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Tube FAQ

High Voltage!

Valve amps work with voltages that in places far exceed the “normal” mains voltage. People who are inexperienced in working with such voltages should rather leave the amp closed, and let an experienced technician perform the work, as the voltages in an amp can be FATAL. One false move and that's the end … Even hours after an amp has been turned off, touching some components can lead to a high voltage discharge. The cause of this is the electrolytic capacitors – primarily in the power filter – which can hold a high voltage charge for a very long time if no 'bleed' resistor is included to discharge them.

Furthermore, an isolation transformer is recommended when working on an amp under power, so that the amplifier is galvanic ally isolated from the mains. This does not provide 100% protection (similar to an airbag in an auto); nevertheless, it does provide a basic protection in case high voltage comes in contact with earth. Furthermore, an FI (fault interrupter) is a very sensible investment in your personal health, even if this costs a bit more. In case of a ground fault, the FI separates the runaway current from the mains voltage and thus contributes actively to preserving your life.

Do valves have to be measured after replacement?

Preamp valves do not normally require measurement to set the bias voltage. They can simply be replaced. Power tubes depend on the type and the construction of the power amp, as well as how the bias voltage is set. For example, auto- or cathode-bias automatically sets the bias voltage. “Non-adjustable fixed bias” also provides no possibility to adjust the bias voltage, and it is recommended to use valves which are measured for these amps. For “adjustable fixed bias” or just “fixed bias,” the bias voltage should be checked and adjusted if necessary. This is described in the bias-setup document.

How long do valves 'last'?

The life expectancy of valves is dependent on many factors such as output power and heat, mechanical stress, environment, and so on, which makes giving a precise life expectancy impossible. Usually, a drop in power output and highs, or degrading bass reproduction indicates the valves are reaching the end of their life. Nevertheless, immediate replacement is not necessarily required. The bias voltage of the power valves should be checked – if possible – and adjusted if required. Also, it can be assumed that not all valves, preamp as well as power amp, will go bad at the same time. As a rule of thumb, the preamp tubes should last through three sets of power valves. BUT: at any time, any given valve could go bad; and so a set of replacement valves should be on hand.

Which valves will turn my low-gain amp into a high-gain amp?

When the amp is designed to use 12AX7/ECC83 preamp valves, then a simple substitution cannot produce more preamp distortion, as the amp is not designed for it. If this is the case, either modifications to the amp are necessary, or a device must be inserted in front of the amp, or a different amp should be used.

… how about less gain?

Here, gain is associated with distortion, and reducing gain is simpler. When the input valve is a 12AX7/ECC83, substituting a 12AT7, 12AY7, 5751, or even a 12AU7 will often produce the desired reduction in distortion, and the amp will stay clean longer.

12AT7 and 12AU7 can be substituted for a 12AX7?

Yes and no. These valves require different bias voltages, but usually the input valve in guitar and bass amps can be switched from a 12AX7 to a 12AT7, 5751, 12AY7, or 12AU7 without changes to the amplifier circuitry. Other preamp positions are more critical; and so only the input or driver valves should be substituted. Although no 100% guarantee can be made that the substitution will work without problem in every amp, we know of no case in which such a substitution damaged the amplifier.

And 12AU7 to 12AX7

No! This substitution should not be made. 12AU7 and 12AT7 valves are normally used where more current is required; for example, as a driver for the reverb tank. The 12AX7 cannot provide enough current; as a result, it will not work at all, or will at best work poorly.

The glass envelope of my valve is partially milky white. What does that mean?

The valve is defective. The vacuum has been broken, and the valve is unusable. Replace it immediately and do not use the defective valve.

What does NOS mean?

NOS = New Old Stock. This term is used for valves which were produced quite some time ago, but have never been used. Valves can be stored for decades with no reduction in performance.

And what does JAN mean?

JAN means Joint Army-Navy. This abbreviation is usually found with NOS valves and means that it was produced for the military. JAN valves often have an improved construction, or tighter selection criteria as the valves produced for consumer usage.

Is NOS always better than current production?

NO! Even in the glory days, there were good and bad lots. Blindly assuming NOS is an indication of high quality is careless; furthermore, this term is often used incorrectly or fraudulently, for example; to get a higher price for the valve.

How can I test my valves myself?

With a special valve measurement and testing device. Testing with a multimeter is not possible. However, there are also large differences in quality among tube testers. The simple devices from the 60s and 70s only rate a valve as 'good' or 'bad,' and are intended as a quick test, but are of no further use.

The filament of one of my valves does not glow as brightly as the other valves.
Is it defective?

No, the valve is not defective, unless the filament doesn't glow at all and the valve remains cold. In this case, the filament is probably broken, and the valve is unusable.

Red plating – What is it?

When the valve is operated too 'hot' – that is, the bias voltage is set incorrectly, and too much current flows, the anode (plates) will start to glow red to orange. If this isn't corrected, and continues for too long, then the valve will be overstressed and fail.


… and a blue glow in the valve?

Blue glow occurs when electrons strike phosphorus atoms in the glass, and cause them to glow, or when ions in the small amount of gas remaining in the glass are struck.


A preamp valve flashes briefly when powering on. Is it defective?

No. The valve is neither defective, nor is it about to become defective. This is a normal occurrence, usually in valves from eastern European production.

Which is the best preamp tube?

This question is often asked, usually followed by “Money is no object!” but there is no general answer. The price of a valve, especially NOS valves, is no indication of its quality. The price is usually a consequence of the valve's scarcity. The best valve is the valve which sounds the best, regardless if it cost 5 or 50 Euros. It has to fit the amp, the setup, and the personal taste. The nice thing about tubes is that they have a more or less strong influence on the tone, which is why, within limits; with various valve combinations the 'sound' may be shaped. More information can be found in the Tube Map.

And which is the best power valve?

Again, no general answer exists, and the situation is similar to that of the preamp tubes. It is unavoidable that a few different sets of power valves have to be tried before you find YOUR setup. What is important for power valves, and has a strong influence on the tone, is the bias current. Instructions how to properly set the bias can be found in the Bias-Setup document.