Preamp Tubes

Preamp tubes are easily identified, in most cases, as the smaller tubes in your amp, and are usually positioned to correspond to your amp’s inputs and early gain and tone stages. They are responsible to progressively amplify the "small signal" from your guitar or other instrument, and gain is added across each gain stage of the amplifier.  Sometimes they are fitted with metal noise shields, which must be removed before you can access them. Since the mid-fifites, preamp tubes have mostly been of the smaller nine-pin “Noval” type, although some older amps will still have bigger eight-pin “Octal” tubes that fit the same size sockets as many types of output tubes. The most common preamp tube by far is the 12AX7. (also known in Europe as the ECC83, or the high-grade US alternative - 7025)

Some other types you will occasionally find in the Preamp look much the same, other than the numbers printed on them. These are: the 12AT7, (or ECC81) often used in reverb-driver and phase-inverter stages; and the 12AY7. (or 6072) These are all known as “dual triodes”, because they contain two independent tubes within the same bottle, and each is responsible to provide the gain within one gain stage of the amplifier. They are mostly differentiated by their gain factor. (i.e. the amount by which they amplify the signal they are given) The 12AX7 has the most gain, and the 12AY7 is a direct substitute but with less gain.  The 12AT7 also has less gain, but requires a slightly different bias voltage for optimal operation (it can be directly substituted in a pinch).

The most common pentode preamp tube used in guitar amps today is the EF86. (or 6267) This is found in early Vox amps, and more recently in models from Vox, Dr Z, and a few others. Whilst having a similar Noval 9 pin base, it is not compatible with the dual triode, and requires different circuitry.

Preamp tubes are normally self-biased and as a rule do not require further bias adjustment when installed.  Always refer to your original equipment manufacturers specifications to confirm requirements for your specific amplifier.

FAQ

How often should I replace preamp tubes?

- Tubes are service parts and do need to be replaced occasionally. They fail for 3 main reasons - they become gassy, (air has leaked into the tube) they develop internal shorts, or they wear out.  Preamp tubes can also become noisy / microphonic and cause unwanted noises or crackling heard via the Amp's loudspeaker. Damaged tubes are easily identified, and if gassy will often develop a white spot inside the glass tube.  Worn tubes will eventually cause tone deterioration and/or become noisy. The best way to confirm tube health is to have a Technician test them using a tube tester.  Amplifiers that are driven hard, used often, or moved around a lot will require tube replacement more frequently than a bedroom setup.  There is no hard and fast rule, but preamp tubes are less stressed than power tubes, and should probably be replaced about half as much as power tubes.  

Do I need to replace preamp tubes with the same brand?

- As long as you are purchasing the same tube number (e.g. 12AX7) different brands can be freely substituted without concern. Subtle tonal differences, as well as differing break-up characteristics are offered by different brands, and ultimately this is down to personal choice.  Presently there are only a handful of tube manufacturers operating around the globe, and many well known tube brands are in fact rebranded by these manufacturers in order to support customer marketing requirements. The "Big 3" tube manufacturers remaining today are JJ Electronic, (JJ Tubes) New Sensor Corp, (EHX, Svetlana, Sovtek, Tung-Sol, Mullard, Genalex) and Shuguang Electron Group.

Do I need to match preamp tubes?

- No!  Preamp tubes are mostly dual triode designs, meaning they have two separate tubes contained in one glass bottle.  As each tube is normally linked to one amplifier gain stage, matching of preamp tubes is not critical and will not make a substantial difference to your tone, or amplifier's performance.  It is true that the phase inverter stage requires a more closely matched tube in order to better balance both halves of the stage, but tolerances on most preamp valves are tight enough that this is not an issue.  In fact most guitar amplifier phase inverters are slightly imbalanced by design, so a slight valve mismatch is not noticeable or critical.

Do I need to adjust bias on preamp tubes?

- Typically no!  Preamp gain stages are commonly designed to be cathode biased, also known as self-biasing. Without changing other circuit component values, it is not possible to adjust preamp tube bias. However always refer to your original equipment manufacturers specifications to confirm requirements for your specific amplifier.

Why do some amps have more preamp tubes than others?

- This is amplifier design specific, and related to the number of gain stages and channels featured in the amp. Irrespective of the number of tubes contained in the preamp, the basic gain stage design principle remains the same. Some preamps also use triode tubes as reverb drivers, and for effects loops.  

Why do I hear a noise in my speaker when I tap on a preamp tube?

- Early small signal gain stages typically have higher gain, and mechanical disturbance of the tube elements will manifest as amplified noise on the speakers. This is most problematic on V1 in the preamp and the condition is known as a microphonic tube.  All tubes do display some degree of microphonics, and is often deemed normal.  Problematic tubes will often howl or ring behind your tone, and they need to be replaced. 

How does a vacuum preamp tube work?

- A preamp tube can be likened to a water valve or tap, where the main flow of water is regulated by how far open or closed the stopcock has been adjusted. A basic preamp tube consists of 4 main components, namely the Anode, Cathode, Control Grid and Heater.  Vacuum tubes rely on thermionic emission of electrons from the hot cathode, to the anode. The heater is not part of the signal chain, and is used to heat up the cathode element.  This is also why tube amplifiers require a warm-up period after turning on, as no electrons can flow from the cold cathode. Electrons are released within the vacuum, and flow towards the Anode which is normally held at high voltage.  This flow of electrons can be controlled by a small voltage applied to the Control Grid. (i.e. the stopcock)  In this way, a small voltage applied to the Control Grid is able to control a much bigger current flowing from the Cathode to Anode, providing amplification or gain.

What are the differences between a Triode and Pentode preamp tube?

- A Triode tube consists of 3 main elements, namely the Anode, Cathode and Control Grid, and is the simplest form of preamp tube, with basic operation as outlined above. A Pentode tube operates in a similar manner, but features two additional elements called the Screen and the Suppressor Grids.  These grids are typically set at predetermined voltage levels by the Amp's circuitry, and are intended to improve tube operational performance and increase gain.  Modern Dual Triode and Pentode tubes often use the same basic nine-pin "Noval" base layout, but are not interchangeable.   

What does the Phase Inverter tube do?

- Low power tube amps use one power tube in a "single-ended" power stage configuration. (Typically below 15W such as Fender Champ or Gibson GA-5) The majority of larger tube amps (typically above 15W) utilise a pair, or multiple pairs of tubes in a push-pull power stage configuration. In a push-pull configuration the signal is "split" into two identical, but oppositely phased signals, and then sent to each power tube. The Phase Inverter or Phase Splitter tube is configured to receive the preamp signal and then split it into the in and out of phase signals, and then pass this onto the power stage. Most Phase Inverters also amplify the preamp signal further. Most guitar amplifiers use a Dual Triode preamp tube in the Phase Inverter, with 12AX7 (ECC83) and 12AT7 (ECC81) the most commonly used types.  Always refer to your original equipment manufacturers specifications to confirm tube requirements for your specific amplifier.  

What is "tube rolling"?

- Tube rolling is the process of trying out a number of different tubes in the same location in an amplifier, and selecting the one that sounds best to you. Differing brands of the same tube type, and also different gain tubes within a common tube type can be substituted, but always subject to manufacturer's guidelines.  This is often done to optimise the tone of an amplifier. Tube rolling will not harm your amplifier, but please remember that tubes run on extremely high voltages, and are also hot.  Only a qualified amp technician should work on your amp.

 

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