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I was playing and heard a weird squeal followed by a pop and then a loud hum.
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08-23-2019 08:07 PM
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It’s a sign from the solid state Gods...
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And this is exactly why I quit relying on Tube Technology. Haven't had any issues since I went Solid State. Although you can bring extra tubes and fix it on the gig,LOL!
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Too many altered chords.
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Nothing intrinsic to the tube does that. It's the result of mechanical damage. For example, storing cables or pedals in the back of the amp can cause breakage when the amp is transported.
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I saw it happen a few times back in the day when everything used tubes, and the first consumer devices with transistors had just started to hit the market. TVs, car radios, phonographs, it was all tubes and we saw all sorts of stuff in the shop. But that type of failure was pretty rare, usually the tube just went completely black inside, or else just died in place. Like Greentone said, it was usually the result of mechanical stress, especially while the tubes were hot. Hot glass gets fragile.
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I've had something similar happen with a newer make tube. Problem with glass is that even a small scratch can create a major fault line with hot and cold cycles. Bummer it happened on a NOS item that probably cost a fair bit more than your regular JJ/Sovtek items.
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... NOS 12AT7
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This is what a hot spare head and a roadie are for, otherwise solid state :-)
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Originally Posted by Greentone
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Originally Posted by wildschwein
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Originally Posted by jzucker
One amp or two? What's the fail mechanism been? The heater or the amp volume dropping? Have the failed tubes been glowing brightly?
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Jack, I think maybe it was unable to keep up with you! :-)
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Originally Posted by GNAPPI
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Are you sure it's not a problem with your amp running these tubes to hot or something? I know it usually is power tubes that burn out first.
If it is the fault of the preamp tube itself, and I burned through 10 of them in a years time. I'd find another source for them.
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Nobody in their right mind would design and put into production a new vacuum tube for Fender amps. The cost for the startup would be astronomical. It is possible, however, to redesign the circuit to lower the voltage, for a few dollars. But then you no longer have a "vintage, all original" amp. It might work better, but the resale value takes a huge hit, for completely illogical reasons. Me, I solved the problems by using modern amps.
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Although it is not as dramatic as causing tubes to break there is a similar problem in Mesa amps where they use a cathode follower circuit. It's usually the fx loop preamp tube. It's known that Russian and Slovakian 12ax7s will burn up in those positions pretty quickly but Chinese tubes can handle it.
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Glass sucks. I sure hope you didn't get cut.
sincerely,
chas.
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Originally Posted by jzucker
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Originally Posted by jads57
- You have no idea whether it's really NOS
- You don't know what physical or electrical abuse it's been through
- It got to my hands being 40+ years old
People on the forums always wax poetic about the original tubes but reliability-wise, JJs have been the most reliable.
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Originally Posted by Franz 1997
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Originally Posted by jzucker
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Originally Posted by Franz 1997
The mid-70s MV Twin Reverb has 440 Volts on the plate.
I suspect that reducing the plate voltage on the driver would be roughly equivalent to turning down the dwell control, if the circuit had one. In my VSA amp that does have a dwell control, I prefer a very low setting. If I were concerned about that high a voltage over-stressing the tube, I’d consider reducing the voltage a bit—unless I wanted to play surf music.
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I looked at the AB763 Twin schem- and yes, you're right, 440v. Apologies, I must have looked at an earlier twin version. Although it seems odd that the voltage on the B+ side of the reverb transformer is shown as only 438v - but that's carping. A quick datasheet look ( Brimar) shows max AT7 anode voltage as 300v. In that case, the 12AT7 is getting both voltage and current stress, somewhat like an output tube being driven at max current , on over-voltage.
Always possible to connect the reverb driver to a lower voltage node, further down the B+ line.
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NOS tubes are the best, the newer ones just aren't built as well. you'll pay more but in the end they'll outlast new ones by a mile.
I think I've had maybe a 1/2 dozen preamp tubes fail on me ever, and that's gigging a few times a week w several amps for many yrs, plus home use. not to mention they were already in the amps when I bought them and they're from the 1960's. I tried some newer tubes and maybe got a year or less out of them.
right back to old RCA's, Mullards, GE's etc and no problems since.
Blowin in the Wind
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