How To Tell If Your Vacuum Tube Is Bad

How To Tell If Your Tube Is Bad

We are often asked how to determine when a tube is bad without access to a tube tester. You’ve probably been there. You’re doing sound check before a gig, something sounds wacky, and now you’ve got to figure out what’s happening. Well although not all tube problems can be found without a tester, many problems can be. We’ll go over some of the most common scenarios that a guitar player might find with his or her guitar amp. But keep in mind the same issues will arise in hifi stereo tube amplifiers and preamplifiers so audiophiles will find this article useful too.

Visible Signs

We’ll start with visible signs. There are a couple very obvious things to look for. First, every tube will have a heater filament that, when working, will cast a nice warm orange glow. In some tubes it’s more intense than others, but this is not a problem. What matters is that it is glowing to some degree. There are some tubes that have their filaments well hidden making it nearly impossible to see. In these cases you can carefully check if the tube is hot or cold. Be careful not to burn your skin. The tube must be heated in order to function. If the filament has failed the tube is useless.

White getter on a bad tube VS a healthy tube getter

The second thing to look for is the condition of the getter. This is the greyish coating usually found at the top of a tube, but can be on the sides, or both top and sides, depending upon the tube type. Any color from grey/silver, to black is healthy. When a vacuum tube develops an air leak (a small crack or bad seal by a pin for example) this getter color will change to pure white. If you see this you know with 100% certainty that the tube is bad.

Bad Purple Vacuum Tube GlowThird, look for a purple glow that is very focused around specific elements inside the tube. Do not confuse this with blue glow that is often cloudy and near the glass. The purple glow around wires or other elements indicates leakage and a tube with this should be discarded.

Perhaps the most obvious thing to look for is any loose parts that have broken off inside the bottle. You can gently shake the tube as well and listen for rattling. All tubes will have some degree of noise when you do this as the grid and screen wiring vibrate so don’t mistake that for broken connections.

Red Plating Power Vacuum TubeWe should also mention that when it comes to power/output tubes another issue can be red-plating. This is sometimes due to wrong biasing of the amp, which really isn’t a tube problem. But there are times when a properly biased amp has a tube that begins to red-plate. This is a sign that that specific tube is failing and in a “run-away” state were the current can’t be controlled by the bias voltage. A tube like this should be replaced. If not, the amp will eventually blow a fuse or worse damage other parts.

Amplifier Will not Power On

In the event your amplifier will not power on, it is almost certainly due to a blown fuse. Check and replace and blown fuse and try again. If the replacement fails immediately it’s often a sign of a failed power/output tube or amplifier fault. If the visual inspections mentioned earlier do not help locate a bad tube we recommend hiring a technician to inspect your amp.

Microphonics and Noise

Problems with preamp tubes are often due to microphonics and noise issues. A microphonic tube will ring and amplify any outside noises such as bumping the amp, tapping on the bottle, or even footsteps as you walk across the floor. All tubes will amplify tapping to some degree but an unusable tube will be very loud and often feedback or squeal. In a guitar amp with many preamp tubes it can be difficult to determine which one is microphonic. This is because tapping any tube near the bad tube appears to be bad as well. Rest assured it is very unlikely to find multiple bad tubes all at once. We suggest you tap each tube gently with a pencil or chopstick (something wooden or plastic, not conductive) and often the culprit will be louder or noisier than the others. Replace that tube and likely all will be quiet. In a home stereo amp or preamp these same steps can be followed. However one more trick that is useful is to swap a suspect tube to the same position in the other channel. If the noise follows to the other channel you know you’ve found the bad tube. If it doesn’t, you know the noise is caused by another tube and you can repeat this procedure one tube at a time until you find it. In a stereo amp you would use this same procedure to find tubes with any noise issues.

Tube noise such as sputtering, hissing, popping can be tougher to find in a guitar amplifier. If you have a spare tube of the same type it’s good to substitute it in place of one in your amp and then listen for the noise. If it’s gone you know you removed the bad tube. If not, re-install the original tube and move your spare to the next position. Repeat until the noise is gone and at that point you’ll know you’ve removed the noisy tube. Some guitar amps with more features can give you clues to where the issue is. For example an amp where the reverb is malfunctioning indicates the tube in that part of the circuit needs to be replaced.

Output tubes can also be microphonic. If you hear a rattle or ghost notes on certain lower notes you likely have a microphonic power tube. One way to confirm this is wear a glove to protect your skin from the heat and then gently hold the tube while you play the note that causing the rattle. Usually gentle pressure on the bottle is enough to stop the vibration and rattling, and will give you a clear answer as to which power tube is to blame. This problem will arise in guitar combo amps but is much less common in heads or hifi stereo amplifiers as vibrations are greatly reduced in these applications.

Loss of Power and other Odd Noises

Sometimes you will hear strange sounds, loss of power, or heavily distort sound. These are signs that a tube is failing. Often power loss that seems like the amp is performing at half power or less will be one or more bad power tubes, or even a dying phase inverter tube.
In a high gain guitar amp the clean channel may be distorted and overdrive channels will be extremely distorted and unusable. Use the isolation techniques mentioned earlier to find the bad tube.
Another symptom is when tone controls seem to have little effect and the sound frequency range has become very narrow. Bass and treble will be greatly reduced. This alone won’t indicate which tube in your amp is to blame but hopefully using the isolation steps we discussed help narrow it down.

40 Responses

  1. Manuel
    Manuel at |

    Great article!

  2. Franky Love
    Franky Love at | Rocks… Head and Shoulders above the competitors who don’t answer the phone. Much love from from Dr. Frank.

  3. Kitt Thompson
    Kitt Thompson at |

    An absolutely good article. all the issues are dealt with
    clearly. I am very impressed, that the TUBESTORE, is
    not only interested in selling their products, but also
    go the distance in helping the consumer, help themselves.
    Good job.

  4. d2g
    d2g at |

    Many thanks – just ordered replacement tubes for a Blues Junior, not sure if that’s the cure yet – amp won’t power on, but the fuses aren’t blown either. Shut down during a gig, it was very hot, master and volume both at about 6, no discernible strange noises before shutdown… tone was getting thin – maybe.

    Now, no light, no noise, nothing. Replaced fuses, no change. Tested fuses, not blown. Power tube labels are faded noticeably. All stock. Low hours on the amp.

    I think it’s likely tubes or a transformer?

    Ideas? Thanks!

  5. james
    james at |

    man im so upset i have a 5150 III 1X12 just replaced power tubes with JJ and bought a sovtek LPS for the PI and 5x JJ preamp tubes and im still getting this like volume drop where ill have it turned up not to loud and after few minutes its like it turns its self down low. The last time i played it today it made like a noise kinda like how when a taser is shot and makes that crackling sound. anyways i been seeing that the 100w 5150 III heads have been having issues with this volume due to faulty soldering with the circuitry. mine is a 2014 model and i have a 5 year warrenty with it i should just sent it in but would like some advice first. ive never done the preamp swap or what ever not even sure if i know how to do it.

    1. tony
      tony at |

      a cap in the audio path could leak DC voltage into the next stage causing that tube to shut down. if its a tone control cap you will hear a very scratchy sound when turning certain controls in the preamp. ive had a few 5150 amps come into my shop, those amps do sound good when functioning well but im not impressed with the build from any companys or different versions of it. bad soldering particularly on cheap fuse posts in the high voltage supply has caused the very issue you speak of in two of those amps in my shop. also “motor boating” from not enough power supply filtering in the preamp.

  6. wes
    wes at |

    I have a 90s era Fender Deluxe that’s been a reliable workhorse for years. Lately, on higher volumes, I get a sudden signal drop, almost as if it is momentarily switching between the clean and dirty channels. Bu it only happens when I hit a chord, so is triggered by there being more signal coming in. My gut tells me it’s tube relate, but before I invest in new tubes, I thought to ask here first. Note the tubes have not been replaced since I got the amp in 2003.

    1. wes
      wes at |

      Further to above note – to clarify – it like the signal is dropped a bit overall, but then when I hit harder it cuts in abruptly then falls off again.

  7. Peter Spinelli
    Peter Spinelli at |

    My Masa Boogie amp has a dying fuse tone to it?
    How can I tell if a transformer is broken?

  8. Alechandro
    Alechandro at |


    I have Fender Pro reissue circa 2004-5 ish. I JUST started getting a throbbing noise coming out of the amp, not guitar related as I’ve changed out guitars and get the same issue. Amp is fine for the first 30 seconds or so and then starts the noise (loud humming on and off almost like a tremolo but with nothing being played). The amp DOES have a tremolo but its switched off. Tubes have been in the amp for a long time but they don’t have more than 400 or so hours on them. (but honestly, thats just a shot in the dark). Yesterday, perfectly fine. tonight, throbbing sound while playing or not playing. sound goes away when in standby. I should mentioned I have purchased matched tubes from you in the past for my Reverend Hellhound amp but not for this amp. I’m guessing its the tubes but no clue which ones. Cursory look at the power tubes while powered on and everything looks fine. preamp tubes perhaps? should I just switch both power tubes out with the ones from the Hellhound and see what gives? or is this something that sounds familiar?



    1. Sandra @ thetubestore
      Sandra @ thetubestore at |

      Finding the source of amp issues can definitely be a daunting task. If you have an extra set of tubes you know are good in another amp than swapping them out temporarily will answer your question if it is a tube causing the problem or if it is something more sinister. Tubes don’t have a shelf life so they don’t degrade without use, however capacitors do so it is entirely possible the issue is on that level. Of course we always hope that it is something easy to fix like a tube. If you have any further questions about your specific situation email us and we can try to walk you through some possibilities.

  9. Jay
    Jay at |

    I have a 2001 DSL401 Marshall. The tubes are new. The reverb isn’t working. If you move the amp around or tap on the reverb tank you can hear the springs rattle through the speaker when amp is on and guitar plugged in. Any ideas. I know they had problems with tube sockets coming loose or unsodered. Amp sounds great clean and on OD. Just no reverb. Thanks

  10. Daniel
    Daniel at |

    Hi Jon! How can we measure how Watts a tube have? For example, if a 6L6 tube have 25w or 30w , there is an equipment capable to measure bias AND wattage of a tube? or based in tube data like plate current and transconductance, we can estimate , for example, the wattage of a vintage RCA?

  11. Dallas L.
    Dallas L. at |

    So I have a fender hot rod deluxe and when I turn it on the only thing coming out of it is a humming noise, ive tried it with multiple different cables and guitars but I noticed that one of the 12axy preamp tubes aren’t lighting up like the rest of the tubes, could this be the problem?

  12. malvaut
    malvaut at |

    excellent article jon

  13. Justin Dodson
    Justin Dodson at |

    Would the article apply to bass amps too? I have an Ampeg SVT Classic I’ve never changed the tubes in since I’ve had it. Lately, in combination with a Darkglass preamp pedal I always have on, my tone starts to get reeaallyy thin with almost no bass and my first guess was the tubes.

  14. Brian
    Brian at |

    Hi Jon,

    I’ve owned a Peavey JSX half stack since 2006. Lately when I play it the tone seems “flat” and not as full as before. I’ve read through all the comments here and it would seem reasonable to suspect the tubes since only 1 has ever been replaced? The amp is only used occasionally and never gigged.

  15. Joey
    Joey at |

    Hi Jon.

    I own a VOX Ad120vt amp that just stopped working. You can turn it on and all the lights glow. But it won´t make any sound when i plug in my guitar. I´ve checked the output on the guitar and the jack, and both the tubes shine orange when i turn on the power. The amp won´t even make a sound when I plug in an out the jack. Do you have any suggestions in what to do?


  16. Claude
    Claude at |

    I have a blackstar ht stage 100 and when im on distortion mode at low volume it sound ok. When raising it up it just sound ugly and moody. The clean chanel seames to be ok. Is that the preamp tube? If so… i live in Montreal. Where could i find good tubes?

  17. Miguel
    Miguel at |

    I have a Mesa Boogie ta-15, in the red channel I don’t have any sound with gain presets, however the clean sound is ok. Can this issue be related to tubes?


  18. Peter
    Peter at |

    I have a old Zoom G9.2tt Multi Effects Processor with 2 -12Ax7
    Tubes. Bought it used.
    It’s hissing, buzzing, and, crackling noise when connected to effects loop and or straight into the Solid State Crate 150 Power Block mini amp head.
    I should change the Tubes right?
    I live in a building made in the 60’s. I use a Hofner Jazzica Custom Archtop jazz guitar with a Bartolini SJ5 floating pick up mainly. I have a 1978 Gibson 335TD also.Great tone. Intolerable noise. I use Mogami cables. Built in ZNR noise gate works but when I play you can hear the hiss noise etc…

  19. Matt
    Matt at |

    Hi, I have a Fender Blues Jr iii SE. After a trip in the car the volume (gain) pot now doesn’t do anything and it is as if it is turned up to about 8 even when on zero. Would that be the tube or something else?


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