New And Improved Version Of The Vacuumtube Bridge

General Radio 1661

Panel view of the new Type 1661-A Vacuum-Tube Bridge.

The Type 501 Vacuum-Tube Bridge has for many years been the industry standard for determining to a high degree of accuracy the low-frequency coefficients of vacuum tubes. It meets the requirements of the IRE Standards on

Electron Tubes' and has also proved useful for transistor measurements.2 Its ruggedness is more than adequate for many production-testing applications.

The basic circuitry, devised by Dr. W. N. Tuttle,3 replaces the usual resistive ratio arms of a bridge by low-impedance voltage sources from transformer secondaries. While phase and amplitude specifications

^Standards on Electron Tubes: Methods of Testing, 1950.

2A. G. Bousquet, "Transistor Measurements with the Vacuu in - T u b e Bridge," General Radio Experimenter, Vol. 27. No. 10. March, 1953.

»W. N. Tuttle, "Dynamic Measurement of Electron Tube Coefficients," Proc. IRE, No. 21, pp. 844-857, June, 1933.

Panel view of the new Type 1661-A Vacuum-Tube Bridge.

require close manufacturing tolerances, ^^ the novel circuit successfully meets requirements peculiar to tube and transistor measurements. The common electrode of the device under test and the power supplies can all be connected to a common ground, and the voltage drop due to electrode currents is negligible. The bridge indicates directly the real component of the coefficient as plate resistance, transconductance, or amplification factor. The out-of-phase component due to interelectrode capacitance is balanced out without affecting the real component value.

The basic design has not changed in successive models, but the method of connecting the device under test has occasionally been modified to adapt to the growing list of tube-base types and to transistors.

A new and improved version, the Type 1661-A Vacuum-Tube Bridge, incorporates many small but significant features.

The general panel layout and the sloping-front cabinet have been retained. The cabinet depth has been reduced somewhat, but other dimensions are left unchanged to permit the new bridge to be mounted in the consoles that many users have found convenient for laboratory use and for production testing with previous models.

Twin Triodes

Perhaps the most welcome new feature is a switching arrangement that permits measurement sequentially of both sections of twin triodes, twin pentodes, etc., without the need for reconnection of the patch cords. This feature will be most appreciated in production testing where formerly two-section tubes had to be run through test twice because the patchcord connection system was adequate only for one tube section at a time.


Tests with self-biasing cathode resistors in the circuit are now required for several tube types, and connections for such tests have heretofore been quite difficult to set up. In the new design, the panel switch that selects the tube section also connects the cathodes to a system of three pairs of binding posts that permit the connection of self-biasing resistors to the separate cathodes or to the two cathodes in parallel.

Tube and Transistor Base Adaptors

In spite of the multiplicity of tube and transistor bases, the bridge has been kept up to date relatively simply, because an adaptor plate is provided for each kind of base. With the adaptor plugged into the panel, the tube electrodes can be connected to the appropriate power-supply and bridge terminals by means of the nine doubly shielded coaxial cables. Four grounded connectors have been added to simplify the grounding of any tube terminals. These connectors and two ungrounded connectors provide convenient anchor points to hold the unused patch cords securely out of the way. The patch cords have been lengthened to insure that they will not interfere with the envelope of a large tube.

A new adaptor plate carries three sockets for transistors. One is for JETEC-30 based transistors; a second is for 3- or 4-iii-line long-lead transistors (or tubes); the third is for 3- or 4-in-cluster long-lead transistors. There are now a total of thirteen adaptor plates including the "universal" plate to which any unusual socket or tube can be soldered. The adaptor plates and other accessories are supplied in a convenient accessory box.

Voltage and Current Limits

The 1500-volt limit has been retained as the maximum allowable "plate" voltage but, because of the growing list of relatively high-current tubes, the "plate" current limit has been raised from 150 to 400-milliamperes de. Because some transistors show a frequency effect even at 1000 cycles, the redesign has extended the operating frequency to include the 270 to 400-cycle range in addition to the 1000-cycle point.

Other Changes

Operation at lower frequencies and at higher currents has necessitated a redesign of the transformers. Magnetic shielding has been improved by mounting the transformers in /¿-metal shields. The input transformer has been eliminated and the input impedance has been reduced (550-2100 ohms).

A new oscillator and a new filter have been designed (see below) to provide both bridge power and detector selectivity at 270 and 1000 cycles.

For the resistance and transconduc-tance measurements, the resist ance standard has been 100,000 ohms. A panel switch now permits a choice between a 10,000-ohm and a 100,000-ohm standard. The use of the lower resistance h

View of storage box, opened to show accessory adaptors and cables.

standard improves the sensitivity of balance and the signal-to-noise ratio by a factor approaching 10 when low "plate resistance" or high "transconductance" devices are measured. By the same token, for a given signal-to-noise ratio the range of resistance or transconductance measurements is extended.

Many of the new features are the direct result of suggestions from the users of the bridge who, after all, are in a much better position than is the instrument designer to evaluate the virtues and the foibles of the equipment.

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