The Museum of T&M Instruments - Hewlett-Packard 434A

"Model 434A is a power meter which bridges the gap between bolometric methodes and conventional calorimetric methodes while retaining the advantagees of both. Bolometrich methods of power measurement using the self-balancing principle are fast, but the upper limit of power measurement is quite low (10 to 100 mw). On the other hand, conventional calorimetric methodes are slow, and are not suitable at powers less than about 1 watt. By combining the self-balancing principle with a calorimetric device, fas response has been obtaied over the range from 1 millliwatt wo 10 watts." (Manual 434A, Hewlett-Packard, 1959)

BRIEF SPECS

Power Range	.01 to 10 watts
Input Resistance	50 ohms
Accuracy	within 5%
Frequency Range	DC to 12.4 kmc
Power	175 watts
Dimensions	20-3/4" x 12-3/4" x 14"
Weight	49 lbs (25 kg)
Technology	tubes & oil ;-)
Price (for 1958)	1,115 $

The hp 434A is a collector's dream: a rf power meter containing a pump, oil and heat exchangers! It'a a good example of an unusual approach of combining different technologies (of that days) to get the desired results. The main reason for using liquid (silicone oil) is to move away the heat from the 50 ohm input resistor. Caution: make no attempt to connect the power meter to any signal source - even not to a ohmmeter to check the 50 ohms - without oil flow: the input resistor may overheat and burn out. Another hint: after transport or turning upside down the instrument must operate a few hours to free the oil from air bubbles. The AC drive signal for the balanced bridge again is the standard hp RC-oscillator. As usual a tungsten lamp is used as a amplitude stabilizing part. While the lamp ist working nearly w/o power it might be defective. It is recommended to replace it with a simliar type - not all lamps in hp instruments are the same (see Service).

You can obtain some good description from "A NARRATIVE HISTORY OF HEWLETT-PACKARD FROM 1939-1990" by John Minck.