>It attaches to the pole piece, eh?
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Correct.
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>I had assumed that the dust cap thingy on the FE107E contributed to the high frequency production, like a whizzer cone.
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It does, though it's normally all breakup modes unless well damped, so it tends to comb filter with output emanating off different areas of the cone, and of course contributes towards the acoustic radiation impedance when it's a point source.
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>Also, is the air gap between the new phase plug and the speaker cone not a problem?
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Not normally. Done right, the gap is tiny since the plug will be the exact same size as the pole piece. Don't want dust to get in the gap.
GM
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Correct.
====
>I had assumed that the dust cap thingy on the FE107E contributed to the high frequency production, like a whizzer cone.
====
It does, though it's normally all breakup modes unless well damped, so it tends to comb filter with output emanating off different areas of the cone, and of course contributes towards the acoustic radiation impedance when it's a point source.
====
>Also, is the air gap between the new phase plug and the speaker cone not a problem?
====
Not normally. Done right, the gap is tiny since the plug will be the exact same size as the pole piece. Don't want dust to get in the gap.
GM
Well, if you replace it with just the pole piece you'll definitely lose a lot. 
WRT attaching a phase plug, it depends.
I haven't done any rigorous research, but relying on my once upon a time keener than average male hearing and a lady friend's >22kHz hearing, when properly done, you lose the extreme top end of the break up BW (harsh/grainy/gritty sound), but the increased smoothness/coherence of what's left sounds more 'real' and far outweighs any measurable loss of HF BW.
Folks who prefer the top end 'air' sound of a titanium dome tweeter or compression driver will probably disagree though, so as always, YMMV, in which case cap off a piezo at 25-40kHz. When I felt the need, I used some series resistance to further flatten the BW and use a bypass cap to lift up the extreme HF, or you can drive them with a tube amp.
Here's a comparison of a few different phase plugs on a Lowther: http://audioprogress.free.fr/
GM
WRT attaching a phase plug, it depends.I haven't done any rigorous research, but relying on my once upon a time keener than average male hearing and a lady friend's >22kHz hearing, when properly done, you lose the extreme top end of the break up BW (harsh/grainy/gritty sound), but the increased smoothness/coherence of what's left sounds more 'real' and far outweighs any measurable loss of HF BW.
Folks who prefer the top end 'air' sound of a titanium dome tweeter or compression driver will probably disagree though, so as always, YMMV, in which case cap off a piezo at 25-40kHz. When I felt the need, I used some series resistance to further flatten the BW and use a bypass cap to lift up the extreme HF, or you can drive them with a tube amp.
Here's a comparison of a few different phase plugs on a Lowther: http://audioprogress.free.fr/
GM
One thing that has become apparent is that the 12dB per octave lowpass filter on the amp I bought does not cut it. I'm running it now using a Richter Scale for the lowpass filter. The alignment of the sub has a rising frequency response in the critical range when unequalized, and the mains are vented, so they have a sharp acoustic rolloff. To run it without the Richter Scale, I definitely need to get a plate amp with a 24dB/octave lowpass filter.
Live and learn.
Live and learn.
GM said:
My hearing doesn't go that high anymore, but for the sake of others, I'd like to get it right. A tube amp is right out! (Religious principles.) Adding series resistance is certainly an option. It already has a BSC circuit with 5 ohms. Maybe I could just put a cap in parallel with that.
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