How to Diagnose Your Amps Bad Tube

Nothing beats the true analog sound of a cranked tube amp!

That feeling when the ‘sweet spot’ is unearthed, and the tubes are creating witchcraft, belting out warm saturated tones when all of a sudden… something goes ‘pop’ and things don’t sound quite right anymore!

The problem is, not many of us have the luxury to a tube tester, leaving us in the dark with our amps diagnosis and treatment. Even if we did, a tube tester is not capable of painting a clear picture with the problem with our delicate tube amps.

Essentially, pre and power tubes are a cluster of fragile components concealed in a vacuum sealed glass chassis. Their length of service hinges on many factors.

These range from how consistently the volume is cranked, road travel, level of maintenance, vibrations from the speaker, wear and tear etc.

I and friends have had our fair share of tube amp issues in the past. Our experiences brought me to write this post to help identify potential tube issues you may be experiencing.

Symptoms of Bad Pre and Power Tubes

  • Unusual sounds coming from the amp..(‘popping’ ‘hissing’ ‘crackling’ ‘humming’)
  • The amps overall volume lower than usual
  • Individual tubes glowing brighter or dimmer than others
  • The amp’s tone has degraded (loss of tonal dynamics)
  • The amp will not power on

What Do Bad Tubes Look Like?

To begin with, it would make sense to identify the visible signs of bad tubes before booking your amp with a tech to uncover any deeper problems.

Check the Filaments Glow!

Luckily it’s very easy to visibly identify what a bad tube looks like without the need for a tube tester. Within your tubes, sits a heater filament and when working optimally, will illuminate with a satisfying warm orange glow.

These filaments when glowing, emit electrons at high temperature producing heat for the tube which is needed to maintain the amplifiers optimal tone.

Depending on the manufacturer, other types will glow more brightly than others. The main point is if the filaments are glowing equally and to a good level, it means they are working and pass the first part of the visible test.

If a filament fails to glow, this is known as ‘filament failure’ and needs a replacement. If one individual tube glows less brightly than the others, it means it has worn down and not functioning optimally which will also need replacing.

Notice the Tubes Illumination Color

Purple glow – A cloudy purple or violet glow around the inside of the components although looks pretty, is a clear indication that there’s a leak somewhere within the tube.

If spotted, replace immediately before attempting to use the amp again. To maintain optimal function, tubes require an airless sealed vacuum. An air leak within the tube generates positive ions causing ionization within the components. Although this looks attractive it’s actually damaging to your amplifiers health.

Blue glow – Not to be confused with a purple or violet glow which is common, a bright blue fluorescent glow is actually the opposite and is sign your tubes are healthy. Nothing to fear with this one, this is a side effect of a normal functioning tube.

Pink glow – Pink is a sign that the tubes have too much gas content inside the vacuum, caused by excessive grid currents within the components.

Check the Condition of the Getter

An aspect most people overlook with their tubes is checking the condition of the ‘getter’. If you are unsure, a getter is a silver or light grey metallic material that is used in any sealed system including vacuum tubes.

They are positioned near the top housing of the tubes, although they can be coated by the side and bottom in other types. The point is their condition can indicate the tubes health and can reveal potential defects such as a slow air leak for example.

A getter with a grey or silver color indicates a healthy tube, this can range from a light grey all the way a metallic chrome which is fine. keep in mind, a black getter is not a burn mark or ‘hot spot’ which many people mistake this for.

There’s no need to replace the tubes, this is actually a by-product of the flash getter. What needs your attention, is a getter that has a pure white finish which is an indication of a leak within the vacuum. If you spot this, go ahead and replace this critter with a fresh tube.

Look for ‘Red Plating’

Burn marks or ‘hot spots’ inside the glass is a sign of ‘red plating’ which is when the tubes glow a constant red even when the amps on standby.

This is usually when a power tube is producing too much voltage and with it, excessive heat scorching the inside of the glass housing.

You should immediately replace with a fresh one as the faulty ones on its way out. This can also be caused by faulty transformers or can be caused by other problems with the amplifier’s circuitry.

Do the Tap Test!

If your tubes appear healthy and pass the visual tests, then it’s time to break out the ‘tap test’. Yes technically this an audible test more than anything, but still the standard when hunting for that pesky tube that’s failed you.

It’s pretty simple, take an object: a pen or pencil and gently tap each individual tube while listening for an inconsistency in sound (see video below).

A faulty tube will sound duller than the other tubes or sound like it has components rattling inside. This is a clear sign that the tube has failed and needs a replacement. Keep in mind, It’s rare that more than one tube will need replacing.

Safely Access your Amplifier First!

For safety precautions,  be aware! In order to access and replace the pre and power amp tubes, you may have to expose your amps circuitry. Be careful, you must be wary of ‘contact points’ that can hold high amounts of voltage.

Before exposing any circuity, I would recommend powering your amp off and unplugging it at the mains.

I would then let it sit for 10-20 minutes. This is to allow any charged components to become discharged. Doing so will make it safe to root around the chassis of your amp.

Do not root around the back of your amp or change tubes, if its switched on or plugged in at the mains. High amounts of voltage can run through components which is asking for trouble.

Can Bad Tubes Cause Volume Loss?

The simple answer to this question is yes! I’ve had this issue in the past but ultimately, when a pre or power tube begins to weaken, it loses its voltage and performance.

This causes a significant loss in volume (as much as half) or cause sudden random volume drops or ‘fade-outs’ when playing.

Weakening tubes also affect the frequencies your amp can produce, wildly degrading the tone which is interpreted as volume loss. Some examples can be your amp losing the low frequencies resulting in a loss in ‘bottom-end’ with the outcome being a ‘thin’ and ‘tinny’ sound.

If your amp has never had a tube change, then the first protocol would be to replace the bad tubes. Usually, power tubes are the first to go as they contain a lot more fragile components, run at higher voltages and tasked with the bigger job of amplifying the speakers signal.

Preamp tubes, on the other hand, are way more reliable as they are compact and only tasked with amplifying the guitar signal to the amp using less voltage, meaning they wear out less frequently than power tubes.

Unusual Sounds from the Amplifier!

It’s an obvious sign your tubes are on their way out to fail when your expensive tube amp begins to sound like a ‘humming fart factory.’ The audible signs are pretty obvious with common symptoms being:

  • Constant ‘fuzzy’ sounds
  • ‘Humming’, ‘pops’ and ‘crackles’
  • ‘Squealing’ noises
  • Volume fades in and out
  • Lack of punch and drive
  • Lack of brightness
  • Flat sound with no sonic character
  • No high-end frequencies
  • High gain amplifier’s clean channel is distorting

Sound Example of Faulty Tubes… (See Video Below)

Any of these are clear indications for a tube change if this doesn’t resolve the problem there’s a bigger issue here and recommend taking the amp to a local guitar tech.

How to Tell if Tubes are Overheating?

The fact is that tubes normally generate a lot of heat which means they are working fine. It’s difficult to tell when tubes are genuinely ‘overheating’ because all amp and tube combinations are different and run optimally at various perimeters. Most tubes will optimally run at a range of 150-250º so that gives a fair amount of variance to work with.

I speak to guitar players who worry about the heat generated from their tubes, pointing a fan at the back of their open back amp to cool them down.

However, I would not recommend this or installing an internal fan. Purely because blowing cool air on your tubes, means they will have to work harder to maintain their operating temperature.

Secondly, do you remember why you’re told not to pour boiling water on a frozen car window on a frosty winter morning? Because you will crack the glass right? This is known as “thermal shock” and for the tubes, it’s the same principle but in reverse.

Trying to quickly cool extremely hot tubes can strain the glass, weakening them which will more likely cause a leak within the sealed vacuum causing them to die a leaky death.

The reality is that the amp will have been designed to withstand the heat for most situations: bedroom playing, rehearsing and performing on a humid, sweaty, stage at some death metal gig for example.

With that said, I would recommend a fan for the amp facing at a 45-degree angle just not directly in front of the tubes.

The main reason why tubes get a little too hot is they have either too much power are too much voltage running through or a combination of both.

Keep in mind, that the heat coming from the amp can be from other components, not just the tubes. However, if you feel that they are raging hotter than usual, take them to your local amp tech to give them a look over.

When to Replace Power/Pre-amp Tubes?

First, let’s get this out the way, this is a hotly debated topic within the tube amp community.

Mostly on gear forums but to tell you the truth, there is no concrete answer as tubes don’t have a specific life span before they give out.

Some tube worshippers will replace them every two years, whereas, others will only replace their current tubes until they die an honorable death. So the question is… who’s right?

In my opinion, the main reason why you will want to replace your tubes relates to tone.

When you first bought your amplifier, you must have been delighted with the tone, that’s why you bought it in the first place right? Over time and usage, however, the sound begins to slowly degrade and is not as crisp and sharp as it used to be.

The change may have been that slow and gradual you may have not even noticed with your ears adapting to the current tone? A full fresh power tube replacement will inevitably give your amp a new lease of life upgrading the tone similar to when you first purchased it.

So how long will your tubes last until they bypass their optimal function? There’s no set time but roughly this is a general standard for most tubes…

Average Pre Amp Tube Lifespan

Most manufacturers = ‘1000-5000 hours of usage’

Average Power Tube Lifespan

6L6, 6V6, 5881WXT, EL34, = ‘1000 hours’ of usage RL84 = ‘500 hours of usage’

As stated this isn’t set in stone, as there are many other factors to consider including plate voltage, load inexpedience, bias current, the age of the amp, hours of use, transportation damage, manufacturer etc.

The point is when you feel the sound and tone is not dynamic as it used to be then a fresh tube change is on the cards.

The main thing to listen for is your amps bass response, level of punch and drive. All these factors will make the bottom-end sound lose and muddy which is usually a clear sign your power tubes need replacing.

It is also important to check the visual signs of wear and tear on your power and preamp tubes as stated above not just audible signs.

Another reason why you will want to replace your tubes is they will inevitably burn out, blow or fail (depending on the reliability of the amp and individual tubes).

keep in mind, when tubes bypass their expected usage time, the probability that they will fail increases, meaning they are much more likely to short out and blow a fuse which is something to keep in mind if you are gigging regularly and don’t want to be caught out on stage.

It’s also important to know that when a tube has failed, you must not use the amp until the failed tube has been replaced as operating with a faulty tube can damage the transformers.

How to Change your Tubes Safely!

Do Different Tubes Change the Amps Tone?

I like to think of this as changing ‘apples for apples’, sure some apples from different farmers will taste slightly different, some sweeter some sourer, but ultimately it will still taste like an apple!

I do agree, swapping power and preamp tubes will give you a slight difference in tone, but we are talking 5-8% deviation here. Now changing the amp’s speaker is like eating an orange!

This can serve up to a 50-80% difference in sound meaning that tubes, although do play a part in tone, don’t need that much attention.

With that said, I do recommend you sample many different pre and power tube combinations with your amp as your budget can possibly afford. The reason being that you will find a combination that suits the sound of your amplifier and tones you want to achieve.

Regardless of what tubes you are using: 6L6, 6V6 EL34, EL38, 5881 etc, Each have their own sonic characteristic which will slightly change the sound and feel of your amp, helping you narrow the search for your ideal tone.


In my mind, tube amps are the still the most authentic sound when it comes to tone. Your tubes are the workhorses of your amp.

Fragile as they are fantastic, they do need some attention to keep your amp healthy and happy. Hopefully, this article has informed and given you a heads up the current condition of your tube amplifier.

Recent Content

link to The Guide to Roller Bridges

The Guide to Roller Bridges

With the many options for guitar hardware. You may be interested to learn about roller bridges? Are roller bridges worth it, and what are the potential benefits of replacing one with a conventional tune-o-matic bridge? So let’s dive in… Roller bridges are worth it if you have a guitar with a Bigsby style tremolo system […]