On Wed, 13 Dec 2006 14:50:55 -0000, "N Cook" wrote:

Stephen Cowell wrote in message
t...

"N Cook" wrote in message
...
Could a Crate BV120H valve amp really blow 4x 12 inch 16 ohm speaker box
?
Amp as received was set for 16 ohm output, I measured the speaker box
impedance at 13 ohm, or so, before connecting up just to be sure.
Very feeble and distorted sound. No signs of the reported burning smell,
output matching transformer primaries at about 50 ohm seemed ok and
secondaries not open circuit in the amp so turned to the speakers. The
amp
o/p switch setting was set for 4 ohms instead of 16 ohms but as I
measured
16 ohms I assumed a switch problem.

It sounds like the speakers were blown before this exercise
started... what you did (with no loud noise) should not have
blown any speakers... *maybe* the amp could suffer, but
not the speakers.
__
Steve
.



Reasonably easy to cleanly remove one of the cones.
If a next time, to cleanly remove the dust skirt I will make a small hole
and pull with a sickle probe to cleanly remove that section.
No sign of burning on the voice coil so I could count the number of turns in
both layers if I was to rewind another. But the cardboard core that this
coil was on has broken up completely . Not knowing how to do precision
origami , looks like the end of that one.

Voice coils, cones and spiders are all replacement parts. Just that I don't know
who to get them from in your neighborhood.

I don't think it is so much an issue of the amp damaging the speakers as in your
original question, but of speakers simply failing as they will. There are a lot
of very cheap speakers out there now. Tube amps don't fail by generating a DC
offset on the output like solid state stuff will.

If there is power for power no audio difference between a 16 and 4 ohm
speaker, is there an advantage to protecting amps or speakers to opt for 2
pairs of parallel 16 ohm for 4 ohms overall or 2 seriesed 4 ohms in parallel
with same for 4 ohms. For 16 ohm setting for the last one of 4 4 ohm in
series then if 1 goes o/c then that would protect the others

There are some tonal differences in how the speakers are connected. I'll suggest
that your series arrangement is a poor choice - true if one speaker goes open it
protects the others - however tube amps really, really dislike throwing signal
at an open load. Often the unterminated output transformer will do a flyback
phenomenon that puts several thousand volts across the primary and arcing tubes,
sockets or the tranny.

I'd stick with series-parallel as traditional for a 4x12, just do it with better
drivers or a higher quality recone on those.

Ron