Electronic Voltmeters

Posted on Feb 4, 2014

The HP Model 400A Vacuum Tube Voltmeter is unusually flexible because of its wide frequency and voltage ranges. AC Voltages as small as. 005 and as high as 300 volts can be simply and directly measured without any precautions over a frequency range of 10 cycles to 1, 000, 000 cycles. Accuracy of readings is assured because the high 1 megohm input im

Electronic Voltmeters
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pedance does not disturb the usual circuit under test. Furthermore, the calibration error of the instrument under all conditions is less than 3 % to 100 Kc and less than 5 % to one megacycle. " The 400A service manual is the oldest HP original document of our collection. It is a 7 pages RONEO document with the schematic diagram handmade on the drawing board and dated 7/28/44. (note that the picofarad value of small capacitors was labeled µ µf at this time. ) Further improvement of the sensitivity can be obtained with the 450A amplifier introduced in the 1948 catalog. The 450A can be used as a stand-alone general purpose amplifier or as a companion for the 400A or 400C Voltmeters. The 450A has a switchable gain of 20 or 40 dB. Used in conjunction with the 400A voltmeter for example, it will increases the sensitivity 100 times at 40 dB gain for a final 300 microvolts full scale. A specially-designed diode, in combination with the HP probe design, makes the exceedingly flat frequency response from 20 cps to 700 Mc of the 410A possible. With this flat frequency response are combined the factors of low input capacity and high input resistance. The input resistance and reactance are high throughout the entire range of the instrument, and thus measurements are made without appreciable detuning or loading of the circuit. The diode developed by EIMAC for this instrument was certainly the first example of an outside...

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