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Re: 6as7 life continued...

> When is a 6as7 not good?

Let's start with the easy ones:

- Internal shorts
- One or more dead filaments
- One or more blown internal links
- Loss of vacuum (gassing)
- Fractured envelope

> What measurement makes it not good anymore?

Without getting into the really wooly world of tube testing (and tube testers) there are two main quality factors:

- Emission (cathode)
- Transconductance

For most small-signal & power tubes, transconductance is the benchmark of quality. The 6AS7 is a special case however, due to its' high transconductance and low Mu. This combination of characteristics creates a problem for transconductance tests; most testers (even the Hickoks..) don't have enough resolution or accuracy (mainly on the control side, not the meter side) to give a meaningful transconductance figure for a 6AS7.

If we need to test an otherwise working 6AS7 at the factory, it's done on a 1940's era Philco emission-type tube tester. This tester has a simple pass/fail readout - if in doubt, we compare meter readings to a known-good (new) tube. This approach has never failed us.

Bill


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