User`s manual

squeal or make other space-like sounds. Mostly, they will make sound when you tap
them. A healthy tube may make a tink or bonk sound or nothing at all. An excessively
microphonic tube will make spastic sounds when tapped which can help isolate an
intermittently noisy tube. The 6BC8 will always exhibit some microphonics because it
is the first stage of amplification and has a relatively tiny signal passing through it.