4 ohms, 400 watts
into 2 ohms." This is meant to maximise, truthfully so, the subject solid-state
amplifier's power capability into (nominal) speaker impedances, because being
current-producing devices, transistors do operate this way (as per Ohm's Law).
It is probably quite clear that we have a preference fbr output-transformer
coupled amplificrs since we have made a specialisation of their design and how
to build them. We go to great expense designing and winding output transformers
TRANSFORMERS AND FEEDBACK 59
up to and beyond the specifications laid down by Williamson; the shorter cuts
and 'quickie' methods of design and execution don't produce the results that we
or you are looking for. Designing an output transformer, particularly the high
power models, presents the ultimate "Catch-z2" t you need to handle very low
frequencies at high power, so mandatorily (in terms of powcr, or watts) you need
to make it big and heavy with high flux-density. As you start to get bigger, so
advances proportionately the squeeze on the high frequencies; understandably, the
magnetic 'window' is increasing with the diameter of the bobbin, the distances
are getting greater, leakage-inductance starts going up and the upper frequency-
response starts to go down out the window, we say. To combat this war that
physics wages on the designer,
- he has to rely on precision hand-work in the actual
winding of the unit's bobbin. In the hand-winding stage of a high perfbrmance
audio transformer (though a motor actually rotates the bobbin for reasons of speed
constancy) portions of the primary (input) winding are interleaved with portions
of the secondary (output) winding and again, sub-portions of the various portions
of a given winding are laid down, like fingers on a hand when moved close
together, and this is known as bi-filar and tri-filar winding (when three windings
are so laid). Doesn't sound all that difficult, does it?
The real problem comes in the planning of the winding-design so as to
split the sections in such a way that the multi-split (ever-changing in diameter,
and therefore length) is evenly shared over the sections so that each row (as in
knining) is exactly and evenly filled. So when we talk about "nineteen sections",
remember also lhat every interleaved section is separated with a thin insulation
dielectric. This must not have the merest hump or surface unevenness to present to
the next winding, because a loss of high-frequency information will occur through
leakage-inductance. All in all, we are talking of a very carefully conceived and
executed piece of art.
Having conquered the problem of size hurting top response through leak-
age inductance and other pitfalls, we routinely build our transformers to exceed
nominal power by hfty percent and the frequency response to go down cleanly at
power to two octaves below what is considered to be the bottom limit: 20 cycles
(Hz) one octave lower than that is 10 cycles, and one octave lower yet is
- the power spec to which we work. The theory and practice is that we
5 cycles,
want the amplifier to produce tight, clean bass at the frequencies that involve
you and the loudspeaker, say 30 cycles; we therefore have to get the unit to start
getting control at the so-called sub-sonic frequencies. Then at the point where the
music enters the loudspeaker's realm, the ampÌifier has gotten rock-solid control
of the bass.
As you will see, negative t'eedback is importantly related to this discussion
because it affects overall perlormance of the transformer and its output impedance.
-
60 THE VTL BOOK
First a little background: the benefits of overall negative feedback were first doc-
umented by a young engineer, Harold S. Black, in 1928. He found thar vast
improvements in linearity and distortion could be achieved by taking a small
portion of the output and re-injecting it back into the front or input of the ampli-
fier. Feedback started to appear in many units made by professional equipment
manufacturers such as Western Electric in theatre amplifiers before World War II.
This idea of inducing feedback was not the first knowledge of the principle,
for when the American Lee De Forest first "upgraded" (you see they were at
it already!) Fleming's simple diode by adding a grid to make the first viable
triode, he soon discovered that the tube inherently had 'intemal' feedback, and the
benefits were noted. Later designers like Williamson (in 1947) showed elegant
ways of sampling the feedback and re-introducing it to the cathode, and also
proved that a discreet amount (under 20 dB) of feedback (like spices in food)
was good, though too much caused phase-shift and 'slowness' or poor transient
response, among other things. Much later still, early designers of solid-state
equipment, struggling with the new technology, quickly spotted thar a sufeit of
feedback (40 to 60 dB) could be very useful in panel-beating lousy circuitry into
some sort of reasonable-looking measurements (on the 'scope, not the ear) and
negative leedback started to gather an unsavoury reputation.
Negative-feedback (always we stress: used in sensible quantities) has a
greater influence than would seem apparent on matters of output and output-
transformers. It majorly reduces output impedance when taken from the 'driving'
secondary winding, in our case reducing the nominal impedance to less than
one tenth of an ohm. This increases the amplifier's damping-factor to what
we feel is optimum at around 25135. (And a very controversial subject is this
matter of damping-factort True damping factor is too complex to go into deeper
in this treatise.) Yet more importantly, by having the feedback coming from
the high end of the winding that is actually connected to the loudspeaker, one
is able to an extent to embrace both the speaker and cable within the feed-
back loop with very beneficial results. This phenomenon arises by 'back-EMF'
(electromotive force emanating backwards from the speakers' magnetic motor)
and is more complicated yet to explain herein.
Because we use this feedback-lowered impedance and transducer-embracing
EMF to good advantage, we do not have our transformers wound 0, 4, 8, 16 or
0,2, 4,8 ohms as presented taps; our system requires that the entire transformer
be included in the loop. When you have a 0, 2, 4. 8 tapping approach and
the speaker is connected to 4 ohms, say, the feedback will be connected to the
'high' or 8 ohm tap, leaving that distance in the transformer secondary. Move
the feedback link down to the 4 ohm point where the speaker is attached and you
TRANSFORMERS AND FEEDBACK 61
have the rest of the transformer "flapping in the breeze" and w,astin,q, valuable
current (Amperes!) that could and should be channelled into - the loudspeakers.
We, like Williamson, have a multi-section secondary and we combine these
sections in series-parallel inside the transformer for maximum effect. In the case
of our transformers of 100 watts and up, we bring these points or taps out to
extemal lugs marked A through H, so that the connection/s can be made to suit
lsee transformer drawings in the Appendrces]. Another reason why we don't
subscribe to the 0, 2,4,8, method, and this is vitally important, is that when then
output impedance is altered to another value so accordingly should the feedback
be changed, to optimise the interface: and you can't do that with a screwdriver.
While it would seem obvious that 4 ohms is half of 8, that unfortunately is not
the way of it: in impedance calculation and measurement, 4 ohms is seven tenths
of 8 ohms and so on.
What of the so-called 'hybrid' designs, which employ a mixture of tube
and solid-state technologies? Well, this leaves us in a head-shaking state of
bemusement! It would seem that one school says that nothing is as good as tubes
in the front-end driving a solid-state output-section while the other school says
that tubes have no equal as output devices (in normal transformer coupled mode)
yet believe that solid-state has the edge as input and driver cicuitry. For the record,
we confess to having experimented with both variants and long ago decided that
neither equals, let alone beats, pure legitimate tube technology. The Radford
company in England brought out their TT 100 some fifteen years ago which had
a transistorised front-end driving KT88's, and the comparison between this beast
and their regular STA l5's and STA 25's was something frightful to behold. Our
knowledge and experience of hybridization in professional equipment, too, has
always seemed to tell us that a gap has been found in design and construction
that saves a bunch of money.
The Ultra-Linear Aspect
All of our power amplifiers (except triode units as noted) employ this excel-
lent output transformer concept. The idea was brought to new attention by
Messrs. D. Hafler (the same David!) and H.l. Keroes in 1951 through the pages of
the joumal "Audio Engineer". Invented rn 1931 by that titan father of stereo itself,
Alan Blumlein, the method was known in Britain as "partial triode operation",
which explains itself rather clearly.
In standard operation, the screen-grid of a tetrode or pentode is connected
to the 'B' or high-voltage rail (or a separate and possibly regulated 'B' is created
for it). The tube will give its maximum power with a somewhat higher (though
very small) amount of distortion. When the screen is joined with the anode or
plate, the tetrode functions in triode mode, yielding considerably less than half
62 THE VTL BOOK
power, but with the triode's lower distortion. "Partial triode" positions the screen
grids at a point (a tap in the transformer primary) somewhere in between the
anode and the centre-tap (the 'B'-rail); the closer this point is to the anode, the
closer the tube behaves as a triode. Going closer to the centre-tap produces more
of the tetrode performance.
What seems to be not generally known in some quarters is that the'magic
point' differs for the various types of output tubes and the class in which they're
positioned; also the extent to which the screen grid then functions as 'more
metal' to the anode and affects the permissible class and voltage maxima of
operation. When the screen is at some mid-point, the tube is functioning as a
tetrode basically but with internal negative feedback applied to the screen. We
have carefully selected the percentage of primary used in our ultra-linear designs
to maximise power output without giving up the triode benefits; though we choose
not to disclose these ratios [see TneNsronusn ScHruelcs].
The Lile and Availability of Tubes for VTL Equipment
As thoroughly trained design engineers who have made a life-long study of tube
equipment, we find this a particularly irksome subject. A question (statement,
sometimes) such as "...and tubes don't last very long, doooo they?" from an
unsuspecting member of the public (who is often under 25 years of age and has
never owned tube equipment, or deflnitely not ours) is apt to make us reach for
the telephone to ask our lawyer if he is prepared to defend a charge of willful
AGB (assault with intent to inflict grievous bodily harm).
Whence cometh this uninformed misconception? Don't people remember
(or talk to older family members who do remember) how reliable and trouble-
free the radios and phonographs of the Thirties, Forties and Fifties used to be?
Many are still in use today by some of our Grannies and Grandpas. Don't people
know that almost all professional musicians use tube amplifiers (and these are
manhandled in the roughest ways) to earn their living on the road? Don't people
know how many cinemas, drive-in movies, small churches, meeting and concert
halls, broadcast stations, ham-radio transmitters, teìephone link-ups, or how much
medical, measuring and nav igation/radar equipment depend on tubes for maxi-
mum failure-proof service? Is it not well enough known that Soviet Russia uses
almost exclusively tube equipment for military and strategic communications? (In
point of fact, their reason for this is mainly the tube's ability to stay operational
in nuclear radiated areas so you'll make sure that when you plan your next
- only with tube equipment.)
fall-out shelter, you'll fit it
Ocldly enough, it would seem to be fixed in some people's minds that
because a tube can be readily unplugged and replaced upon failure, it follows
that frequent failure must be part of the scheme of things. Yet wouldn't the
TUBES FOR VTL EOUIPMENT 63
same folks bc absolutely thrilled if they knew that a brand-new motor could
be instantly plugged into their automobiles at low cost and without tools or
specialised knowledge? That they could in fact perform the switch themselves?
Yes, it is true that there has been unreliable tube equiprnent (as there
have been unreliable cars, computers, refrigerators, solid-state amplifiers, cas-
sette decks, and so on). But those products have, by and large, fallen into the
lollowing categories:
I )
equipment from smaller firms which have not had sufficient design knowl-
edge, nor the money to get such knowledge;
2) equipment that has been vastly over-complicated (often with much solid-
state hybridization), thereby getting away from the inherently advantageous circuit
simplicity;
3) equipment that has been "designed down" to a price point, thereby com-
promising the quality of many components that surround the tubes;
4) equipment that has utilised poor-quality tubes, which, make no mistake, did
and do exist.
If one looks back to the time when manufacturers such as Marantz, Fisher,
Scott, Brook, Mclntosh, Harman-Kardon, Dynaco, Leak, Lowther, Luxman, and
Lansing (to name a few) were in full amplifier production, can one honestly say
that unreliability was ever really a major factor?
You see, if one takes a given topology for, say, a givcn gain stage (e.9., an
input triode) which would consist of a tube, a socket, an anode resistor, a grid-load
resistor, a cathode resistor and probably a decoupling capacitor, plus a coupling
capacitor to the next stage, there is no way it can or will fail (provided, of course,
that the wattage and working voltages of the resistors and caps are correct). Now,
though, try surrounding the tube with solid-state technology: current-source the
anode with a Darlington-pair, ditto the cathode and top it off with a complex
regulator for that stage only. Then when (not ifl) it breaks down, you'll be able
to say that tubes are unreliable! Moreover, it will not sound like a tube while it
is working.
The same is true for an output stage (or stages) if the tube is being asked
to run at a higher current (heat, therefbre) than that for which it was intended.
It is very true that more poor-quaiity tubes abound nowadays due, obviously, to
the tube's being the less commonly used technology and because some top-class
manufacturers have shut down. Sadly, it was the so-called "smart" countries that
raced out of tubes first. Good tubes are damned hard to make and are akin to
analogue mechanical \ /atches. [Remember them? You used to wind them up and
they never gave you any problems.l This is because the factory must be staffed
64 THE VTL BOOK
by a large number of nimble-fingered, dedicated gnomes wielding tweezers and
wearing eye-pieces.
Tube Types We Use, And Some We Don't
Fear not, very good tubes are still being made; in America, we have become
very close to MPD (successors to the General Electric plant) whose products are
distributed exclusively be Richardsons, a multi-million, enterprising firm that has
offices and warehouses world-wide. The Richardson Corporation saw the vital
importance of continuous quality-tube supply early on in the game and, as and
when certain plants ceased production, Richardsons has picked up the tooling,
moved it to La Foxe, Illinois, and kept strategic types continuously in production.
One of the key aspects of VTL equipment is that we design for tube types
that are easily available from known premium-quality sources; and besides the
premium sources we prefer, our chosen types also have altemative (but lesser
quality) options available world-wide from other factories.
The GE-MPD factory worked continuously with us to produce a VTL SQ
(Special Quality) 6550A the "A'version is 15 to 20 percent more
-
in anode dissipation (42 Watts) than the regular 6550 (35 Wafts) and GE-MPD
are the only people in the world able to build it. We Iìt this tube standardly as
well as the GE 6CA7lEL34. The latter piece is even more rugged than the historic
British Mullards we started with. GE also builds our favourite 6201's (the military
version of the l2AI7) and our l2BH7 driver tubes.
The mainland China Shugang factories (and affiliated brands) build an ex-
cellent l2AX7A to a continuous quality that has almost become an industry stan-
dard. We have put hours of testing into their output tubes too, but so far remain
unimpressed with them: they have neither constancy (sample to sample) nor
longevity when used at or near rated specification. They rlo not offer a genuine
6550 yet, preferring to use a KT88 variant-structure in a bulb marked 6550. (The
M-O Valve company authorised the Chinese to build the KT88 when the British
factory closed but the new version doesn't get close to the original on any
parameter. In any- event, the 6550 and KT88 are different, though they do have
some similarities. )
One can often fit a KT88 (a beam tetrode) in place of a 6550, but seldom
the other way round. The KT88 was designed in Britain using the American
6-550 as a model, but was built more ruggedly and can take very much higher
voltages on its screen grid and anode. The KT66 has similar ruggedness and can
approximately be compared to a 6L6 and an 807, though both the KT66 and 807
types can handle higher voltages and power than the 6L6. The 70274 was closer
in beef to the '66 and '88, but is now hard to come by in its original form, having
been approximately replaced by the newer 8417.
TUBES FOR VTL EQUIPMENT 65
A word here about tube qualities, even those bcaring the identical type
number: the EL34I6CA1 is a prime example. This tube was one ol the last to be
designed, and went into near-simultaneous production all over the world. There
was an industrial version made by Siemens & Hatske of Germany that was so
ruggedly built one could push the anode voltage up to around 1000 volts and get
close to 100 watts lrom a single pair! The average tube manual shows a maximum
design-centre voltage of 800 volts: but this will refer to a premium American or
British piece. However, there are two or three East European examples of these
types that are visibly slimmer and lighter in intemal construction, known in the
trade as "Continental Slimmies". Some of this style of EL34 are fairly well-made,
but simply not comparable with the best of British or American items. Apply
800 volts to the anode and, as they say on Brock's Crystal Palace lìreworks,
"stand well clear!"" We run EL34's at 500 or 525 volts.
It is because all of our designs run their tubcs so conservatively that they
run so cool and stay working so long. If in a pinch you'fit some "Slimmies" to
a VTL unit however, you will get neither the power output nor the longevity of
the original tubes fitted by us. Other tubes we have used are:
8417: A truly fine output tube though not, in our view, widely enough available.
EL84: (US equivalent 6BQ5.) A nine-pin miniature Noval, commonly regarded
as the smaller brother to the EL34, and electrically similar to the older, octal 6V6.
The EL84 was used in some of the early VTL models (16, 25 and 30 watts) and
we nill relurn to it at sotnc timc.
7868: A tube in a large-miniature Novar nine pin base which we highly favour.
No other type substitutes for this base-wise, and were becoming harder and harder
to obtain.
KT66: (Well, not exactly, but...) we have recently sourced a Russian military-
grade substitute close to a 5881, but with higher maximum permissible volt-
ages. We are very
- pleased with this tube and feature it in our "Custom" 80 Watt
monoblocks.
KT88: Oh, what a tube this was in its prime version: only made in Britain by
IVlullard-Osram and co-marketed as the Genalex "Gold Lion". Even in the final
couple of years before M-O's closing in June 1986, the quality had started to slip
due to imperfect machinery and dwindling personnel.
EF86: One of the best miniature pentodes ever. We do not favour pentodes as
an input or preamplifying device because of noise and distortion.
6L6: (Also available in heavier duty versions suffixed 'G' and 'GC', as well as
the smaller-enveloped 5881.) The 6L6GC is as tough as the proverbial army boot
and will deliver good clean power for ten years and more in musical instrument
amplifiers. Yet, apart from older equipment such as Mclntosh, almost no current
high end items utilize it. Most likely