KT66 Push-Pull Power Amplifier Pt.2

Simple configuration for use with either KT66 or 6L6 tubes; Class A operation delivers 13W output. Designed and Built by Katsu Nagashima

Measured Performance

The following measurements were taken using GEC KT66 output tubes. The term “completed amplifier (with NFB)” refers to the configuration shown in Figure 3, which includes 6.0 dB of loop negative feedback—a moderate amount chosen for musical balance. The resulting overall voltage gain was 23.7 dB.

[Fig. 3] Amplifier circuit of this unit (R channel only; red text indicates voltages for R channel / common to both channels, blue for L channel voltages, and green for current values)

The residual noise (Table 3) measured 0.86 mV (right channel) and 0.36 mV (left channel) with the input shorted—excellent values for an A-class push-pull amp.

[Table 3] Residual Noise of the Completed Amplifier (8Ω)

■ Frequency Response
Measured at 1 V output / 8 Ω load (see Figure 6):
・Without NFB:
–1 dB = 20 Hz – 50 kHz
–3 dB = 8.1 Hz – 108 kHz
・With NFB:
–1 dB = 9.8 Hz – 157 kHz
–3 dB = 4.2 Hz – 204 kHz

This exceptionally wide bandwidth confirms the output transformer’s high quality and stability.

■ Distortion
As shown in Figure 7, the minimum THD was 0.047 % at 10 kHz / 3 V output (≈ 1.1 W)—an outstanding figure.