VAC PA100/100 POWER AMPLIFIER
Operating Instructions
DONOTOPENTHISUNIT-NOUSERSERVICEABLEPARTSINSIDE. DONOT
REMOVETHEBOTTOMPLATE. LETHALVOLTAGESAREPRESENTWITHIN
THE CHASSIS. DO NOT EXPOSE TO MOISTURE OR OPERATE IT IF WET
VACUUMTUBESBECOMEHOTENOUGHTOCAUSESERIOUSBURNS. NEVER
TOUCHATUBEWHENTHEUNITISON. ITMAYTAKESEVERALMINUTES
FOR THE TUBES TO COOL DOWN AFTER THE UNIT IS SWITCHED OFF.
DO NOT PLUG INTO AC UNTIL ALL CONNECTIONS ARE COMPLETED.
Introduction
TheVACPA100/100isasinglechassiswide-bandlow-feedbackstereo
amplifierconservativelyratedat100wattsperchannel. Itistobeoperatedonly
fromthevoltagevisibleontheconfigurationcardlocatedunderthefuse. Please
contactVACoryourdealerifthevoltageinyourareadoesnotagreewiththe
label. Thefuseisaslowblowtypeofthesizenotedonthebackofthechassis.
A tube cage is available as an option.
TheinputcircuitofthePA100/100isderivativeoftheWilliamsondesign, which
wasfirstpublishedinEnglandin1947, andwasthemainstayoftheoriginal
highfidelitymovement. Notablefeaturesofthiscircuitincludeadirect-coupled
inputamplifierandphasesplitter, resultinginlowphaseshift, andinherent
precisebalanceofthephasesplitter(ifproperlyexecuted)atallfrequenciesand
levels, resulting in excellent push-pull balance, stability, and low distorti
TheoutputstageisderivedfromtherenownedPA90C, featuringindividually
adjustable "fixed-bias" and partial-triode ("ultra-linear") operation.
Premiumpartsareemployedthroughout, including14pound22sectionbifilar-
woundultra-widebandoutputtransformers, polypropyleneandpolystyrene
signalcapacitors, highcurrentrectifiers, lowESRpowersupply, and
proprietarywiring. Thequalityofpartsandcarefullayoutallowstheuseofvery
little feedback for high sonic purity and low distortion.
Download from Www.Somanuals.com. All Manuals Search And Download.
Operation
Continuous operation is not recommended. If the amplifiers will not be auditioned for a few hours it is best to
turn them off. This avoids undue stress to the KT88 output tubes.
As with all high fidelity products, the sound characteristics of the VAC amplifiers change somewhat as they
warm up. Best sound will be achieved after 15 minutes of operation, with subtle changes occurring for up to
one hour. However, we advise against leaving the equipment on at all times because of the attendant
acceleration of output tube wear. Life of the output tubes averages between 2,000 and 8,000 hours depending
upon brand fitted and random variations within the tubes themselves.
Any time that the VAC Power Amplifier has not been used for a few weeks the sound may be different. This is
also normal for high resolution audio equipment. Optimum sound should return after a few hours of operation,
preferably with an audio signal.
Please note that although your VAC amplifiers have been run for 48 hours at the factory, they will continue to
"break in" for approximately 150 hours. Also be aware that many components display the need for a new break
in period after being transported in unheated cargo aircraft.
Bias Adjustment
Your VAC Power Amplifier has been shipped with output tube bias preset. This should be checked when you
install your amplifier, and periodically thereafter. It must also be set whenever an output tube is changed.
Adjustment of the output tubes is quite easy, and requires only a small screwdriver. On the front panel are four
bi-color LEDs, each with an adjacent adjustment control. View the LEDs from directly in front, not from above
or below. Adjust the LEDs to be green when the amplifier is not playing music. To set, turn the control
clockwise until the LED just begins to change to an amber/red color, and then back the control off just slightly
to restore the pure green color.
As music plays, the LEDs will vary from green to red. Check the bias if you ever see an LED stay red while
music is not playing, or if an LED does not tend toward amber or red when music is playing. If the flashing of
the LEDs is bothersome while listening, the switch on the front labelled “Bias Test” can be set to “0", disabling
the LEDs. The switch must then be set to “1" to check the bias of the output tubes.
Any tube that can not be adjusted to the correct bias point should be replaced. Any tube which requires constant
readjustment should be replaced immediately. Never operate the amplifier with an LED that shows red when
music is not playing.
When installing new tubes, set the bias controls counter-clockwise. Check the bias of both channels as the
amplifier warms up - don't wait ten minutes.
Bias levels should be checked monthly to ensure optimum sound quality. It is not unusual for the bias to change
with time, particularly when tubes are new. The greatest amount of drift occurs during the first 200 hours of
a tube's life. The drift may change direction periodically, such that the bias control must be increased and later
decreased, or vice versa. Check bias if the sound seems lacking in detail or dynamics.
Tube Types
The input/splitter tube and the driver tube may be type 12AU7A/ECC82/E82CC or type 12BH7 (typically richer
sound). The output tubes are type KT88. Please feel free to contact VAC if you require additional information
or advice.
Download from Www.Somanuals.com. All Manuals Search And Download.
Installing New Output Tubes
Replacement tubes are available from VAC and other sources. Output tubes are KT88 beam power Kinkless
Tetrodes (6550 may also be used - see Tips & Advice, Output Tubes). It is not necessary that they be matched
pairs, although a slight improvement in measured performance may be achieved in this way. Make certain that
each tube fits firmly in its socket. A tube that fits loosely may not make correct contact on all pins and might
"run away" (read on). It is best not to mix brands of tubes.
ALL POWER MUST BE OFF. Remove the old tubes after they have cooled down (TUBES BECOME HOT
ENOUGH TO CAUSE SERIOUS BURNS WHEN IN OPERATION AND MAY TAKE SEVERAL MINUTES TO
COOL DOWN). Install the new tubes firmly and fully in the sockets, taking care to observe the direction of the
locating ridge on the plastic center pin of each tube.
Follow the normal turn on procedure and begin the BIAS procedure. While doing this, keep an eye on the plate
(the outermost metal structure) of the output tubes. SWITCH OFF IMMEDIATELY IF THEY BEGIN TO GLOW
RED. This indicates that the tube is "running away", being destroyed rapidly by conducting excessive current.
(Note: with some KT88s a slight dull orange glow may occur over a very small section of the plate, usually at
an edge. This is acceptable and not the same as running away, in which most of the plate will become bright
orange or red.)
Tubes may run away for several reasons:
1)
2)
The tube is not fully inserted in the socket.
The tube fits loosely in the socket and thus can not make correct contact. Such a tube is unusable and
should be returned to its seller.
3)
4)
5)
The tube is defective.
The bias is misadjusted.
There is a problem with the amplifier. Contact VAC or your dealer to arrange service.
In the event that trouble is encountered, try another tube. Stop if the problem persists and consult with your
dealer or VAC.
Follow the BIAS procedure described previously in this manual to set the idle current at 70 milliamperes per
tube, checking the idle current at 1 minute, 30 minutes, 1 hour, and 2 hours.
Replacement of Low Level Tubes
All power must be switched off. Allow tubes to cool down. Remove and replace with new tubes of the
appropriate types, noting the location of holes in the socket and pins of the tubes.
Replacement tubes are available from VAC and other sources.
Ultra-linear or Triode?
The input amplifier, phase splitter, and driver stages are triode tubes operated in Class A with no cut-off or grid
current. The output stage is connected for partial-triode, well known for low distortion, good damping factor,
and high efficiency. This technique was invented by Alan Blumlein of EMI in England, and was popularized in
the US as "ultra-linear." VAC can convert amplifiers to triode connection for a small charge.
Download from Www.Somanuals.com. All Manuals Search And Download.
Care of Chassis & Face Plate
VAC chassis are aluminum for superior electromagnetic performance. The finish is a tough textured powder
coat paint. This finish is durable but can be scratched or chipped, just as that of a fine automobile may be
damaged. Cleaning with a damp cloth WHILE THE AMP IS SWITCHED OFF AND UNPLUGGED should suffice.
DO NOT use cleaning solutions containing harsh detergents or solvents. They may damage the finish or
silkscreened lettering.
Tube Cages (Optional)
Perforated metal cages are planned as a future option to enclose the vacuum tubes. Mounting directions will
be supplied with the tube cage kit.
Need Help?
Please call us with any questions you may have. It is better to ask than to guess. Our telephone number is
(USA) 941-359-2066. Our fax number is 941-359-2057. We can be reached by e-mail at [email protected].
Download from Www.Somanuals.com. All Manuals Search And Download.
Warranty
This amplifier is warranted for a period of thirty (30) days from the date of purchase. In addition, if the
registration card(s) is received by VAC along with a copy of your sales receipt from an authorized VAC dealer
within this thirty days, a service contract will be extended to cover your equipment for three (3) years (except
tubes). Receipt of your registration card will be confirmed in writing by VAC.
This warranty applies only to units sold to and operated by private individuals in the United States of America
through authorized VAC dealers. For warranty information outside of the U.S. contact the importer of VAC
equipment for your country. Units sold outside of the U.S. should still be registered with VAC.
Valve Amplification Company
4366 Independence Court, Suite C
Sarasota, FL 34234
Tel 941-359-2066
Fax 941-359-2057
-------------------------------------------------------------------------------------------------------------------------------------------
Mail this form along with a copy of your sales receipt as soon as possible to:
REGISTRATION FORM
VAC PA100/100
Name
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_______-_______-__________________
Address
Telephone
Dealer name
__________________________________ Salesperson ____________________
Dealer address _________________________________________________________________
Purchase date ___________________________ Serial Number___________________
How did you first learn of VAC products? __________________________________________
Please provide any comments on VAC products or your dealer _________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Download from Www.Somanuals.com. All Manuals Search And Download.
Tips & Advice
A Word About Tubes in General
It is a truth that each brand of tube sounds different in a particular high resolution circuit. This is because no
two manufacturers make a tube type in quite the same way, and the central tendencies of the performance
parameters will differ slightly with each maker. To emphasize the point, examine the plate structure of any two
12AX7 from different manufacturers. You will probably find that they may not even the same shape and size.
(Be careful here, as often a tube is made by a firm other than indicated on its label. In the heyday of tubes it
was common to crossbrand between major labels, such as GE and RCA. Today many labels do not manufacture
their tubes at all, including Gold Aero and RAM.)
This sonic variability may at first seem a liability, but further thought will reveal that it is an advantage, just
like the ability to adjust VTA on a tone arm. The owner of a tube amplifier can select those tubes which sound
like the real thing in his/her specific system. Of course, if the manufacturer you prefer is rare you may want
to purchase a few spare tubes for the future.
How long should tubes last? It has long been known in professional circles (and probably now forgotten) that
a tube such as the 12AX7 will display better performance characteristics after two years of continual operation
than when it was new. In normal use it is not unusual for a low level tube to last 10 years or longer. Output
tubes are another story, as they are continually providing significant amounts of current. Here the sound is your
best guide. Certainly tubes should be replaced when the amplifiers can no longer meet specifications or when
(if you have access to a tube tester) the tube's emission is significantly down or its transconductance is
substantially out of specification. In normal use, output tubes will last at least 2 years and perhaps more than
10 years.
VAC will be happy to test tubes for concerned customers. For appropriate fees we can provide
transconductance, idle current and/or a photographic record of the tube's plate family curves.
Download from Www.Somanuals.com. All Manuals Search And Download.
TIPS & ADVICE
A Word About Output Tubes
Your VAC Amplifier can use 2 different output tube types: the KT88 Beam Power Kinkless Tetrode and the
6550A Beam Power Tetrode. We consider the sound of a good KT88 to be superior. Properly manufactured
6550/6550A will work, but may have a slightly higher incidence of failure in the PA100/100. Feel free to
experiment with different brands and types to customize the sound to your tastes.
As with interconnects and speaker cables, each tube manufacturer's KT88 tends to have a distinct sound when
used in VAC amplifiers, and of course the 6550As sound different as well. Here is a brief summary of our
experiences with currently available tubes from different sources.
M-O Valve Company/Genalex/GEC KT88 (U.K.) PRE-1970:
An accurate and sweet tube with good bass quality. Quality is surprisingly variable for a premium tube.
World wide supply is limited. Highly recommended.
M-O Valve Company/Genalex/GEC KT88 (U.K.) POST-1980:
Hard, glaring sound, only fair detail. Not recommended.
VAC Tested KT88-T:
The best of the versions from China, and our current first choice. Good combination of punch, air,
sweetness, and detail. Reliability is very good.
Golden Dragon KT88-M:
No longer available from production, but an excellent tube. A bit more matter-of-fact than the KT88-T,
having more of an NOS US tone, but still preserving the airiness of the KT88.
Golden Dragon KT88, KT88 Classic, KT88 Super:
Generally fairly good for sound; reliability somewhat inconsistent.
National NL-KT88-USA:
A version produced by the US arm of the Richardson's organization. Visually very similar to
the original and sonically satisfying, slightly lightweight sound, but sweet. Very expensive.
Svetlana KT88:
Several versions have been made, so it is difficult to give a blanket comment. The newest version (as
of summer 2000) shows promise, with a sound somewhat similar to the GE 6550A.
US made RCA and Tung-Sol 6550:
Early 6550 type in the "coke glass" bottle. Pretty good sound.
GE 6550A (applies to those manufactured in USA by MPD):
Somewhat soft and lean, with a slightly buzzy character, but fairly sweet.
Phillips/ECG/Sylvania 6550A:
Similar to the GE tube; later production samples may have a somewhat hard upper midrange.
Sovtek 6550:
Samples tested do not bias correctly, having insufficient current flow. Not recommended.
Download from Www.Somanuals.com. All Manuals Search And Download.
TIPS & ADVICE
A Word About Low Level Tubes
The small tubes (Voltage Amplifier/Phase Splitter and Driver) in the VAC Amplifiers are type 12AU7. This tube
is essentially the same as types 12AU7A, 5814, 6189, CV4003, ECC82, and E82CC. The quality of the tube
used is by far more important than which of these (equivalent) types is used.
The preferred 12AU7A is the British/Chinese Golden Dragon. Acceptable alternate versions include the Brimar
CV4003 (ladder plate version), Mullard CV4003 (very midrangy), generic Chinese 12AU7 (brighter), Sylvania
6189 (darker sound), the GE 12AU7A (slightly forward and grainy) and the Yugoslavian 12AU7 (cohesive and
somewhat coarse).
Many classic tubes worth trying if you have access to them, such as those from Telefunken and Amperex,
although a caution is in order, as we have recently seen East German EL34 relabelled "Telefunken West
Germany." Tubes actually manufactured by RCA, Westinghouse, and Sylvania can be quite good, but the name
on the tube doesn't always indicate who actually made it. Other names to watch for are Valvo, Mazda, Tung-Sol,
Bendix, Mullard, Brimar, and Raytheon.
TIPS & ADVICE
A Word About Bias Levels
The output stage is designed to operate in rich Class AB1 at an idle current of 70 milliamperes per tube, at
approximately 535 VDC across the tubes. For more technical information about classes of operation see VAC
Technical Monograph 90-8.
Some designers erroneously market a lower voltage and current operating point (as little as 26 mA @ 550
VDC) as "Class A ." Be assured that the VAC and its tubes are strong enough to handle the more demanding,
1
richer bias level.
Download from Www.Somanuals.com. All Manuals Search And Download.
TIPS & ADVICE
A Word About Impedance Matching
We strongly suggest that you experiment with the three available impedance connections for the best sonic
match with your system. Since no loudspeaker represents an unchanging impedance at all frequencies, it is
impossible to assert with certainty which output tap is appropriate to use. In many systems an amazing
difference in sound will exist between the various impedance taps.
You should consider the output impedance markings on your VAC Power Amplifier as follows:
"8 ohms" matches loads between 4 ohms and 8 ohms
"4 ohms" matches loads between 2 ohms and 4 ohms
"2 ohms" matches loads between 1 ohm and 2 ohms
Most loudspeakers vary outside of any one of these ranges, which is why experimentation is essential. We often
find that matching a speaker's minimum impedance is more important than matching its nominal (average)
impedance.
If you bi-wire your system (run separate speaker leads from the amplifier to the high and low frequency
transducers) you may discover that two different impedance taps work best. For example, with early Martin
Logan Sequel II we find that the bass speaker is best matched with the 4 ohm tap, while the electrostatic panel
is best controlled by the 2 ohm tap. To achieve this connection, the black leads of both speaker cables connect
to "G", the red lead for the panel connects to "2", and the red lead for the woofer connects to "4". With later
Sequels we use the 4 ohm connection for both drivers.
Contrary to popular misconception, no power is lost due to unused output taps. Also, the amount of negative
loop feedback in the amplifier does not change, being fixed at approximately 6 decibels for all output taps. For
more information consult VAC Technical Monograph 90-9.
TIPS & ADVICE
A Word About Output Stage Operating Mode (triode internal wiring option)
Triode amplification is the oldest form known, while partial triode ("ultra-linear") operation of pentodes and
beam power tubes dates to 1937. From the engineering perspective, the triode mode differs from the ultra-
linear mode in the following ways:
1)
2)
3)
4)
5)
Somewhat more distortion at low power levels
Somewhat less distortion at moderate power levels
Reduced maximum power output
Output impedance matching is somewhat less critical
Increased damping factor.
The ultra-linear mode applies negative voltage feedback from the primary of the output transformer to the
screens of the output tubes. The ultra-linear circuit displays different characteristics (efficiency, distortion
components and levels, etc.) depending upon how much of the primary winding is common to both the screen
and plate of the output tubes. The VAC implementation of the ultra-linear circuit has been selected to achieve
performance quality as close to pure triode as possible while still increasing power output and efficiency.
VAC can provide directions allowing a skilled technician to convert the output stage to triode operation.
Download from Www.Somanuals.com. All Manuals Search And Download.
SPECIFICATIONS
The VAC System has been developed with the critical ear as the major arbiter of quality, with both conventional
and unique measurements providing insight and guidance as necessary. The lack of emphasis on measurements
is due to the fact that engineering's arsenal of equipment and techniques do not operate on the pattern
recognition principles that control human perception of sound.
In the immortal words of Daniel von Recklinghausen, if it measures good and sounds bad it is bad. If it
measures bad and sounds good you've measured the wrong things.
For those concerned with test bench performance, the following describes typical measured performance of a
PA100/100 operated at 120 VAC, 60 Hz.
Power Output:
100 watts continuous average power at 1 kHz into 8 ohms connected to the 8 ohm
tap, both channels driven; 103 watts one channel driven.
Frequency Response: down 0.5 dB at 5 Hz and 64 kHz, ref 0 dB = 1 watt @ 1 kHz.
down 3.0 dB at 3 Hz and 115 kHz, ref 0 dB = 1 watt @ 1 kHz.
Power Bandwidth:
down 0.5 dB at 12 Hz and 50 kHz, ref 0 dB = 100 watts @ 1 kHz.
down 3.0 dB at 8.5 Hz and 85 kHz, ref 0 dB = 100 watts @ 1 kHz.
Distortion:
< 0.25% THD at 1 kHz 10 watts
Noise:
< 1.5 mv at the output, S/N ratio > 84 dB
Sensitivity:
0.775 volts input for full power output
Negative Feedback:
Absolute Polarity:
Fuse:
Loop feedback is fixed at approximately 6 dB regardless of output tap selected.
Does not invert signal polarity.
Slo-Blow type, 4A for 100 & 120 Volt configuration, 2A for 220 & 240 V.
300 watts at idle, 516 watts with both channels driven to full power.
18 " width x 15 " depth x 7.5 " height
Power consumption:
Dimensions:
Download from Www.Somanuals.com. All Manuals Search And Download.
|