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Scott Dorsey Scott Dorsey is offline
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Default Simple Test Circuit for Tube Noise

wrote:
Can anyone suggest a simple circuit to measure vacuum tube noise?


There are some details in the Radiotron Handbook. You might also find
some information in a VERY early version of Motchenbacher's book on low
noise electronics.

However, you're better off testing in-circuit, or in a jig that emulates
the circuit that the tubes will be used in, because the different noise
sources are of different importance in different applications.

How
about negative (emission) grid current?


If you ever have to worry about grid current in an audio circuit, something
has gone horribly wrong. From the standpoint of the input, the tube is a
big capacitor. The only time the tube ever draws current is when the Miller
capacitance is so high that it's acting like a capacitive load on the input.
You might see that in a big power tube occasionally, but that's the last place
to worry about noise.

Also, what are some of the
quieter tubes out there? Thanks!


For what application? Part of the problem is that the input and output loads
have a lot to do with noise performance. Part of the problem is that some
applications have big issues with microphonics and others don't. And if you
are worried about RF noise, all the rules change.

In general, I'll point out that for low level signals with minimal drive,
nuvistors are hard to beat. And there are some really impressive compactrons
out there; people ignore them because they are viewed as TV tubes, but that
means the prices are low and the numbers are good. And in a pinch there is
always the EF86, the 6DJ8, and other frame grid types.

A lot of it has to do with quality of construction and cleanliness of
assembly too. There are very quiet 6AU6 tubes out there... but there are
also lots of very, very noisy ones.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."