Noisy 12AX7 or 12AU7
Random noises that sound like popcorn popping in the background is almost always a 12AX7 or 12AU7 small signal tube. It’s usually isolated to one channel so it is simple to figure out which one is the culprit. Swap one pair at a time left to right, until the problem moves to the other channel. Once the noise problem changes channels, mark the problem tube immediately with a marker or tape so it won’t get mixed up with the good tubes, then get a replacement.
Money saving tips
Sometimes simply swapping tubes channel to channel can cause the noise to disappear. This means there was an intermittent connection between the tube and the socket, and simply the act of re-seating the tubes cleared it up.
Here's another tip. The two front center tubes in power amps and integrated amps are input tubes and they need to be lower in noise than the tubes flanking them which are less picky driver tubes. If this noisy input tube is a 12AU7 in your model amp, you can swap it with a 12AU7 from the driver position and see if the noise disappears.
If you have lost a channel, it could be a “dead” small signal tube. Use the same troubleshooting procedure as above.
Be wary of sharp crackling noise
A sudden sharp crack lasting only a second at start up or shut down may be one of the large power output tubes (EL34, KT88, etc). This is more serious than a noisy small signal tube. If you hear this, keep a close watch on the power output tubes to see if any flash, and immediately turn your amp off and replace any bad tube.
Bad Tube Indicator
When you turn on your PrimaLuna, the Bad Tube Indicator may flash for a moment. This is nothing to be concerned about. Do NOT cycle the amp on and off. Always wait three minutes before turning power on again.
If the Bad Tube Indicator stays on, it will put the amplifier into protection mode and there will be no sound (except in the case of the HP amp and integrated which has an Adaptive AutoBias board for each channel). Before replacing the tube, you can swap it to another socket to see if it was merely a case of having bad contact between the tube pins and socket. If the suspect tube causes another BTI to activate, STOP and replace the tube before turning the amp on again. It's not a good idea to take a chance by moving it again.
Small signal tubes like 12AX7 and 12AU7 typically last between 5,000 and 10,000 hours. You don’t need a tube tester to determine if your tubes need replacing. Simply plug in a new pair and let them settle in for at least 100 hours. If they sound a lot better to you, leave them. If they don’t, put the old ones back in and run them until such time you don’t find the sound acceptable.
If you see one small signal tube glowing a little brighter than the other, that is nothing to worry about. The part that glows is the heater, which is similar to a filament inside a light bulb. So it either lights up or it doesn’t. If it looks dimmer, that’s because it’s slid up into a sleeve called a cathode, and during assembly the heater may stick up and out of the top a little more or a little less. This has zero effect on performance.
This can also occur with power tubes. KT150 tubes, for example, exhibit almost no glow because the heater is completely obscured by the cathode. However, if a power tube (EL34, KT88, KT120, etc) begins to glow red on the main structure of the tube (called the plate) stop and contact your dealer or PrimaLuna. This is extremely rare with PrimaLuna amps, as they run the tubes so easy and have Adaptive AutoBias to keep the tubes in check.
When changing tubes you may want to take the opportunity to try different brands or vintage NOS (New Old Stock) tubes. Changing tubes is another way to fine-tune your system to get the last 10% of tonal balance perfect. Please go to our section on tubes swaps and visit www.upscaleaudio.com where you will find the largest inventory of the highest quality current production and vintage tubes in North America.