Power Tubes

Power tubes are normally the biggest tubes in your amp and are found in the output stage.  Important - a rectifier tube can also be mistaken for a power tube due to its similar size. If fitted, there is typically only one rectifier tube, but at least two or four similar power tubes in any amp.  The exception would be in small low wattage “single-ended” amps such as a Fender Champ or a Gibson GA-5 that use only one power tube. (These amps typically have less than 15W output)

Power tubes are normally Pentodes, or Beam Tetrodes, and unlike the smaller preamp tubes typically contain only one tube. So called “British” amp designs have favored the Power Pentodes EL34 and EL84, and the “American” amp designs the Beam Power Tetrodes such as 6L6GC, 6V6GT, KT66 and 6550.

Modern guitar amp manufacturers regularly use only about six different types of power tube, and only four of these are used in significant numbers. The four most common power tube types are the 6L6GC, 6V6GT, EL34, and EL84.  

The majority of power tubes feature 8 pin “octal” bases.  Whilst they might appear interchangeable given their common socket base, they are not, each having different voltage and circuit requirements.  The EL84 is smaller than other power tubes, featuring a 9 pin “noval” base similar to the shorter preamp tubes.  Always refer to your original equipment manufacturers specifications to confirm the power tube requirements for your specific amplifier.

Correct biasing of power tubes is critical to the optimal performance of any amplifier, and incorrect adjustment can affect sound quality and amplifier / tube reliability.  Re-biasing is normally required when the power tubes are changed.  “Fixed bias” amps generally have an adjustable bias level that needs to be checked and reset when you change tubes. “Cathode-biased” amps, (often called “Class-A” amps) are self-biasing and do not require adjustment.  Always consult your original equipment manufacturers specifications to confirm biasing requirements, and make use of a suitably qualified tube amp technician to perform the correct biasing procedure.

As the majority of tube amp output stages work in a push-pull configuration it is important to replace power tubes in matched sets. (Normally two or four tubes) Matching tubes with similar electrical and gain characteristics ensures optimal balance and efficiency of the output stage, ensuring the best tone and optimal tube life.  

Unless ordering singles tubes, or otherwise requested, Tone Tubes always ships power tubes in matched sets. There is no additional charge for this.

FAQ

How often should I replace power 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. Power tubes can also become microphonic and can cause unwanted noise or crackling heard from the amplifier's speakers however this is rare.  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. Blown fuses are often a sign that one of the power tubes has developd an internal short. 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 the more and the louder you play will determine how often power tubes need to be replaced. Power tubes are the hardest working tubes in any amplifier, and should last between six months and two years in most regularly used amplifiers.  

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

- As long as you are purchasing the same tube number (e.g. EL84) then different brands can be substituted without concerns. However it is not advisable to run different brands of power tubes in one set as this can compromise optimal push-pull performance. 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 change all power tubes at the same time?

- It is highly recommended to change all power tubes at the same time and in matched sets. (Typically two or four per amp)  Matching is often difficult to achieve when mixing old and new tubes in one output stage.  Most guitar amps use push-pull output stages, and rely on the power tube sets to be closely matched and balanced to achieve optimal performance. 

Do I need to match power tubes?

- Typically yes! Power amp tubes mostly operate in sets of two or four and require matching as the general rule. There are exceptions so always refer to your original equipment manufacturers specifications to confirm requirements for your specific amplifier.  Unless ordering singles tubes, or otherwise requested, Tone Tubes always ships power tubes in matched sets.

What is power tube biasing?

- In the same way as adjusting your car’s engine idle speed, many tube amps require the idle current of the power tubes to be set within certain limits in order to ensure optimal performance and tube life.  Tube bias has a significant effect on tone and tube life! Tubes are hot-biased when their idle current is set too high and cold-biased when their idle current is set too low. Hot-biased tubes will distort earlier due to decreased signal headroom, and also run hotter resulting in shortened tube life as they are operating beyond their plate dissipation specifications.  Hot-biased tubes often glow bright red or with "red spots", a sure sign of melting anode plates!  Cold-biased tubes will often sound thinner and more sterile. Tube life is not affected when tubes are cold-biased, but your tone can suffer.  Adjusting the bias is a procedure to set the correctly specified negative voltage on the power tube control grid. This in turn controls and regulates the current flow thru the power tube at idle. Always refer to your original equipment manufacturers specifications to confirm biasing requirements for your specific amplifier.  

Do I need to re-adjust bias when replacing power tubes?

- This depends upon the specification of your amp, so essential to always refer to your original equipment manufacturers specifications to confirm biasing requirements for your specific amplifier.  Cathode or self-biasing amplifiers do not require re-biasing when power tubes are replaced, but most fixed-bias amplifiers do require re-biasing to ensure optimal performance.  Some amp manufactures such as Mesa-Boogie design fixed-bias amps, but do not make provision for bias-adjustment.  Instead they rely on fitment of tubes specially selected to fall within their tolerance criteria, including idle current consumption. 

Why do some amps have more power tubes than others?

- This is amplifier design specific. Small wattage guitar amplifiers (<15W) often only use one power tube in a single-end output stage. Medium and higher wattage amplifiers almost always use push-pull output stages, requiring at least two and often four power tubes, also depending upon the power ratings of the specified tubes.  Guitar amps with 6 or more power tubes in their output stage do exist, but are rare!  

How does a vacuum power tube work?

- A power 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 power 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 signal voltage applied to the Control Grid is able to control a much bigger current flowing from the Cathode to Anode, providing amplification or gain. Additional Screen and Suppressor Grids further optimize a power tube’s performance.

What are the differences between a Pentode and Beam Tetrode power tube?

- A Pentode has 5 active electrodes, namely the Cathode, Control, Screen, Suppressor Grids, and the Anode.  It is closely related to the Beam Tetrode which features 4 active electrodes, namely Cathode, Control and Screen Grids, and Anode, but instead utilizes a beam forming plate instead of the Suppressor Grid.  This beam forming plate is normally connected internally to the Cathode.  Both the suppressor grid and beam forming plate have essentially the same function of suppressing secondary electron emissions, but the Beam Tetrode is defined as having a more uniform transfer characteristics.  As such the Beam Tetrode is often referred to as a “kinkless” tetrode. The majority of power tubes used in guitar amplifiers feature 8 pin “octal” bases.  The EL84 power pentode is the exception, featuring a 9 pin “noval” base similar to the shorter preamp tubes.

What does the Phase Inverter tube do?

- Low power tube amps (typically below 15w) use one power tube in a "single-ended" power stage configuration. 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.  Next to the Power Tubes, the Phase Inverter tube also works extremely hard, and as a rule it is advisable to always replace this tube when replacing the power tubes. Always refer to your original equipment manufacturers specifications to confirm tube requirements for your specific amplifier.     


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