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Anybody blow a ceramic woofer with an OTL amp?

I would appreciate anybody's experience with transients (such as a tube blowing) in an OTL amp that results in breaking the ceramic driver in a Kharma 3.2. I just had this happen to me - a driver tube blew, terrible noise from the woofer and it broke. The Kharma distributor said this was only the second he had heard of with an Atma-Sphere amp, but that it was much more likely with an OTL amp because of the lack of transformer. Ralph is interested in the frequency with which this occurs, as he feels it only happens with ceramic woofers, which are much more likely to blow than non-ceramics. Have any other OTL owners had this happen to Kharmas? I guess the lesson is to keep testing tubes or keep a careful eye on them to prevent this happening, but I would like to know if this was a freak happening or more common than has been generally accepted? Judging from the number of Tenor-Kharma combos out there, it should be more common than I thought if the combination of OTL and ceramic woofers is a problematic one.
springbok10
rushton's avatar
rushton
2,445 posts
 
I think Springbok is asking about the frequency of occurrence, as in number of events, as opposed to audio spectrum frequency.
"the frequency is DC"
Springbok, sorry to hear that a blown tube took out your driver. It's rare for those Russian 6AS7Gs to blow rather that simply shorting their internal fuse with little other fuss.
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honest1
586 posts
 
I think he means how frequently it occurs.(not the frequency of the signal that did the damage.)
springbok10 OP
711 posts
 
Actually, it was a 6SN7 that blew. (RCA Black plate NOS) Very unusual. Yes, I am enquiring about occurrence frequency.
trelja
3,223 posts
 
I'm also sorry to hear this news, Spingbok10.

I have had 6AS7G and 12SX7 tubes go bad on me. The 6AS7 continually builds up blue gas inside until they pop (and REALLY violently), which relieves the pressure. Then, the whole thing repeats. The fuse arrangement provides no defense. However, neither the amp nor my speaker was damaged in the slightest way. Replacing the tube corrected the situation, and bias was still spot on.

This being the only 6AS7G go down on me, I have been fortunate. Others have told me about the potential, ala the Soviet era QA/QC mindset. This being the reason several at the old ASOG site developed means of testing, run in, and burn in for the tube.

The 12SX7GT would send a really disconcerting pop through the loudspeaker at turnoff. Ralph's initial diagnosis was an output tube, but after going through each, I was still experiencing it. Trying the same thing with the input tubes found the culprit. I'm just glad it wasn't a capacitor or some of the other things we discussed.

For the record, I have never had a 6SN7 go bad on me.

As far as ceramic, in this case, alumina (Al2O3) drivers go, despite being very hard, they are brittle. It is not unknown for them to be easily damaged via shipping, handling, and now this reason. The one advantage a more conventional paper or polymer driver would have in this situation would be their compliance and resilience.
avguygeorge
2,300 posts
 
Would not a bias resistor have prevented this?? Meaning the resistor takes the hit??
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