|[AMRadio] Tube ID|
k4kyv at charter.net
Mon Feb 24 14:09:00 EST 2014
I don't understand what could be so mysterious or complicated about ID-ing
a VR tube. The miniature versions and octal versions all have the same
pin-out. Just plug the tube into a piece of equipment and measure the output
voltage with a voltmeter. Then look in the tube charts to see which tube
type regulates to that voltage.
The tube is normally wired in series with a dropping resistor, usually a
wirewound power resistor, and the series string is connected from the output
of the voltage source to ground. 90% of the time, the tube will fire,
regardless of the nominal rated output voltage. Even if the wrong tube is
plugged in, it is highly unlikely to damage anything in the short time
necessary to take a meter reading.
If the tube fails to fire, then it is either bad, or else the voltage rating
is too high for the voltage source, something I have rarely encountered,
although a VR-150 sometimes won't fire in a circuit designed to use a VR-75.
Another simple test would be to clip-lead a resistor in series with the
tube across a known voltage source and read the voltage that appears across
the tube. (Use some maths and calculate the appropriate resistance to avoid
damaging the tube or power source by drawing too much current).
Most VR tubes give off a purple or bluish glow, but the 75-volt ones use
neon gas, and give off an orange glow.
VR tubes, especially the 75-volt variety, work almost as well as neon lamps
as RF indicators.
An often overlooked VR tube is the type 874. It was the original
mass-produced gaseous regulator tube, rated for 75 volts, and dates back to
the late 1920s, but can be found in equipment manufactured well into the
post-WWII period. It is designed to draw up to 40 milliamps at minimum load,
while the octal and miniature versions are rated for only 30 ma, allowing
the 874 to regulate over a wider range of source voltages and output loads
than the lighter duty VR-75 and its miniature equivalent .
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