Tubes FAQ
Contents
High Voltage!
Tube amplifiers operate at voltages that can be significantly above normal line voltage in some places. If you are not experienced in handling such voltages, it's best to stay away from the internals and trust a technician who knows what they're doing, as there is a potential danger to life. A wrong move, and that's it...
Even with the amplifier turned off, high-voltage discharges can occur hours later if you touch the components. The cause of this is often capacitors (primarily in the rectifier) that can store their charge for a long time if not discharged with a bleeding resistor.
Furthermore, when working with live circuits, it is recommended to use an isolation transformer to ensure electrical isolation from the amplifier to the ground. While this doesn't provide 100% protection (similar to an airbag in a car), it does offer basic protection against contact with live, voltage-carrying components. Additionally, a Ground Fault Circuit Interrupter (GFCI) is a very sensible investment in your safety, even if it costs a few extra bucks. A GFCI disconnects the line voltage in case of a ground fault, actively protecting lives.
General Questions and Answers
Where can I find more information about matching and selection?
For further information on matching and selection, please visit the relevant FAQ.
How long do tubes "last"?
The lifespan of tubes depends on many factors, such as power and heat, mechanical stress, environment, etc., making it nearly impossible to provide an exact estimate of a tube's lifespan in hours.
Usually, a loss of power, high frequencies, or imprecise bass response may indicate a slowly deteriorating tube. However, replacement is not always immediately necessary. For power tubes, you should check the bias setting, if available, and adjust it if needed. It can usually be assumed that preamp tubes should outlast approximately 3 sets of power tubes.
BUT: Tube failure can occur at any time, so it's advisable to keep a matching set of replacement tubes on hand at all times.
What does NOS mean?
NOS stands for New Old Stock. It refers to tubes that were produced a long time ago but were (still) unused. Tubes can be stored for decades without affecting their function or quality.
And what does JAN mean?
JAN stands for Joint Army Navy. This designation is often found on NOS tubes, indicating that the tube was manufactured for military use. JAN tubes often have improved construction and/or tighter selection criteria compared to tubes intended for civilian use.
Is NOS always better than current production?
NO! Even during the heyday of tube production, there were good and bad runs. Relying blindly on high quality just because a tube is labeled NOS is risky, as this term is often used mistakenly or deliberately to inflate prices during sales.
Tube Selection
What is the best preamp tube?
This question is often asked, usually with the addition: "Money is not an issue." However, there is no one-size-fits-all answer. The price of tubes, especially NOS tubes, does not necessarily indicate quality. Prices often depend on the availability of a particular tube type.
The best tube in this case is the one that suits you best, regardless of whether it costs 5 or 50 euros. It must match the amplifier, the setup, and your personal taste. The beauty of tubes is that they have more or less influence on the sound, allowing you to shape the "sound" within certain limits through various tube combinations.
More information about the sound characteristics of different preamp tubes can be found in the Tube-Map (PDF).
And what is the best power tube?
Here, too, a blanket answer cannot be given, and a similar statement applies as with preamp tubes. It is often inevitable that you have to try out several different tubes until you find YOUR setup. However, the bias point is also crucial for power tubes and often significantly affects the sound. Keyword: bias adjustment. Instructions for correct bias adjustment can be found in the Bias Adjustment Manual.
What tubes do I need to use to turn my Low-Gain amp into a High-Gain amp?
If the amplifier's preamp is based on 12AX7 / ECC83 tubes, simply replacing a few tubes will not necessarily result in more "distortion" – the design of the amplifier may not be suitable for that purpose. In such cases, modifications to the amplifier itself are necessary, or the use of preamp devices or a different amplifier.
... but can it go for less gain?
Reducing gain is usually simpler, as gain is often equated with distortion, and this direction is easier. If the amplifier's input stage uses a 12AX7 / ECC83 tube, changing it to a 12AT7, 12AY7, 5751, or even a 12AU7 often achieves the desired reduction in distortion, allowing the amplifier to stay "clean" longer.
Can I replace 12AT7 and 12AU7 with 12AX7?
Yes and no. These tubes have different operating points, but in most cases, the input tube in guitar and bass amplifiers – going from 12AX7 to 12AT7, 5751, or 12AU7 – can be swapped without the need for modifications to the amplifier itself. Other positions in the preamp may be more critical, so it's best to stick to changing the input or driver tube. While we cannot provide a 100% guarantee that this will work flawlessly in all amplifiers, we are not aware of any cases where it has caused damage or failure.
And from 12AU7 to 12AX7?
No! That should not be done. 12AU7 and 12AT7 tubes in the preamp are usually used where more current is required, such as for driving spring reverb. The 12AX7 cannot provide enough power, so it would not function properly or at all.
Tubes in Practice
Do tubes need to be biased after replacement?
Preamp tubes are usually not biased, as the bias point is set. They are simply replaced. For power tubes, it depends on the type and design of the power amp and how the bias point is set. There are auto-bias and cathode-bias systems, where the bias point is set automatically.
For "non-adjustable fixed bias" amplifiers, there is also nothing to adjust, but it is advisable to use tubes matched to this circuit/voltage.
For "adjustable fixed bias" or simply "Fixed Bias" amplifiers, the bias setting should be checked and corrected if necessary. Instructions for bias adjustment can be found in the Bias Adjustment Manual.
How can I test my tubes myself?
With a specialized tube testing and measurement device. A direct test of the tube and its quality is not possible with a regular multimeter.
However, there are significant differences in tube testing devices. The simple devices from the 1960s and 1970s, which often only distinguish between "bad" and "good," are certainly helpful as quick testers but not much more.
When Problems Arise
My new preamp tube is defective
A common issue: I received a new preamp tube, installed it, and found that it's defective. Either there is no sound coming from the amplifier, or it hums and crackles. When I reinstall the old preamp tube, everything works.
It is understandable that the user thinks the preamp tube is defective, but in most cases (> 90%), this is not the case. The cause of this behavior is usually bent or worn-out contacts in the tube socket. Over time, the contacts have "adapted" to the diameter of the pins of the installed tube but have also lost their elasticity. When a different tube with thinner pins than the old one is installed, the contacts no longer make proper contact, resulting in a significant resistance between the spring contacts and the tube pins. Alternatively, the contacts may be so bent that no connection is established at all. The result, in such cases, is as described above. In such cases, the solution is either to carefully bend the contacts in the socket (CAUTION, HIGH VOLTAGE POSSIBLE!) or to replace the tube socket. Also, using a tube with thicker pins can help, but this does not address the root cause of the problem.
The glass of one of my tubes is partially milky white.
What is this?
This tube is defective. The vacuum has been compromised, rendering the tube unusable. Replace it immediately and do not continue to use it.
The filament of one of my tubes does not glow as brightly as the others.
Is this tube defective?
No, the tube is not defective unless the filament does not glow at all, and the tube remains cold. In that case, a filament break is likely, and the tube is unusable.
Red Glowing – What is it?
If the tube is operated "too hot," meaning the bias point is set incorrectly, and the current through the tube is too high, the anode plates start to glow reddish to orange. If this condition is not corrected and persists for a certain period, the tube becomes overloaded and fails.
... and blue glowing inside the tube?
Blue glow can occur on the inside of the glass envelope, where electrons flying around can rip phosphorus atoms from the glass and make them glow or inside the tube, where existing gas ions are excited to glow.
One of my preamp tubes flashes briefly when I turn it on.
Is this tube defective?
No, this tube is neither defective nor damaged. This is a normal behavior, often observed with tubes from Eastern European production.