Gaps between tubes, and in the funnel-to-tube coupling, should be sealed with caulk.
One transistor and a 386? A mere 66 dB of gain.
Coupled to a high-output piezo mic like the Mouser model specified, this demure piece will give anyone wishing to the polytube shotgun a fighting chance.
In fact, bypassing the emitter resistor with 10pF and placing a 10pF cap on 386
pins 1 and 8 will boost gain beyond 90 dB-and the device is fully stable if built on a neat,
spacious circuit board…
