Acoustic B410 mkII tweeter

[Randall]

Got an Acoustic B410 mkII in to replace a speaker. Got it installed and noticed the bullet tweeter isn't doing much, if anything. If I unscrew it, and put it close to my ear I can barely hear something. Guy wants to replace it. Problem is, I don't know what to replace it with or where to look. There are no markings on it whatsoever. I don't know how these even work with regards to power handling, and frequency. Can I just throw any generic bullet tweeter in there if it fits, and call it good? This one is square with corner screw holes at 2.75".

The cab is rated at 600W 8 ohm. That's another thing, are piezos rated in ohms, and would that effect the cab impedance?

[Enzo]

Piezos are like capacitors that make noise. They don't come in ohms.

There is apparently a switch to turn it off, but is there a crossover? I mean cap or cap and coil, not just a resistor. No crossover means likely piezo, as they cross themselves over naturally.

Does it have screws? Take it apart gently and see what it is made of. it is a piezo or it has a diaphragm with a voice coil.

it is a present day product, contact the company and find out what they ask for the part. Hmmm, looks like their web site says they don't offer parts for their amplifiers.

Disconnect it and measuer with an ohm meter, does it have some low resistance or is it open like a cap?

[Randall]

No crossover, just a resistor. Opening it destroyed it as the diaphragm came unglued from the cone. The terminals measure 133 nF, and it looks like a cap charging up when measuring Ohms, until it settles in like an open.

And yeah, I have not phoned Acoustic yet after seeing the thing about not supplying parts. I'd rather do it myself if I can.

And it seems like it was crossing over too high, because I could only hear some hiss, and not much from the instrument, but I don't understand how that would work.

[Enzo]

Then it is probably just a cheap piezo, and find one that looks like it. The old Motorola ones, now made by CTS had a common driver - they call it a "motor" - It had three screws holding it together. I could buy the basic little bullet job for 90 cents, and remove the motor and put it on a more expensive piezo horn, like the dual piezo divided horn in an old KB300 PV combo.


The things naturally don't respond to much below 2000Hz, so a lot of bass doesn;t make sound from it, the horn increases the percussive edge and other zingy parts of the sound.

These are pretty cheap cabs, seems to me, a piezo sounds about right.


The piezo is a ceramic crystal that deforms when voltage is present across it: it shrinks in one dimension and grows in the other. SO they use that phenomenon to move the cone. You can drive it without the cone, those sonalerts and similar piezo buzzers are just a flat disc. The disc can crack and they then barely work, or the stop altogether.

[The Dude]

If you could post some pictures, it would help.

[Randall]

[Enzo]

It is not the same molding, but it looks to be a knockoff of the old Motorola basic piezo. Apparently CTS sold the Moto tolling off to yet another company some years back, but they are still making them.

GRS PZ1005 3-1/4" Piezo Horn Tweeter Similar to KSN1005A

[Randall]

Funny, I just came here to ask about that very same page! Thank you! I'll grab a few to have on hand. Good mark up on those, I'd say.

One last question, how is it they can handle 400 watts when they are rated at 50/75 watts?

[Enzo]

They don't, any more than a common compression tweeter handles 700 watts in a 700 watt PA cab.

The piezo doesn;t respond to lows much anyway, it only has to handle that which it puts out. The resistor is there to drop some volts.

There isn't generally 400 watts of highs going on when your bass amp puts out 400 watts of audio. or any audio signal really. That is why a big PA cab might have a 400 watt 15 woofer, and a mid, and a 50 watt horn driver for highs. Put some music on your scope. The bass is the large signal, while the highs are the furry little stuff between the kick drum and bass spikes.

[The Dude]

They don't actually have to handle 400 watts as they only respond to higher frequencies.

On a side note, many of the piezo tweets have the same bolt and rear assembly patterns, including some of piezos with larger horn exits. You can remove the screws on the cheaper ones like in Enzo's link and stick the rear "driver" assembly on many different exits.

<simulpost>

[Randall]

Not the one I have. The cone is glued to the exit, and the diaphragm is attached to the terminals on the back cover. Looks to be assembled with fresh glue.

[The Dude]

Well, certainly not true every time, but often. Much of the time, the cone is just sandwiched in and attached only to the crystal.

[olddawg]

Look on MCM. The have a whole selection of piezo tweeters. Cheap.

[J M Fahey]

1) confirm itīs a piezo, same as the common "6 slices" one, but with a different shape horn which makes it vaguely resemble a bullet horn,

2) to save on assembly. the cone is glued to front and piezo disk, which must float to vibrate freely is epoxied to cone apex.Then tinsel wires go through eyelets and are soldered.

They "should" have been desoldered and solder sucked before pulling rear cover, since you didnīt, they tor the piezo disk off.
Oh well, itīs already done.

Some piezos do not bother to glue the cone to horn and sandwich edge between 2 cardboard rings, but saving them save 1 cent which in the great scheme of things means a lot.

Here I can buy spare cones and disks and repair piezos, doubt it pays to do so in USA.