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wrote:

Just to throw this out for a 25 + year old speaker. I have Acoust
model X with the servo amps...

Any one reading these threads who has Acoustats, do not let them
divert your attention. They are worth the effort.

The X, if I recall, used tube output to directly supply high voltage to the
panels. Taking some time, I found (from a 1977 Audio Critic) that the
panels were driven from the plates of 6HB5's. Tone bursts revealed
substantial ringing around 1 KHz and 4. Standing waves in the large panels
were suggested as a possibility. A 0.35 msec pulse was unable to be
accurately reproduced. FR was good, and reasonably flat. Vertical beaming
was severe. A 35 Hz bass bump was present on large signal feed. Both amps
blew up during testing. However, given the price, it was an OK speaker if
one could live with low signal levels.

Michael