COLIN LAMB wrote:
The RCA Tube Manuals have an excellent summary of tube testers.

Many transconductance tube testers test the tube under static conditions,
using dc on the grid. This is a simpler technique, but not as accurate as
tests made under actual conditions.

I have never seen a DC transconductance tester. All of the Hickok units
that I am aware of are "dynamic mutual conductance" testers.

That is the reason that the best test
is to install the tube in the circuit and see if it works properly. Some of
the very expensive tube testers, like the Western Electric card tester, will
test the tube under conditions likely to be seen in the actual circuit. The
problem with one of those testers is that the tester is one large heavy
suitcase and the cards used in the tester is one large heavy suitcase.

Agreed, but I'd sure love to have one anyway.


The dynamic tube tester uses ac on the grid and an ac ammeter of the
dynamometer type. Transconductance is equal to the ac plate current divided
by the input-signal voltage.

That is the method used by all of the testers based on the Hickok designs.
Testers designed by Hickok, but made by others, include, I177, TV2, TV7,
and I believe the RCA suitcase testers.

The summary of the RCA text is as follows:

"The tube tester, therefore, cannot be looked upon as a final authority in
determining whether or not a tube is always satisfactory. Actual operating
test in the equipment in which the tube is to be used will give the best
possible indication of a tube's worth."

That is, of course, the reason why any tube tester is basically a luxury. They
do a good job of showing when a tube deviates drastically from the norm, but
say nothing about whether the tube will/won't work in an actual circuit. Tektronix
warns technicians not to replace tubes just because they test weak, but rather
to replace tubes that prevent the scope from meeting alignment specifications.

-Chuck

Hi,
The static transconductance test is more commonly known as the
grid-shift method. It's very old, dating from the 1920s, but can be
extremely accurate, depending on the amount of grid shift (easy with
modern digital meters). Grid-shift went out of favor in the US in the
1930s but most of the British AVO models use it, except with rectified
AC on the plate instead of DC.

The Hickok AC-47 from 1930 had a dynamometer movement (6.25mA F.S.)
and is actually a very fine instrument but was dropped in the mid-30s
because of expense and the difficulty of keeping it up to date with
adapters. Other than one Sensitive Research design, the last
dynamometer model was Hickok's "laboratory model" 700. That beast was
obsolete the day it appeared in 1951.

The Hickok Cardmatic models---123, military USM-118B, Western
Electric KS 15874---can measure either emission or transconductance,
sometimes both (on TV sweep tubes for instance where extra cards are
provided for each test).

Alan