No.26
Hi-Fi Audio
6AS7

This amplifier is designed to provide the best possible electrical and sonic performance from a Svetlana 6AS7 in a basic single-ended Class A1 circuit. The driver stage is a Svetlana 6BM8 in mu-follower connection, which this tube is well-suited for. The upper cathode follower drives a cathode-biased 6AS7 directly coupled to an Electra-Print JT-1K output transformer, which was specially designed for this service. This circuit will also accept a One Electron UBT-1 transformer, with a small loss of power output and performance.

Since the two triodes in a 6AS7 sometimes vary in biasing characteristics,a 500-ohm rheostat (or wirewound resistor with sliding adjustable tap) is used to balance them. The 10-ohm resistors in the plate circuits allow attachment of a DC voltmeter from plate to plate. Simply adjust the rheostat until the meter reads close to zero volts DC. The unbypassed rheostat also provides some degenerative feedback, linearizing the 6AS7 without a global feedback loop. The resulting performance and sound is comparable to or superior to some 300B SE amplifiers, at a much lower cost and with minimal feedback circuitry.

The power supply uses the Electra-Print SP2-5, which was specially made for this design. Conservative component ratings were followed throughout the power circuits. The upper rectifier tube may be a 5Y3, 5V4, 5R4, 5AR4 or 5U4, while a 5U4 or 5AR4 is recommended for the lower rectifier. The two power resistors marked (*) must be determined or adjusted in order to drop the voltage as required. This depends on local power-line conditions and other factors. Try using 250-ohm 10-watt wirewound resistors with adjustable sliders.

DC heater power is not required for this amplifier circuit. Some 60-Hz hum may be "bucked" out with the 100- ohm potentiometer shown. It is not necessary for some users and may be left out if desired. The other 6.3v winding MUST float with respect to ground, to prevent damaging the heater-cathode insulation on the 6BM8s. A film capacitor to ground is recommended to assist with possible noise suppression; this capacitor's value may need to be adjusted for best noise rejection.

OBSERVED PERFORMANCE (measured on prototype amplifier)

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Frequency response:

10 Hz to 28 kHz, -1 dB

Distortion into 8-ohm speaker load, 1000 Hz:

0.25% at 1 watt

0.80% at 5 watts

clipping onset at 8 watts

Figure 1

Figure 2

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