COLIN LAMB wrote:
Many people believe that a tube tester needs to be a transconductance type
to be accurate. I have numerous examples of both.

My experiences through hundreds of tests are that the emission tester is
accurate in almost all cases. As a matter of fact, I usually grab whichever
tester is handy. If a tube is bad, either tester will usually tell you.
Emission testers may actually be better in some situations - such as
determining life left in the tube.

Poppycock! Transconductance testers combine the functionality of an emissions
tester with a simple test for low frequency AC gain.

Further, transconductance testers have a sensitive test for gassy tubes.
Something that an emissions tester cannot do (except in extreme cases).

Your experience just shows that old tubes are rather reliable.

I can recall only two tests in hundreds where the transconductance tester
led me to a problem hidden by the emission tester. In one case, a Drake
transceiver had a tube that had developed cathode interface and had about a
1 megohm internal leakage, which had developed because of a circuit designed
by Drake, which had a very high impedance grid circuit that recommended by
tube designers.

The condition known as "cathode interface" comes about because the interface
between the metal cathode substrate, and the highly em missive oxide cathode
layer, becomes degraded. The interface develops a higher DC resistance than
it is supposed to have. The problem is somewhat vexing because the capacitance
of the cathode interface remains as high as when the tube was new. The most
noticeable characteristic of "cathode interface" is that the tube has reduced
DC gain, and normal HF AC gain. If you put a square wave through a DC coupled
tube that has "cathode interface" the the top and bottom horizontal lines of
the square wave will droop towards zero. If you compensate the tube circuit
so that no droop occurs, there will be a pronounced spike at the leading and
trailing edge of the squarewave's rise and fall time. Cathode interface was
rarely a problem before the advent of high performance vacuum tube oscilloscopes.

What you discovered was a gassy tube. When gas enters a vacuum tube, the gas
causes a current path from the cathode to the grid. This makes the grid go
more positive than normal. In a circuit that has high grid impedance, the
tube will significantly change its bias point. A fairly important thing in
grid leak biased tubes (that are often found in receivers).

Although I had substituted another tube, it also had
cathode interface. The transconductance tester allowed me to do some
detective work to find the problem. A third tube solved the problem.

In the 30's, many people built their own tube tester. I have done that.

I may get a lot of flack for my opinion, but I have my flack shield on and I
can only report what I experience.

Colin K7FM

If you are going to spend the money and waste the space on a tube tester,
you ought to get one that will actually do a good job of testing tubes. Today
that is even more important than it was 40 years ago... tubes have become
obscenely expensive, and most tubes you will be exposed to have long since
passed their expected useful lifetimes. Conditions like gas are quite common,
and should be tested.

-Chuck