Microphonic Tube??

[Ron Evans]

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