VAC PA35 User Manual

VAC PA35/35 TRIODE POWER AMPLIFIER

Operating Instructions

DO NOT OPEN THIS UNIT - NO USER SERVICEABLE PARTS INSIDE. DO NOT
REMOVE THE BOTTOM PLATE. LETHAL VOLTAGES ARE PRESENT WITHIN
THE CHASSIS. DO NOT EXPOSE TO MOISTURE OR OPERATE IT IF 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.

DO NOT PLUG INTO AC UNTIL ALL CONNECTIONS ARE COMPLETED.

Introduction

The VAC PA35/35 is a single chassis wide-band variable-feedback triode stereo amplifier
rated at 35 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 PA35/35 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 levels, resulting in excellent push-pull balance, stability, and
low distortion.

The output stage operates pentode versions of the EL34 with all grid elements, except the
control grid, tied to the anode, thus securing the electrical and physical characteristics of
single grid triodes.

Premium parts are employed throughout, including 14 pound 22 section bifilar-wound ultra­wideband output transformers, polypropylene and polystyrene signal capacitors, high current
rectifiers, low ESR power supply, and proprietary wiring. The quality of parts and 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

It is suggested that you begin listening with the feedback control (front left of chassis) set to
"A" (zero feedback) and the audio ground switch (rear panel) set to "Normal."

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 EL34 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.

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 (frontmost) tube may be type 12AU7A/ECC82/E82CC or type 12BH7 (typically richer
sound). The driver tube (next toward the back) is type 12AT7A. The output tubes are type EL34, pentode
version only. Some 6CA7/EL34 have been made as "beam power" tubes, and will damage the PA35/35, as
will any output tube that internally ties pins 1 and 8. Golden Dragon tubes are generally recommended and
have been used extensively during the development of this amplifier. 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 EL34 pentodes only. Some

6CA7/EL34 have been made as "beam power" tubes, and will damage the PA35/35, as will any output tube
that internally ties pins 1 and 8. 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.

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.