I've been building electrical and
electronic devices since I was a
child. Only recently did I develop an interest for vintage vacuum
tubes. One of my friends is an avid audiophile. He brought
me over to his house and showed me his $30,000 system: Cary Amplifiers,
oversized cables, preamps, designer CD player and monstrous audio
speakers. I saw his 211 and 300B tubes glowing in the dimly lit
room; their glow pulsating with the clearest audio I have heard in a
long time. I was definitely impressed with his stereo audio
system. He regretted, though, that he did not know if his
tubes were properly matched. You can imagine that if someone
spends 30k on their system they probably would like to have the tubes
function in an identical manner to within a few percentage points.
While we were at work he asked me how hard would it be to design and
build a 211 tube tester. He knew his Western Electric 300B tubes
were matched because he bought them from the factory as a "factory
match". They even came with their own plate-current curves that
were generated on a curve tracer. However, General Electric and RCA
stopped making 211
tubes long ago and there was no way to test his tubes.
I went on the internet and found various vintage tube testers: Hickok,
Heathkit, Tripplet and Sencore were a few that I found. Almost
none could test the 211; only one had the ability to test current
conduction at one grid and plate setting. None of them tested
transconductance. This would not work for my friend.
I started thinking about how to tackle the problem. I have to
admit that initially I was a little lost. Most of my experience
was with capacitors, inductors, diodes, transistors and
transformers. Who used vacuum tubes anyway? I started
re-educating myself on vacuum tube theory and how they worked.
The vacuum tube is the predecessor to the transistor; it acts as a
valve controlling the amount of current that travels across the plate
to the filament. Next, I studied the specifications of the 211
tube. The tube handles a lot of voltage, and when conducting
maximum current it can put out 100 - 200 Watts. I wouldn't drive
the tube to this extreme as this will shorten its life span.
Next, I had to design an electronic unit that would operate the tube
within it parameters. Except for the casing, I wanted to use
vintage parts. The voltage and current meters had to be vintage
analogue devices. The circuit design had to be simple, but
effective. I got my solder gun, solder and flux ready. My
workbench with all its resistors, capacitors and dials was ready. The
problem was that there were no schematics
available or any suggestions on any web sites on how to build a circuit
board for this application. I finally realized that the key to
building the tube tester truly understanding how the
vacuum tube works.

