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.
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.
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.
INSTALLATION Physical requirements: 1) 2) 3) 4) 5) 6) Provide adequate ventilation - allow at least 3 inches above and 1/4 inch between VAC chassis. Do not place in a completely enclosed cabinet. Do not stack other equipment on top of the VAC units. Do not operate on carpet or any other surface that might block air flow. The chassis of the PS90 will become hot in normal use. Do not allow the chassis of the VAC components to touch any metal parts, such as the frame of an equipment rack.
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.
BIASING Your VAC Power Amplifier has been shipped with output tube bias set for an idle current of 54 milliamperes (mA) per tube. This should be checked when you install your amplifier, and approximately every month thereafter. It must also be set whenever an output tube is changed. Refer to Figure 3. Bias levels must be set with accessory PM1 milliammeter. Insert the phone plug of the PM1 into the socket labelled "METER" on the top front of the PA90.
AC BALANCE ADJUSTMENT The AC balance adjustment, located on the top of the PA90 between the low level tubes, allows the amplifier to be adjusted for maximum symmetry of the push-pull signal. It has been set at the factory for lowest distortion performance with the supplied tubes. Ordinarily, you need not change the setting of this control, even when the low signal tubes are changed, for two reasons.
SELECTION OF OUTPUT STAGE OPERATING MODE All VAC amplifier output stages may be operated in partial-triode (ultralinear) or full triode modes, at your discretion via a single switch. Although the mode may be changed with the amplifier in operation, we strongly suggest that you turn the OPERATE switch to off before operating the mode switch. To change the mode, insert a flat blade screwdriver into the switch opening between the output tubes (be careful not to burn yourself on the output tubes).
INSTALLING NEW OUTPUT TUBES Replacement tubes are available from VAC and other sources. Output tubes may be KT77 beam power tubes or EL34/6CA7 power pentodes. It is not necessary that they be matched pairs, although an 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). DO NOT MIX TYPE KT77 AND TYPE EL34 TUBES.
REPLACEMENT OF LOW LEVEL TUBES Refer to Figure 4 for the location and types of 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. AC BALANCE may be readjusted for minimum measured distortion if desired, but most tubes will perform correctly without adjustment. Replacement tubes are available from VAC and other sources.
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 will probably find that they may not even the same shape and size.
TIPS & ADVICE A Word About Output Tubes Your VAC Amplifier can use 2 different output tube types: the KT77 Beam Power Tetrode and the EL34/6CA7 Power Pentode. From the engineering perspective, beam power tubes have sharper "knees" to their plate characteristics, more second harmonic distortion, less third harmonic distortion, and are somewhat less tolerant of load impedance mismatches.
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.
TIPS & ADVICE A Word About Output Stage Operating Mode Triode amplification is the oldest form of amplification known, while partial triode ("ultra-linear") operation of pentodes and beam power tubes dates back to 1937. Both modes of operation are available with your VAC Power Amplifier.
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 principals that control human perception of sound. In the immortal words of Daniel von Recklinghausen, if it measures good and sounds bad, it is bad.
WARRANTY Your equipment 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 (tubes excepted). Receipt of your registration card will be confirmed in writing by VAC: it is important that you call VAC if you do not receive this confirmation.
