VAC Phi 70 - Manual (Page 4)

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INTRODUCTION
The Phi Seventy is a unique power amplifier. It allies purist amplifying devices with the power and
solidity required to control modern audiophile loudspeakers. Extreme linearity is achieved without
resorting to loop negative feedback, for a more natural, dimensional sound.
The most noticeable feature of this amplifier is the 300B filamentary triode output tube. Originally
designed by Western Electric in 1935 (Footnote 1), it has several extremely desirable features that
have kept it in continuous production since that time. An examination of its dynamic curves (Figure
1) reveals it to be a naturally linear device, more so than any transistor device. It gives this superb
performance with voltages on the order of 450 VDC, not the 1000-1500 VDC required by other
output triodes like the 211 or 845. This lower voltage and lower output impedance allows superior
output transformer design, control of the loudspeaker, and safety.
Each 300B tube has its own separate heater power supply and independent cathode self-bias
network. This has several major advantages. No user adjustment of any kind is necessary to maintain
proper balance of the amplifier. The stabilizing action of this technique upon the idle current (bias
point) of each tube is exceptionally strong, and self corrects for large mismatches among tubes or
drift with age. This circuit also results in real-world output capability much greater than the test-
bench rating would indicate (Footnotes 2,3,4).
A total of four 300B are used in Class A1; that is, they are not operated into cutoff (as in Class AB
or B) or driven into grid current (suffix “2" operation) in normal operation. This is the purest, most
linear technique known.
The input circuitry of the Phi Seventy is derivative of the unique Williamson circuit, which provides
pure, direct coupled, inherently balanced input amplification and phase splitting. Type 6SN7/5692
octal twin triodes do the honors.
The superb VAC output transformer provides superb voltage/current translation, allowing
impedances from eight ohms to as low as one ohm to be driven.
The main chassis is machined from thick aluminum, which is then plated with nickel for enhanced
RFI rejection. The separate power supply allows e.m.f. fields, switching transients, and mechanical
vibration to be isolated from the audio circuits. The high voltage supply dual choke pi filtering for
extremely low noise.
The Phi Seventy is designed not to the latest fad but to substance, for the highest possible sound
quality. Time spent familiarizing yourself with this manual will be well rewarded.