VAC PA90 User Manual
POWER AMPLIFICATION SYSTEMS
by
Valve Amplification Company
Model PA90C1 Power Amplifier
& Model PS90C1 Power Supply
Operation & Maintenance
CAUTION
DO NOT OPERATE THESE UNITS WITHOUT THE GROUND STRAP ATTACHED
BETWEEN EACH AMPLIFIER AND ITS ASSOCIATED POWER SUPPLY.
OPERATION WITHOUT THE GROUND STRAP POSES A POTENTIALLY LETHAL
SITUATION.
THESE UNITS CONTAIN NO USER SERVICEABLE PARTS. DO NOT REMOVE
THE BOTTOM PLATES OR TOP COVERS. LETHAL VOLTAGES ARE PRESENT
WITHIN THE CHASSIS. DO NOT OPERATE THE UNITS IF THEY ARE WET.
VACUUM TUBES BECOME HOT ENOUGH TO CAUSE SERIOUS BURNS. NEVER
TOUCH A TUBE WHEN THE UNIT IS ON. IT MAY TAKE SEVERAL MINUTES FOR
THE TUBES TO COOL DOWN AFTER THE UNIT IS SWITCHED OFF.
INDEX
Introduction
Unpacking
Installation
Operation
Biasing
AC Balance Adjustment
Selection Of Output Stage Operating Mode
Installing New Output Tubes
Replacement of Low Level Tubes
Care Of Chassis
Tips and Advice
A Word About Tubes in General
A Word About Output Tubes
A Word About Low Level Tubes
A Word About Bias Levels
A Word About Impedance Matching
A Word About Output Stage Operating Mode
Specifications
2
INTRODUCTION
Your VAC Power Amplifier is designed not to the latest fad but to substance, providing the
highest quality of sound. Time spent familiarizing yourself with this manual will be well
rewarded.
Figure 1 demonstrates the way to connect the VAC amplifiers and power supplies. A single
PA90 is connected to a PS90 to form a 120 watt monophonic amplifier.
Some of the features of your power amplifiers include:
The maximally balanced push-pull circuitry, with push-pull conversion (phase inversion)
occurring in the first stage, better exploits the inherent advantages of push-pull operation and
allows improved performance of the driver stage.
Modest loop negative feedback (8 dB) ensures better dynamics, gentle clipping, and superior
overload recovery. Simple feedback arrangement avoids unnecessary multiplication of the order
of distortion products.
Completely hand wired in three dimensional space for the shortest possible lead lengths,
allowing passive components to perform much nearer to the theoretical ideal.
All chassis are machined 1/8 inch thick aluminum, creating an extraordinarily rigid nonmagnetic platform that is unusually resistant to vibrations that could mar the sound.
Mechanical vibration from the power supply is completely isolated from the amplification
circuitry due to the multiple chassis design.
Energy storage is distributed among the chassis to place it nearer to the point of demand and
dedicated to particular uses, ensuring superior isolation of the various stages.
Output stage is based on the KT77 Beam Power tube or the EL34/6CA7 Power Pentode
operated in Class A at 54 milliamperes idle current per tube.
Output stage may be operated in partial-triode (ultra-linear) or full triode mode, selected via a
single switch.
Impedance matching taps are provided for 2, 4, and 8 ohms.
3
UNPACKING
Each amplifier and power supply chassis is packed in double box cartons for the greatest
possible protection during shipping. Included in the power amplifier carton are vacuum tubes,
a power cable for attachment to the PS90 Power Supply, and a separate grounding strap
which must be attached to the PS90 for safety. Tubes are packed individually to prevent
damage, and must be fitted to the PA90 Power Amplifier before installation can proceed.
Each tube socket is covered by a small round sticker. On this sticker is a number that
corresponds to a numbered sticker on each tube. Fit each tube into the matching socket, first
removing
been factory set, but should be checked during the installation procedure.
DO NOT CONNECT TWO PA90 POWER AMPLIFIERS TO A SINGLE PS90 POWER SUPPLY.
the sticker from the PA90 and the tube. The bias levels for the output tubes have
4
INSTALLATION
Physical requirements:
1)Provide adequate ventilation - allow at least 3 inches above and 1/4 inch between VAC
chassis.
2)Do not place in a completely enclosed cabinet.
3)Do not stack other equipment on top of the VAC units.
4)Do not operate on carpet or any other surface that might block air flow.
5)The chassis of the PS90 will become hot in normal use.
6)Do not allow the chassis of the VAC components to touch any metal parts, such as the
frame of an equipment rack. This might create a parallel ground path that will degrade
the sound of your system.
Electrical connections (refer to Figure 2 to identify the connectors and leads referred to in these
directions.):
1)FOLLOW THESE DIRECTIONS CAREFULLY. FAILURE TO CONNECT THESE UNITS AS
DESCRIBED MAY RESULT IN INCORRECT OPERATION AND MAY POSE A DANGER OF
LETHAL SHOCK.
2)The PS90 Power Supply must be unplugged and turned completely off.
3)Connect the heavy gauge grounding cables between the ground terminal of the PS90
and the ground terminal of the PA90 Power Amplifier. The binding posts provided for
this purpose have a nut that can be completely removed, and so accommodates the
circular lug (ring tongue terminal) used on the ground cables. Remove the nut, place the
lug over the binding post shaft, replace the nut and tighten firmly (an 11 millimeter open
end wrench may be used). The ground cable ensures a ground return for the power
supply in the unlikely event that the two redundant ground paths in the power cable
between chassis should fail.
4)Connect the power cable provided into either socket marked "To Power Amplifier" on the
back of the PS90 Power Supply. Connect the other end of the power cable into the
socket marked "Power Supply In" on the PA90. NOTE THAT THE POWER CONNECTORS
ARE DIFFERENT ON EACH END OF THE CABLE, ARE KEYED, AND HAVE A KEYED
LOCKING COLLAR. You should feel a slight "snap" when the locking collar has been fully
turned. Never attach two PA90s to a single PS90.
5)Connect signal inputs to the power amplifiers. BALANCED (XLR INPUT) AMPLIFIERS
CAN ONLY WORK CORRECTLY WITH FULLY BALANCED INPUT SIGNALS - OTHERWISE
DAMAGE WILL OCCUR.
6)Connect loudspeakers between the appropriate pair of binding posts on the back of the
power amplifiers. The binding post nut accepts an 11 millimeter open end wrench. Be
careful not to over tighten. Note that the positive terminal is the same for all impedance
connections. For further information see Tips & Advice: A Word About Impedance
Matching.
7)Connect the PS90 to the power source indicated on the rear panel, either 120 volts AC
or 220 volts AC, 50 or 60 Hertz.
8)Follow the BIAS procedure described in this manual to set the idle current at 54
milliamperes per tube, checking the idle current at 1 minute, 30 minutes, 1 hour, and
2 hours. Also, read the section entitled INSTALLING NEW OUTPUT TUBES.
CAUTION FOR GROUNDED SPEAKER SYSTEMS OR COMMON GROUND SWITCHING
SYSTEMS. The PA90 chassis and "-2 ohm" output connector are at ground potential with
respect to the 120 volt AC source and the signal input shield. NEVER CONNECT ANY OUTPUT
TAP EXCEPT "-2" TO THE GROUNDED SIDE OF A GROUNDED LOAD, such as may be
encountered with some switch boxes and certain speakers. In such a system, the only output
impedance available from the PA90 is 2 ohms. Special modifications for grounded and
common loads are available. Contact VAC for details.
5
OPERATION
Set the STANDBY switch on the PS90 to on. This provides power to the filaments (heaters) of
the PA90 tubes and charges the bias supply. After a delay of 2 minutes the OPERATE switch
may be set on, providing full voltage (approximately 480 volts) to the plates of the tubes. The
amplifier is now on and ready for listening.
Following the turn on procedure described above will help ensure long tube life. Having two
power switches provides additional control over the amplifier.
To turn the unit off, first turn the OPERATE switch to off, followed by the STANDBY switch.
(Actually, they may be thrown simultaneously. In addition, switching off STANDBY alone will
cut both supplies, but OPERATE must be switched off before the next turn-on.) Sound
(distorted) will continue for several seconds after the power is switched off. The slow sound
decay is caused by the slow discharge of the large energy storage capacitors in the power
supply and power amplifier. (The sound would continue for much longer except for the bleeder
resistors in the PS90.)
To mute the unit for a short time (15 minutes maximum advisable), turn the OPERATE switch
to off but leave the STANDBY switch on. The unit will play immediately when OPERATE is
again switched on.
Before disconnecting your amplifiers from their power supplies allow them to sit with power
off for five minutes.
power supplies. Failure to do so may result in damage to the regulators when the units are
reconnected.
Do not use the STANDBY mode for periods exceeding 15 minutes.
This allows energy stored in the amplifier chassis to drain back in to the
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 one hour of operation, with
subtle changes occurring for up to two hours. 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 3,000 and 10,000 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. Refer to the discussion
of break in contained in the INTRODUCTION section of this manual for further information.
Please note that although your VAC System has been run for 48 hours at the factory, they will
continue to "break in" for approximately 150 hours. The break in is most pronounced on the 90
Watt Amplifiers, particularly in the ultralinear mode. Note that the triode and ultra-linear modes
do not require separate break in periods, and that the ultra-linear mode will improve even if you
only operate the amplifiers in the triode mode (this is in most cases the most listenable way
to break in the PA90).
Also be aware that many components display the need for a new break in period after being
transported in unheated cargo aircraft.
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