[Id ita]

Group: rec.audio.tubes Date: Sat, Aug 11, 2007, 4:01am (PDT+7) From:
(maxhifi)

"Id ita" wrote in message
...

**So, the current fave 45 is suited to meet current real-world
facts: my modest SPL
requirement, a preference for small-ensemble music, an acceptable
driver on hand...


What kind of driver is it? I built a stereo 2A3 amplifier, using
EF37A drivers, and a 6CA4 rectifier, and love the sound of it,
however, it's too weak to use full time with my main speakers.


It's a driver from Frankenstein's lab. A Jensen 855A 8" with the outer
two-thirds of the cone and untreated paper surround removed, and
replaced with the outer cone and cloth surround of a Radio Snack
(40-2086?), carefully glued to the remaining inner cone.

A whizzer was removed. The inner cone and
dust cap were treated with an acrylic sealer. Strips of 1" carpet felt
were glued to the inside of the basket struts. The result is
considerably smoother than the stock Jensen, but the top octave is
pretty much gone. I've used a small horn tweeter to fill it in, and
gotten an even match, but the horn is too aggressive no matter how it is
adjusted. With the horn gone, it's back to "better than hi-fi" despite
the soft highs.

The enclosure is a 3.6 cu. ft. sealed box. A port would extend the
lows, but complicate the impedance curve to the distress of my SET. I
prefer smooth rolled bass to a "thump" anyway.

A good match to this kind of driver might be an SE pentode with no loop
feedback, working as a constant-current amplifier. The voice coil Z
typically starts to rise at the frequencies where the cone mechanically
rolls off. A current source puts higher voltage across the higher Z, and
compensates the roll-off. But then the bass is utterly undamped, and an
open baffle will likely work better.

Regards,
Barry