Microphonic Tube??

[Ed Lambert" "K1ZOK]

Actually, it's pretty easy to find a microphonic tube.

Advance the volume control to a point just before it begins to howl.
Then tap all the tubes with the eraser end of a pencil.  The one that
makes the noise is bad.

Ed
K1ZOK

On Sat, 13 Jan 2001 15:12:54 -0600 Ron Evans <cosmos41 at IX.NETCOM.COM>
writes:
> I recently bought a Hallicrafters S-38D (I know...very minimal
> receiver!) which was my first receiver back in 1957.
>
> This one has a few problems, some of which I can repair but I need
> your
> help on a pretty simple (for you guys!) problem.  Although I've been
> a
> ham since I was 16 years old, the technical side of it just never
> was my
> forte.  But now, I'm enjoying working on old radios and really just
> in
> the beginning stages.
>
> Question:  When I advance the Volume control to a certain point,
> there
> is a loud "howl" for want of a better word.  It sounds like it
> would
> continue getting louder and louder (like feedback) if I didn't
> decrease
> the volume setting.  Is this possibly a microphonic tube?  If so,
> would
> it be more likely to be the Detector & Audio Amp 12SQ7 whose grid
> is
> connected to the volume control, or could it be the Audio Output
> 5Ol6-GT?
>
> I don't have a tube tester, so I'm kinda shooting in the dark.  I
> have a
> lot to learn; I know that!  I guess the first thing I've learned is
> that
> I need a tube tester!  I could substitute known good tubes if I had
> any
> but hate to buy a full set on speculation.
>
> Anyone have a buoy they could throw a drowning senior?!
>
> As always, my gratitude.
>
> 73,
>
> Ron - K5MVR
>