The C70 KT-88 Vacuum Tube Stereo Amplifier

This vacuum tube amplifier is a Class-AB design using KT-88 output tubes to produce 35 watts per channel. Its important features are summarized as follows:

• 35 Watts per channel
• KT-88 output tubes, Class-AB
• Pentode operation with fixed bias
• Over-tubed design with 8000-ohm output transformer
• 500-Volt power supply with 500 uF capacitance and choke
• Regulated screen, input stage, driver and fixed-bias supplies
• Fully balanced differential design with no explicit phase splitter
• Cascoded transistor current sources for long-tailed pairs
• No electrolytics in the signal path
• Only one coupling capacitor stage within the feedback loop
• Extensive input, output and power supply RFI immunity enhancements
• 3 Hz to 100 kHz, +/- 3 dB
• Damping factor of 20

The C70 amplifier is a bit unusual in that it employs 8000-ohm output transformers in combination with a 500-volt plate supply, operating in Pentode mode, to produce the maximum output power of 35 watts. In a sense, this is what I call an "over-tubed" design, in that the KT-88's are a much larger and more powerful tube than is normally needed to produce 35 watts. In essence, they are loafing. This, in combination with the 8000-ohm output transformer's larger turns ratio, provides a higher output current capability and an intrinsically lower output impedance than one would normally obtain. The effects of output transformer leakage inductance and stray capacitance are also reduced in this arrangement. Tighter bass and better bass extension and low-frequency power output are among the advantages of this approach.

The power supply provides 500 volts from an over-sized power transformer (actually a Sixties-vintage television power transformer) and 500 uF of capacitance and a choke in a pi configuration. The power supply is heavily bypassed and snubbed with smaller-value capacitors.

The screen and all input and driver circuit supplies are regulated by the use of power MOSFET-based voltage regulators. Regulated screen supplies in a Pentode design are especially advantageous, but rarely found in practice.

The Class-AB output stage employs fixed bias, and this is provided by a regulated negative supply employing an LM337 voltage regulator IC. Transistor-based current sources are provided in the long-tailed differential pairs in the input and driver stages, and these are provided a -15 V regulated supply utilizing a 7915 regulator.

The signal path is fully differential from input to output, and does not employ an explicit phase splitter. Instead, the 12AX7 input differential pair provides implicit phase inversion by having the input applied to one side and the negative feedback applied to the other side of its input. The use of a very high impedance cascoded transistor current source for the tail of the pair provides exceptional common mode rejection and outstanding inherent phase splitting action, even out to very high frequencies. There is no capacitor in the feedback shunt return path, and instead a d.c. balance adjustment is incorporated in the input stage. There are therefore no electrolytic capacitors in the signal path.

The driver employs a 12AU7 differential amplifier, which is direct coupled from the input differential pair. Indeed, there is only one coupling capacitor (per differential side) in the signal path, coupling the driver outputs to the KT-88 grids. The presence of only a single capacitive coupling in the signal path greatly enhances low frequency stability, tightness and extension. The driver differential pair also employs a cascaded transistor current source for exceptional common mode rejection and further enhances the balance and symmetry of the phase-split signals provided by the input stage.

Both a.c. balance and d.c. balance adjustments are provided for the KT-88 output stages.

The C70 is a medium feedback design with a damping factor of about 20.