[R-390] (I found the problem) NOT!!

[Barry Hauser]

> Ha, your'e right!  The sound from the tapping sounds like when a
microphone
> is tapped.  Almost like an echo.
>
> Jim

Yup -- that's why they're called microphonics. Now try turning up the audio
and yelling at it.  If you can hear, however faintly, your own voice through
the headphones or speaker, the microphonic is almost definitely in a tube.
If not, it still might be a tube.

Some tube types are microphonic prone.  I think the 6GH8 is one, not sure
about the 6DC6.  That's why it isn't far fetched to have a bad batch.  (It
can happen during manufacturing or in transit.)  Can you "borrow" another
6DC6 from some other rx?  (I'd still go over the solder connections.)

Some tube testers have output terminals for a speaker or amplifier-speaker,
so you can tap the tube and listen.  It probably won't show on the meter.

Barry