Troubleshooting Tubed Components

Basic Troubleshooting of Tubed Components

I have isolated the component that is causing the trouble. Now what?

Once you have isolated the component with the dead or noisy channel, try swapping all the tubes from left to right. If the offending channel moves, then we know at least one of those tubes is bad. Isolate the bad tube by swapping tubes left to right one pair at a time, until the offending channel moves again. Then we have the bad tube and can get a replacement. Typically, moving the small tubes within an amp, pre-amp, or any other component does not require adjustments like you may need to do when moving large power tubes. But always check what the manufacturer says.

Power tubes, biasing, and other general answers regarding power amps

Typically, power tubes do not cause noise the way a small tube might due to their "job description." There are two exceptions to this: The first one is in some tubes like the Chinese KT88 or JJ KT88, there can be a sharp and quick "crackle". In the Chinese tube it may be no biggy, however in the JJ it may be the screens bending and touching. This CAN be a problem...which is why we don’t use JJ here.

It’s important to regularly check the bias of your amps. Once you get used to it, it’s like petting your dog or cat... it becomes enjoyable. This doesn't apply to amps with autobias circuits (some are really cheesy, others are very sophisticated... In this case there is no need to monitor bias. Otherwise, you should check the bias of your amp every 2-3 months. If the bias is significantly higher or lower than the default setting shown in your manual, then adjust the bias accordingly. You may not need to make adjustments each time you check your bias, but this helps to ensure you are getting optimal performance from your amp, and helps keep your tubes working properly. Bias that is set too low can make the amp sound thin or veiled, while running the bias too high may shorten the life of your tubes, and in some cases may damage the tube or the amp.

There are a few common mistakes that come up occasionally:

  • Always have speakers hooked up when checking the amp.
  • Make sure you're not listening to music through the amp you're biasing. You're checking the bias while the amp is at idle, and running a signal through the amp is not "idle."
  • Make sure your multimeter is set to the correct test measurement. Check your manual to see if your amp reads bias in millivolts (mV) or milliamps (mA) and ONLY use the test measurement outlined in your manual.
  • Before adjusting one channel, do a quick read on the other. If both are off by 20% or more you MAY be reading the meter wrong. Contact the dealer or manufacturer where you bought the amp. Contact us only if you bought the amp here… I can’t keep up on how to bias every amp ever made, so please don’t be offended if I say "I don’t know."

You must always adjust bias when installing new power tubes. Unless your amp has an automatic bias circuit, there is no exception to this rule. As a precaution, turn the bias all the way down on your amp before plugging in new tubes. Fresh power tubes do not need as much negative bias to reach the amp's bias setting as older tubes do, and this helps to prevent the new tubes from shorting out.


One channel on my amp is out. I checked the bias for that channel and I'm not getting a reading...

If your amp has fuse protection for the tubes, and one channel is out, check the fuse on that channel. Fuses are safety valves that are meant to blow before your amp does. Typically a faulty power tube is to blame for a blown tube fuse. Replace the bad fuse and power your amp back on. Pay close attention to all the power tubes on the side which had the fuse go out. You may want to disconnect your speakers just as a precaution. Check each tube for tell-tale signs of failure: flashing, glowing red plates, one tube glowing significantly brighter than the others. Also, monitor the bias in that channel. It could be all the tubes are fine, but your bias is too high and just needs to be turned down.


Some of my power tubes have this eerie blue glow on the glass. Is that normal?

Yes, that is normal for power tubes. That blue glow is just stray electrons bouncing off the glass. Some tubes will have a more noticeable glow than others, even amongst a matched set of tubes. Sometimes the glow will burn off, that is normal too.

What is NOT normal is a bright blue or purple glow coming from inside the plate structure of the power tube. If you see a bright blue or purple glow coming from inside the plates, turn your amp off immediately and replace the tube.


The bias reading on my multimeter keeps bouncing around no matter what I do...

When biasing a power amp, the bias number will always jump around. On a Cary SLI-80 for instance, the bias should be set to 75mA, or milliamps. Some amps use mV, or millivolts as a bias reading. This is just used as an example. Once the bias is set, immediately it may jump to 77mA, then to 74mA, then maybe 72mA, then maybe back up to 76mA. This is normal. The bias may change more when the amp is given a signal. Also normal. When you are adjusting bias, the amp should not be receving a signal. You are biasing the idle current. Take it from me guys... don't sit there watching the bias while your listening. All that will do is stress you out and that's no fun. You're supposed to be enjoying the music, right?

Now, it is NOT normal if the bias jumps up and down dramatically. If the bias on the SLI-80 were to jump from 75mA to 105mA, down to 42mA, up to 88mA, that is NOT normal, and you should shut the amp down. If you get bias readings like that, chances are one power tube has lost a screen and needs to be replaced.