Model 400B vacuum tube voltmeter, 1948
At the time of their introduction, HP voltmeters offered unprecedented reliability for their price.
The first voltmeter, the 400A, was introduced early in 1942 after a design by Dave Packard in 1941. It was manufactured until 1958. Notable for stability (it was one of the earliest vacuum tube voltmeters to need no initial adjustment for zero and none for drift) and for wide high input impedance over 1 MHz bandwidth, the 400A became an industry classic. A key to achieving its performance was application of negative feedback in amounts then unheard of.
This excerpt from the HP instrument catalog describes its features:
The HP Model 400A vacuum tube voltmeter, a recent development of HP laboratories, possesses all the important desirable features. It is one of the best available yet its accuracy is unexcelled and it has extreme sensitivity over a wide frequency range. One of the outstanding features of the HP 400A is that the voltage indication is proportional to the average value of the full wave. This is a feature not found in most electronic meters on the market today.
The 400B was based on a design originally made by Dave Packard in 1941.