Overview
The Hewlett-Packard 400C is a precision AC vacuum tube voltmeter (VTVM) introduced around 1949 as an improved successor to the earlier HP 400A series. The instrument was designed for laboratory, communications, and electronic service applications requiring accurate wide-band AC voltage measurements.
The 400C became well known for its high sensitivity, wide frequency response, and rugged Hewlett-Packard construction. It was capable of measuring extremely small AC signals while maintaining high input impedance to minimize circuit loading.
Technical Specifications
| Specification | Details |
|---|---|
| Manufacturer | Hewlett-Packard |
| Model | 400C |
| Type | AC Vacuum Tube Voltmeter |
| Introduced | Approximately 1949 |
| Measurement Range | 1 mV to 300 V full scale |
| Frequency Range | 20 Hz to 2 MHz |
| Input Impedance | 10 megohms |
| Meter Type | Analog RMS-reading meter |
| Technology | Vacuum tube |
| Power Requirement | 115 VAC |
| Weight | Approximately 15 lbs |
| Cabinet | Metal bench-top enclosure |
Physical Description
The HP 400C features a compact metal cabinet with a large illuminated analog meter and rotary range selector switches on the front panel. Earlier examples used a sloped-front cabinet style common to late-1940s HP instruments, while later production units appeared in several cabinet variations.
Internally, the unit contains multiple miniature vacuum tubes and a regulated power supply designed to maintain stable measurements despite line voltage fluctuations.
History
The HP 400-series voltmeters were among Hewlett-Packard’s earliest electronic measuring instruments. The original HP 400A was designed by Dave Packard during the early 1940s and became one of the company’s foundational products.
The 400C improved performance substantially over earlier versions by:
- Extending frequency response to 2 MHz
- Increasing sensitivity to 1 millivolt full scale
- Improving overall stability and accuracy
The instrument was widely used in:
- Radio engineering laboratories
- Military electronics facilities
- Broadcast engineering
- Television servicing
- Research laboratories
- Communications testing
The 400C was eventually superseded by the HP 400D and later members of the 400-series family offering improved bandwidth and accuracy.
References
- HP 400C Operating and Service Manual
Original Hewlett-Packard operating and service manual for the HP 400C. - Radiomuseum HP 400C Reference
Technical specifications, photographs, and historical information for the HP 400C. - HP 400C Restoration and Technical Notes
Detailed technical discussion and restoration information for the HP 400C VTVM. - HP Memory Project: Early Electronic Voltmeters
Historical information covering the HP 400-series electronic voltmeters and company development.