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  1. #1
    I was playing and heard a weird squeal followed by a pop and then a loud hum.
    Look what happened to my NOS 12AT7-20190823_181531-jpg

  2.  

    The Jazz Guitar Chord Dictionary
     
  3. #2

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    It’s a sign from the solid state Gods...

  4. #3

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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!

  5. #4

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    Too many altered chords.

  6. #5

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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.

  7. #6

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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.

  8. #7

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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.


  9. #8

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    ... NOS 12AT7
    I guess it is not "NEW" any more....

  10. #9

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    This is what a hot spare head and a roadie are for, otherwise solid state :-)
    Regards,

    Gary

  11. #10
    Quote Originally Posted by Greentone View Post
    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.
    never done that in my life and in fact, that tube was never out of the house since I put it in the amp.

  12. #11
    Quote Originally Posted by wildschwein View Post
    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.

    actually the NOS Jan Phillips AT7s are regularly on sale for $10.95 through tubedepot which is about the price of a JJ. I just ordered another 10 recently. Surprising how quickly I've run through them though. I thought the previous 10 from a year ago would have lasted me a lifetime.

  13. #12

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    Quote Originally Posted by jzucker View Post
    actually the NOS Jan Phillips AT7s are regularly on sale for $10.95 through tubedepot which is about the price of a JJ. I just ordered another 10 recently. Surprising how quickly I've run through them though. I thought the previous 10 from a year ago would have lasted me a lifetime.
    Gads, pre-amp tubes going that fast? I'd have a DMM or scope on that pre-amp circuit in a heartbeat.

    One amp or two? What's the fail mechanism been? The heater or the amp volume dropping? Have the failed tubes been glowing brightly?
    Regards,

    Gary

  14. #13

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    Jack, I think maybe it was unable to keep up with you! :-)

  15. #14
    Quote Originally Posted by GNAPPI View Post
    Gads, pre-amp tubes going that fast? I'd have a DMM or scope on that pre-amp circuit in a heartbeat.

    One amp or two? What's the fail mechanism been? The heater or the amp volume dropping? Have the failed tubes been glowing brightly?
    fail mechanism is them getting squeally and microphonic. Not sold on the NOS tubes. I also have been having problems with the expensive tung-sol reissues. I think I'm going to go with JJs for preamp tubes from now on.

  16. #15

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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.

  17. #16

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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.

  18. #17

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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.

  19. #18

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    Glass sucks. I sure hope you didn't get cut.

    sincerely,
    chas.

  20. #19

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    Quote Originally Posted by jzucker View Post
    The reverb driver in a fender amp has 440v on it. Way over spec. It's always been a problematic tube. Too bad nobody can just design a new tube that can take that kind of heat.
    That can't be right. Even on a Twin the schematic will show 300v or less on the reverb driver tube. If you have measured 440v then something is wrong.

  21. #20
    Quote Originally Posted by jads57 View Post
    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.
    i didn't burn through ten of them with a single amp. This was through several different fender style amps and ten was an exaggeration. I bought ten and still have a few left. It's probably more like 5 of them and 3 with this amp. And honestly, the issue with NOS tubes has always been that:

    1. You have no idea whether it's really NOS
    2. You don't know what physical or electrical abuse it's been through
    3. 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.

  22. #21
    Quote Originally Posted by Franz 1997 View Post
    That can't be right. Even on a Twin the schematic will show 300v or less on the reverb driver tube. If you have measured 440v then something is wrong.
    Yes, you're right. I was looking at the nearby B+ line on the schematic. Still, this tube is a known problematic tube in all fender amps. You can't just use any old AT7 in that position.

  23. #22

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    Quote Originally Posted by jzucker View Post
    Yes, you're right. I was looking at the nearby B+ line on the schematic. Still, this tube is a known problematic tube in all fender amps. You can't just use any old AT7 in that position.
    I agree it's a problem, but I think it's cos the driver tube draws very high current through the driver transformer. Like many, I have a soft spot for NOS valves/ tubes, but find they are less reliable than modern tubes and can fail for no apparent reason. The one exception is ( real) Mullard rectifier GZ34/5AR4 tubes. They seem to go on for ever.

  24. #23

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    Quote Originally Posted by Franz 1997 View Post
    That can't be right. Even on a Twin the schematic will show 300v or less on the reverb driver tube. If you have measured 440v then something is wrong.
    From the Deluxe Reverb AB763 schematic:

    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.

  25. #24

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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.

  26. #25

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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.

  27. #26

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    I think it's generally agreed that NOS tubes were better built with better QC, I think the issue may be - if you've had them fail - that they would have been rattling around for decades, possibly tossed around in transit, maybe passed around amongst dealers during trades, and may have developed mechanical problems. I've never had an NOS tube fail electrically, it's always been mechanically; glass breaking, heater winding fracturing etc. Also it's in the nature of things that todays's NOS tubes could well have been at the bottom of the QC pile when they were originally made 50/ 60 years ago, who knows. In the 80s NOS tubes were very cheap, in the 'military surplus' category, so they hardly got feather-bedded treatment.

  28. #27

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    Quote Originally Posted by jzucker View Post
    i didn't burn through ten of them with a single amp. This was through several different fender style amps and ten was an exaggeration. I bought ten and still have a few left. It's probably more like 5 of them and 3 with this amp. And honestly, the issue with NOS tubes has always been that:

    1. You have no idea whether it's really NOS
    2. You don't know what physical or electrical abuse it's been through
    3. 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.
    See #1, #2 and #3 above.

  29. #28

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    This is a forum, no point in saying things just once

  30. #29
    Quote Originally Posted by wintermoon View Post
    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.
    That has not been my experience and I think franz is right. When they were new, they were way better made for the most part than their modern counterparts. But the NOS Jan Phillips AT7s are junk IMO. I just bought 10 more of them but I wish I hadn't. Another one started squealing last week in the PI position where voltage and current aren't a concern. When I first got the 10 that I bought a year ago, I rotated them through several days worth of playing in either the PI or reverb driver positions and they all checked out ok but as time went on, most of them have gone belly up in a relatively short period of time in several different amps. A lower voltage vibrolux, a fargen, a morgan rca35r and a vintage sound 35.

    Conclusion, electrically they check out ok, but mechanically they have been jostled around for 40+ years and you cannot depend on them. I think I will probably go with JJs from here on in. I just got a pair of NOS rca AT7s which sound great in the vintage sound but who knows how long they will last?

  31. #30

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    Amend!!!

    Quote Originally Posted by jzucker View Post
    People on the forums always wax poetic about the original tubes but reliability-wise, JJs have been the most reliable.

  32. #31

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    Quote Originally Posted by jzucker View Post
    fail mechanism is them getting squeally and microphonic. Not sold on the NOS tubes. I also have been having problems with the expensive tung-sol reissues. I think I'm going to go with JJs for preamp tubes from now on.
    A tech recently told me that he sees a lot of problems with the JAN Philips tubes. His theory is that the ones that are around now were made when production was winding down and QC was less rigorous. But they do sound good.

    John

  33. #32

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    yeah, I never liked the JAN Phillips preamp tubes.
    But I love the Phillips STR-387 and 415 double getter 6L6 power tubes, they're all I use and sound superb.
    Those RCA's should last forever Jack and if they don't it's probably because you play too hard

  34. #33
    Quote Originally Posted by wintermoon View Post
    yeah, I never liked the JAN Phillips preamp tubes.
    But I love the Phillips STR-387 and 415 double getter 6L6 power tubes, they're all I use and sound superb.
    Those RCA's should last forever Jack and if they don't it's probably because you play too hard
    They sound better than anything else I've had in there. I'll probably buy some more if I can find them. The guy only had 2. There's another guy who sells pairs of NOS Mullard 4024s for $75. I got a good deal on the RCAs though. It was about 1/2 that.