[HAM] Best 12AU7

[Scott Hawthorn]

At 08:58 AM 3/11/2008, David Anderson wrote:
>Scott,
>You can use a tube tester for both purposes.
>You can, however, use the amplifier itself to do this as well.
>
>Leslie driver tube sections share a cathode resistor, but if (and
>only if) you're sure that the 56k plate resistors are well-matched to
>each other, you can tell by the plate voltages if both sections are
>drawing equal bias currents.
>
>And if you have an oscilloscope that can sum the readings for two
>channels, you can test the transconductance balance of the 12AU7 by
>measuring the signals at the twin triode plates. If you sum the 180
>degree out of phase signals, you should get zero or close to it. The
>same goes for a 147 circuit, though the 12AU7 is also acting as a
>phase splitter here. (If you wanted to, you could implement AC
>balance in a 147 amp by installing a pot to vary the voltage dividing
>point where the inverted signal is fed back to the 2nd 12AU7 section. )
>
>You could theoretically do something similar with output tubes, but
>the plate voltage swings of an output tube are usually too large to
>read directly with most scopes unless you protect them somehow. You
>can test for balanced output tubes by running the amp into a 16 Ohm
>load resistor and looking for symmetrical clipping.

Very useful information David, thanks! Some of it I knew, some not.