Results 1 to 24 of 24

Thread: Will a cracked tube cause a fuse to blow?

  1. #1
    Supporting Member
    Join Date
    Jul 2019
    Location
    Northern California
    Posts
    290
    Thumbs Up/Down
    Received: 30/0
    Given: 22/0
    Rep Power
    0

    Will a cracked tube cause a fuse to blow?

    Does a crack (loss of vacuum) in a preamp or power tube cause current to rise enough to blow a fuse? Would it be correct to say that a crack causes the circuit to open?

    0 Not allowed! Not allowed!

  2. #2
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    33,468
    Thumbs Up/Down
    Received: 2,534/7
    Given: 0/0
    Rep Power
    57
    I guess. I don't think about it that way. I see a cracked tube, I think. "Hey, that's a broken tube." It needs to be replaced. it may or may not be causing whatever you are trying to fix, but it has to be replaced regardless. Install a new tube and FIND OUT if there is more wrong or not.

    1 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  3. #3
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,465
    Thumbs Up/Down
    Received: 3,084/5
    Given: 3,725/0
    Rep Power
    32
    Quote Originally Posted by Enzo View Post
    I guess. I don't think about it that way. I see a cracked tube, I think. "Hey, that's a broken tube." It needs to be replaced. it may or may not be causing whatever you are trying to fix, but it has to be replaced regardless. Install a new tube and FIND OUT if there is more wrong or not.
    This^^^^^^^^^^^^^^^^^^^^^^

    There are mechanical structures and connections in the tube that may or may not be open or incorrectly connected in the event of a catastrophic failure. There's always a likelihood that any temporary short could cause circuit failures in the components as well. A broken amp starts with troubleshooting 101. In this case, busted tube.

    1 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

  4. #4
    Supporting Member
    Join Date
    Jul 2019
    Location
    Northern California
    Posts
    290
    Thumbs Up/Down
    Received: 30/0
    Given: 22/0
    Rep Power
    0
    I understand that a blown fuse could be caused by a short in a tube but why would a cracked tube cause a fuse to blow?

    0 Not allowed! Not allowed!

  5. #5
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,875
    Thumbs Up/Down
    Received: 599/1
    Given: 550/2
    Rep Power
    19
    Normally, HT current flow relies on the vacuum. So if the envelope of a functional tube breaks, thereby losing the vacuum, in the absence of any other faults, I canít see why a regular fuse should blow.
    Maybe if itís shared cathode bias and each tube has a fuse but weíre well outside the remit of a generic query with that.

    1 Not allowed! Not allowed!

  6. #6
    Supporting Member
    Join Date
    Dec 2009
    Location
    UK
    Posts
    3,828
    Thumbs Up/Down
    Received: 507/1
    Given: 102/0
    Rep Power
    14
    I've had white-topped power tubes that have lost their vacuum, shorted and blown the HT fuse. I don't know the mechanism, other than the heater depends on the vacuum to keep it from evaporating material and burning out. Maybe the vacuum leaks the heater gets too hot and distorts the cathode so that it shorts against the screen or whatever, depending on the tube construction. A preamp tube short won't pull enough current under normal conditions to blow the HT fuse due the plate load resistor.

    0 Not allowed! Not allowed!

  7. #7
    Stray Cap DrGonz78's Avatar
    Join Date
    Oct 2011
    Location
    Scottsdale, AZ
    Posts
    2,227
    Blog Entries
    1
    Thumbs Up/Down
    Received: 402/5
    Given: 308/0
    Rep Power
    11
    Had a Mesa Boogie mkiii where one tube lost its vacuum whited out and the other tube on the other side had zero conductance. The two middle tubes were still good and the customer claimed he had played a show just recently with no problems. He had switched the amp in multiclass and other modes while doing the show and the amp worked either way. No fuses were harmed.

    0 Not allowed! Not allowed!

  8. #8
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    33,468
    Thumbs Up/Down
    Received: 2,534/7
    Given: 0/0
    Rep Power
    57
    Tube failure isn't generic. It is possible the tube would just sit there and cause no problem other than not functioning. The heaters run at super high temperature. In air they can burn up. That can cause shorts between elements.

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  9. #9
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,875
    Thumbs Up/Down
    Received: 599/1
    Given: 550/2
    Rep Power
    19
    I was thinking that with air in the tube, the heaters might run a little cooler, because in addition to radiation and conduction, they would then have convection to assist in dissipation.

    0 Not allowed! Not allowed!

  10. #10
    Supporting Member
    Join Date
    Dec 2009
    Location
    UK
    Posts
    3,828
    Thumbs Up/Down
    Received: 507/1
    Given: 102/0
    Rep Power
    14
    Well, with light bulbs the filament oxidizes in the presence of air. The oxide is non-conductive so reduces the CSA of the wire and the resistance goes up. As this happens the temperature increases and it burns out. I was thinking a tube would do similar. Just my flat-earth theory.

    1 Not allowed! Not allowed!

  11. #11
    Supporting Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    3,222
    Thumbs Up/Down
    Received: 2,352/1
    Given: 1,337/2
    Rep Power
    8
    Quote Originally Posted by Perkinsman View Post
    I understand that a blown fuse could be caused by a short in a tube but why would a cracked tube cause a fuse to blow?
    Yes. The visible crack is probably a consequence of a thermal runaway. This is caused by a somewhat leaky/gassy tube (and/or by too high value grid leak resistors). The gas molecules get stripped /ionized by the fast electrons and and the positive gas ions produce a positive grid charge which in turn rapidly increases plate current. Increasing plate current and temperature lead to catastrophic failure.

    https://archive.org/details/Tomer_19...ge/n1/mode/2up

    See pp 28/29.

    1 Not allowed! Not allowed!
    Last edited by Helmholtz; 02-08-2020 at 03:38 PM.
    - Own Opinions Only -

  12. #12
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,641
    Thumbs Up/Down
    Received: 2,186/24
    Given: 5,509/11
    Rep Power
    24
    I'd think whether it's running or not when it cracked would be a factor.
    I'd expect the heater of an already cracked tube to burn up during warm up like Mick outlined above (say a tube that cracked during transport or a new tube with bad vacuum).
    For a tube that cracked during use, I'd think many different things could happen, including faults that would blow a fuse.

    0 Not allowed! Not allowed!
    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

  13. #13
    Supporting Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    3,222
    Thumbs Up/Down
    Received: 2,352/1
    Given: 1,337/2
    Rep Power
    8
    I'd expect the heater of an already cracked tube to burn up during warm up like Mick outlined above (say a tube that cracked during transport or a new tube with bad vacuum).
    Yes, a cracked envelope might cause the heater to burn up, but this as well as atmospheric air pressure inside (electrons can't travel in air) would stop the plate current and not blow the fuse.
    So I think that thermal runaway was the reason for blowing the HT fuse and the cracking of the bulb.

    0 Not allowed! Not allowed!
    Last edited by Helmholtz; 02-09-2020 at 02:01 PM.
    - Own Opinions Only -

  14. #14
    Senior Member Pedro Vecino's Avatar
    Join Date
    Jul 2006
    Location
    Spain
    Posts
    718
    Thumbs Up/Down
    Received: 236/2
    Given: 122/0
    Rep Power
    14
    Many times the fissure is generated by the continous rubbing of the metal protector with the glass. Typically in the contact area between the metal fin and the spring. I mean the protectors that hold the top of the tube. Using heat shrink tubing in that area prevents it.

    Click image for larger version. 

Name:	tube2.jpg 
Views:	20 
Size:	359.8 KB 
ID:	56912

    1 Not allowed! Not allowed!

  15. #15
    Supporting Member
    Join Date
    Jul 2019
    Location
    Northern California
    Posts
    290
    Thumbs Up/Down
    Received: 30/0
    Given: 22/0
    Rep Power
    0
    If thermal runaway is the reason for the fuse blowing, is it an anomaly or is it a symptom of another problem? When I replaced the cracked tube and the fuse, everything seems to work fine, voltages were in spec, and sounds good.

    0 Not allowed! Not allowed!

  16. #16
    Supporting Member
    Join Date
    Dec 2009
    Location
    UK
    Posts
    3,828
    Thumbs Up/Down
    Received: 507/1
    Given: 102/0
    Rep Power
    14
    You can't assume it's thermal runaway any more than you can assume any other reason. A tube failure in the absence of any firm evidence to me is just a random tube failure. I received an order of brand-new, boxed tubes where some had a partial thickness crack in the class envelope, about 4mm long. So that isn't thermal runaway but rather mechanical damage and I bet if they were used the heat cycling would have caused the tubes to fail. While I was wrangling with the supplier trying to get them replaced one began to turn white on the gettering.

    I have some difficulty in visualizing thermal runaway in a preamp tube, or fuse blowing due to a crack. As previously noted, the current draw is limited by the plate load resistor and also the resistor chain that drops the main B+.

    If replacing the tube and fuse fixes the problem and it stays fixed, you'll perhaps never know what the reason was.

    0 Not allowed! Not allowed!

  17. #17
    Supporting Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    3,222
    Thumbs Up/Down
    Received: 2,352/1
    Given: 1,337/2
    Rep Power
    8
    I have some difficulty in visualizing thermal runaway in a preamp tube, or fuse blowing due to a crack. As previously noted, the current draw is limited by the plate load resistor and also the resistor chain that drops the main B+.
    You're right. I missed that the question included preamp tubes. As you say, a cracked preamp tube would not explain a blown HT fuse.

    Of course there are several possible reasons for glass cracks. And thermal runaway may be started by a micro crack, which let some air get inside.

    I was looking for an explanation for a blown HT fuse with a cracked power tube. Excluding other amp problems, I think it leaves essentialy 2 explanations for the excessive tube current: Either an existing crack blew the heater and caused some metal part to produce an internal short between electrodes.
    Or the power tube initially had no (visible) crack but was leaky and over time developed a thermal runaway caused by increased gas content. In this case extreme tube dissipation and redplating might have cracked the glass.

    0 Not allowed! Not allowed!
    - Own Opinions Only -

  18. #18
    Supporting Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    3,222
    Thumbs Up/Down
    Received: 2,352/1
    Given: 1,337/2
    Rep Power
    8
    Quote Originally Posted by Perkinsman View Post
    If thermal runaway is the reason for the fuse blowing, is it an anomaly or is it a symptom of another problem? When I replaced the cracked tube and the fuse, everything seems to work fine, voltages were in spec, and sounds good.
    Still not clear to me if you are speaking of a preamp or power tube.

    Thermal runaway is typically caused by a damaged or poor vacuum/quality tube. But too high grid resistance increases the risk with power tubes.

    0 Not allowed! Not allowed!
    - Own Opinions Only -

  19. #19
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,465
    Thumbs Up/Down
    Received: 3,084/5
    Given: 3,725/0
    Rep Power
    32
    Quote Originally Posted by Helmholtz View Post
    But too high grid resistance increases the risk with power tubes.
    As does a loss of bias. And in an old amp this is what I would suspect first. An intermittent grid connection would be both a high (or infinite) grid resistance AND a loss of bias. This could be a bad solder joint or dirty/oxidized tube socket or tube pins.

    I think it's clear that we're talking about a power tube if only because the fuse was blown. Not to assume these aren't coincidental circumstances that just happened together. That sort of thing does happen. But as it relates to the opening question, if the cracked tube DID cause the fuse to blow then it's a power tube.

    1 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

  20. #20
    Supporting Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    3,222
    Thumbs Up/Down
    Received: 2,352/1
    Given: 1,337/2
    Rep Power
    8
    As does a loss of bias. And in an old amp this is what I would suspect first. An intermittent grid connection would be both a high (or infinite) grid resistance AND a loss of bias. This could be a bad solder joint or dirty/oxidized tube socket or tube pins.
    Good point! Loss of bias would cause another kind of "thermal runaway" showing the same symptoms. An open grid in a fixed bias amp corresponds to infinite grid resistance. (In post #17 I explicitely excluded "other amp problems" just trying to answer the original question.)

    So even if the amp is working fine with the new tube it still makes a lot of sense to look for intermittent contacts.

    1 Not allowed! Not allowed!
    Last edited by Helmholtz; 02-09-2020 at 04:28 PM.
    - Own Opinions Only -

  21. #21
    Member dugdiamond's Avatar
    Join Date
    Oct 2009
    Location
    corona
    Posts
    32
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    Rep Power
    0
    YES IT WILL.
    The tube no longer has a vacuum and it will simply flashover and if you are lucky it will blow the fuse before it does any serious damage.
    Simple as that.

    0 Not allowed! Not allowed!
    Phineus J. Whoopy, you are the greatest! May just get one more peek at that three dimensional blackboard please?

  22. #22
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,465
    Thumbs Up/Down
    Received: 3,084/5
    Given: 3,725/0
    Rep Power
    32
    Quote Originally Posted by dugdiamond View Post
    YES IT WILL.
    The tube no longer has a vacuum and it will simply flashover and if you are lucky it will blow the fuse before it does any serious damage.
    Simple as that.
    I think it's important to qualify this statement by specifying a power tube as the culprit.?. A preamp tube would be subject to enough resistance that it wouldn't stress the mains fuse.

    1 Not allowed! Not allowed!
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

  23. #23
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    33,468
    Thumbs Up/Down
    Received: 2,534/7
    Given: 0/0
    Rep Power
    57
    I find myself having a similar discussion about preamp tubes in other contexts. Pointing out that even if the plate dead shorts to ground, the 100k plate resistor limits current to two or three milliamps, which won't blow any fuses. Exception might be a cathode follower, but then plate to cathode shorts are exceedingly rare.

    1 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  24. #24
    Member dugdiamond's Avatar
    Join Date
    Oct 2009
    Location
    corona
    Posts
    32
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    Rep Power
    0
    Quote Originally Posted by Chuck H View Post
    I think it's important to qualify this statement by specifying a power tube as the culprit.?. A preamp tube would be subject to enough resistance that it wouldn't stress the mains fuse.
    Thank you for recognizing what I had specifically had in mind, but failed to elaborate.
    For whatever reason, I was indeed only thinking in terms of a power amp tube. I really didnt consider the case of other types of tubes or their functions within a circuit, audio amplifier or any other type of circuitry.
    But yes loss of vacuum in a power amp tube or rectifier tube usually will result in some sort of flashover that likely will take out a properly rated fuse.
    Also many amplifiers will have two separate fused circuits. The "HT" fused circuit being the fuse that would likely go open in the case of the power tube losing vacuum and flashing over.
    At least in my always limited but ever expanding experience and understanding.
    I am always learning and it is always my preference to be correctly taught why, rather than discover why not, all on my own! Experience can be a harsh lesson plan if unprepared. I am thankful for the many that share their experience so freely.

    0 Not allowed! Not allowed!

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Hartke A70 blow the fuse
    By Fabrizio in forum Guitar Amps
    Replies: 31
    Last Post: 08-17-2016, 04:26 PM
  2. Is it possible and/or likely for a preamp tube failure to cause a mains fuse to blow?
    By mort in forum Maintenance, Troubleshooting & Repair
    Replies: 37
    Last Post: 10-31-2015, 11:56 PM
  3. Heaters blow fuse
    By jussi in forum Debugging Your Build
    Replies: 5
    Last Post: 09-07-2013, 12:30 PM
  4. Rectifier tube/Power tubes causing fuse to blow?
    By MattB930 in forum Maintenance, Troubleshooting & Repair
    Replies: 8
    Last Post: 10-09-2010, 10:34 PM
  5. newbie bf champ blown fuse, cracked resistor
    By sheppadizzle in forum Maintenance, Troubleshooting & Repair
    Replies: 5
    Last Post: 12-04-2007, 06:43 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •