||The 400B was based on a design originally made by Dave Packard in 1941.
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
for measurements below 1 megacycle. It is extremely easy to operate,
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.