Function Generator: Resistance-Capacity Tuned Audio Oscillator with Lamp Amplitude Control (Hewlett, US 2,268,872, 1942)

The first Hewlett-Packard product: a test oscillator tuned by resistors and capacitors instead of coils. Two valve amplifiers feed their output back to their input through a series and a parallel R-C pair, which passes only one frequency in phase, so the circuit oscillates there. A second, negative feedback path through a small lamp holds the amplitude steady: as the swing grows the lamp warms, its resistance rises and the gain falls. One dial covers ten to one in frequency. Result: a sine-wave signal generator covering 20 to 20,000 cycles in three ranges, about 0.25 % distortion. Source: W. R. Hewlett, US 2,268,872 (1942). Unknown: valve types and component values. Picture: the patent drawing.

上級者

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手順

Build the two-stage amplifier

Wire two amplifying valves in cascade with resistance-capacity coupling, the first's anode to the second's grid, run as class A amplifiers. Not given: valve types, values, the supplies. Ours: pentode valves in sockets.

このステップの材料:

五極管2 個

真空管ソケット2 個

電気抵抗1 セット

コンデンサキット1 coupling and by-pass

アルミ筐体箱1 case

はんだ線1 as needed

必要な工具:

はんだごて

Fit the tuning network

From the second valve's anode, through a coupling capacitor, feed back to the first valve's grid through a resistor R1 in series with a variable capacitor C1, with a resistor R2 and variable capacitor C2 from the grid to ground. Make R1 = R2 and C1 = C2: the frequency is then 1/(2 pi R C), set by the capacitors, giving 10 to 1 on one dial. For the capacitors use a four-gang tuning capacitor with its sections paired in parallel. Switch three sets of resistors in for the ranges 20-200, 200-2000 and 2000-20,000 cycles. Ours: a rotary switch.

このステップの材料:

可変コンデンサー（空気誘電体）1 four-gang

電気抵抗1 three sets of R1, R2

ロータリースイッチ1 range switch

必要な工具:

はんだごて

ドライバーセット

Fit the lamp feedback and use it

From the same output, through the coupling capacitor, feed back to the first valve's cathode across a resistor R4 and a small incandescent lamp R3. As the oscillation grows, the lamp heats, its resistance rises, the negative feedback increases and the gain falls, so the amplitude settles below the point of grid current with no clipping. The negative feedback also steadies the gain against valve and supply changes. Take the output from the terminals; Hewlett's test unit held its output within about 10 % over 20-20,000 cycles, with about 0.25 % distortion and under 0.1 % frequency shift with supply changes.

このステップの材料: