Mazda VAC PA100-100 User Manual

VAC PA100/100 POWER AMPLIFIER

THE CHASSIS. DO NOT EXPOSE TO MOISTURE OR OPERATE IT IF WET.

FOR THE TUBES TO COOL DOWN AFTER THE UNIT IS SWITCHED OFF.

levels, resulting in excellent push-pull balance, stability, and low distortion.

Operating Instructions

DO NOT OPEN THIS UNIT - NO USER SERVICEABLE PARTS INSIDE. DO NOT
REMOVE THE BOTTOM PLATE. LETHAL VOLTAGES ARE PRESENT WITHIN

VACUUM TUBES BECOME HOT ENOUGH TO CAUSE SERIOUS BURNS. NEVER
TOUCH A TUBE WHEN THE UNIT IS ON. IT MAY TAKE SEVERAL MINUTES

DO NOT PLUG INTO AC UNTIL ALL CONNECTIONS ARE COMPLETED.

Introduction

The VAC PA100/100 is a single chassis wide-band low-feedback stereo
amplifier conservatively rated at 100 watts per channel. It is to be operated only
from the voltage visible on the configuration card located under the fuse. Please
contact VAC or your dealer if the voltage in your area does not agree with the
label. The fuse is a slow blow type of the size noted on the back of the chassis.
A tube cage is available as an option.

The input circuit of the PA100/100 is derivative of the Williamson design, which
was first published in England in 1947, and was the mainstay of the original
high fidelity movement. Notable features of this circuit include a direct-coupled
input amplifier and phase splitter, resulting in low phase shift, and inherent
precise balance of the phase splitter (if properly executed) at all frequencies and

The output stage is derived from the renowned PA90C, featuring individually
adjustable "fixed-bias" and partial-triode ("ultra-linear") operation.

Premium parts are employed throughout, including 14 pound 22 section bifilar­wound ultra-wideband output transformers, polypropylene and polystyrene
signal capacitors, high current rectifiers, low ESR power supply, and
proprietary wiring. The quality of parts and careful layout allows the use of very
little feedback for high sonic purity and low distortion.

Unpacking

Each tube socket on the amplifier is covered by a small round sticker. The color of this sticker corresponds to
the color of the sticker on a tube. Fit each tube into the matching socket, first removing the sticker from the
tube and socket. The bias levels for the output tubes have been factory set, but should be checked during the
installation procedure.

Installation

Physical requirements:

1) Provide adequate ventilation - allow at least 3 inches above and 1 inch to each side.

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 and power transformers 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:

1)Complete all installation steps before plugging the amplifier into the AC mains supply.

2)Connect signal inputs to the power amplifiers. Single ended interconnect cables equipped with RCA
phono plugs provide the input to each channel.

3)Connect loudspeakers between the appropriate pair of binding posts on the back of the power
amplifiers. Most popular speaker cables can be accommodated by the output posts. Output impedance
matching is available for loads of 2, 4, or 8 ohms. Connect one lead of the speaker cable to the "G"
position, and the other to either 2, 4, or 8. The "G" terminal is at ground potential, and the amplifier
does not invert phase.

4)Do not connect two amplifier outputs to each other.

5)Connect the AC cord to the power source indicated by the voltage configuration card under the fuse
(100 volts AC, 120 volts AC, 220 volts AC, or 240 volts AC at 50 or 60 Hertz).

6)Follow the BIAS procedure described in this manual to set the idle current. Also, read the section
entitled INSTALLING NEW OUTPUT TUBES.

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.

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)The tube is not fully inserted in the socket.

2)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)The tube is defective.

4)The bias is misadjusted.

5)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.