The physical aspects of the amplifier make a huge difference too. The volume of the amp is measured in watts of power, as well as the speaker size. For practice purposes, a smaller amp with a power between 10 and 30 watts and a speakers range between 8 and 10 inches would be fine.
A model with a power of 50 watts and a speakers range of 12 inches will usually do the trick. These models start with a power of watts and a speaker range that exceeds 12 inches. So if the tube amp doubles the original signal, it has a gain factor of 2.
In the case of audio devices, the gain factor can also be described in decibels. Think about the weight of the guitar and your physical build too. A small or a medium-sized amp might be what you need if carrying heavy equipment might strain you. You should also think about the material used to construct the cabinet of your amp, which houses all the tiny yet crucial electronic parts. The cabinet can be made of wood, and everything from its type to thickness will have a huge effect on how the sound resonates.
Metal cabinets are also popular, so you can find one that is made of steel or aluminum. Nowadays, tube amps come with special effects and features that add more character and dimension to your music. Effects like loop and reverb can be added to several music creations by manipulating the amp just a little. You can also get one of the amps that allow you to easily switch from clean to distorted sounds. In some cases, you can find an amp with an equalizer control for every channel.
This will give you more control over the sound effects as you can precisely fine-tune the volume and distortion. Instead, they will be more suitable for someone who plays guitar for a living because they will be able to understand the full potential of the amp.
Before using your tube amp, you need to give it time to warm it up, so you can use it safely. This will allow you to maintain a good tone with your guitar and will guarantee that the amp is working the way it should. Warming up the amp will make it easier to control, especially with harsher and more challenging notes, as the amp will allow you to control the tones effortlessly. It is a must if you have an important gig or want to impress everyone with your musical performance.
In general, you need to spend at least 20 to 30 minutes warming up your tube amp before pushing in full throttle. In most cases, your tube amp will arrive with special instructions that tell you how much time it has to be warmed up before it can be safely used. However, exceeding this time specification is actually a better idea. A thorough warm-up of your tube amp will guarantee a flawless performance for a longer period.
By doing so, your guitar will sound its best and give you the right tones throughout the whole music performance, regardless of how long it is. The next few paragraphs will help you visualize the flow of electrons through simple circuits. Learning to visualize the flow of individual electrons was a breakthrough for me in understating tube amplifier electronics. Electric guitars generate an alternating current AC audio signal.
The guitar's pickups are small electric generators. Pickups have magnets poles that magnetize the metal guitar strings.
The movement of the magnetic field surrounding the magnetized strings generates electricity in the pickup's coil. The coil is simply a thin insulated wire wrapped around a spool and when a magnetic field cuts through a coil of wire it generates an electric voltage electronic pressure and current electron flow in the coil's wire.
The black and white wires leaving the guitar pickup are the two ends of one long coil wire. A humbucker pickup is simply two of these coils connected end-to-end in series. The Pickup on the left is a wire coil that generates the guitar signal.
The Tone Control bleeds high frequencies to ground. The Volume Pot is wired as a variable voltage divider. The Volume Pot bleeds guitar signal to ground to lower the guitar's output volume. I've been asked many times, " What is voltage? Like electrical charges repel the same way like magnetic poles repel. So if you cram a bunch of negatively charged electrons together onto the metal plates of a battery their negative charges repel one other--they want elbow room.
The tighter they are packed together the higher the pressure so I like to think of negative voltage as electron pressure. A quick note about ' Conventional Current Flow. That's right, the electricity in your car flows from the battery's - terminal through the ground wire, through the car's body, through the radio's ground wire to the radio and then through the positive power wire back to the battery.
With tube electronics it's easier to think in terms of how the electrons are really moving in order to understand them. If you connect a wire across a battery's terminals the jammed together electrons in the negative terminal see the wire as a pipe with lots of room so they flow down the wire. When you have an 'excess' of electrons tightly packed together you have negative voltage. When you have a 'scarcity' of electrons, or electrons are pulled apart from one another, you have a positive voltage.
When I think about a wire with very high positive voltage on it I imagine the wire as an empty pipe with very few electrons in it with lots of 'elbow room' so electrons really want to flow into that wire. Ground or earth represents an unlimited supply of electrons at zero volts or neutral voltage. Touch that high voltage wire wire to a ground and the electrons hanging out there will rush in to fill the void of electrons in the wire.
Voltage is the force of electrons wanting to move from a conductor crowded with electrons to a conductor with fewer electrons and more elbow room. Current is the measure of how many electrons are flowing through a conductor--the more electrons flowing, the higher the current. Keep this in mind when thinking about amp circuits, high voltage is an extreme scarcity of electrons and ground represents an unlimited supply of electrons. As a guitar string vibrates it moves one way and generates a positive voltage in the pickup coil, then as the string reverses direction the voltage is reversed and a negative voltage is generated.
This occurs with every string vibration so an alternating current positive-negative-positive-negative. I'll repeat that because it's a very important concept, as the guitar string moves one direction over the guitar pickup coil it generates a negative voltage excess electrons , then as the string reverses direction the electron pressure voltage and electron flow current reverses too and a positive voltage is generated scarcity of electrons and this repeats with every vibration of the string creating an Alternating Current AC electrical signal.
This tiny little AC signal is what the guitar amp will amplify until it's strong enough to move a speaker cone in and out. The speaker cone alternates in and out with the alternating current from the guitar's pickup coil.
For every guitar string movement there is a corresponding speaker cone movement. An AC guitar audio signal on a wire alternates between positive and negative voltage. The positive half of the AC guitar signal pulls electrons apart and creates a scarcity of electrons. If you graph a guitar audio signal the pitch of the guitar string's sound is expressed as wave spacing frequency and loudness is expressed as wave height amplitude.
A high frequency sound will have tight wave spacing and a low frequency sound will have wide wave spacing. In the graph below the high E string is on the left and the low E is on the right. A quiet sound will have short waves and a loud sound will have tall waves. The direct relationship between string movement and electricity generated in the pickup coil is the key to understanding guitar amplification. Our guitar amplifier will simply make the electric audio waves taller to boost their loudness.
When multiple strings are played the multiple electric waves are summed into complex waves. See my short youtube video to see guitar audio on an oscilloscope. Fender Stratocaster connected directly to an oscilloscope. Each 'wave' on the oscilloscope is caused by one string vibration.
The highest voltage peak of wave is created by the fastest string speed when it's moving directly over the pickup. The zero voltage point center of graph is when the string stops moving and reverses its direction.
The left side of the graph shows a high open E string pluck followed by a pluck of the low open E. Voltage is on the left scale and time runs along the bottom. The top half of the signal is positive voltage and the bottom half is negative. The signal's voltage and current alternate between positive and negative. The tight wave spacing on the left is an indication of high frequency and pitch. The wave height is an indication of power and loudness. The speaker cone will move just like this graph.
Every little twitch on that line makes the speaker cone twitch. When the graph goes high with a positive voltage the speaker cone moves outward, when the graph goes low with a negative voltage the cone moves inward. Amplifiers have large capacitors that store enough electricity to kill even when the amplifier is unplugged.
If you open an amplifier you MUST verify no voltage remains in the capacitors before working inside it. The guitar cable's tip conductor connects to the input jack's "T" tip terminal. The cable's sleeve connects to the "G" ground terminal. The guitar signal travels down the wire and through grid stopper resistor R3. The guitar's alternating current audio signal enters the amplifier at guitar input jack 1 or 2.
Resistor R1 on jack 1 is the ' input resistor. A grid leak drains off unwanted DC voltage to keep the tube's control grid near 0 DC volts.
See more on impedance here and see this web page for grad school level information on how Fender multiple input jacks and jumpering channels works. The signal moves from the guitar jacks down the yellow wires to resistors R2 or R3 , which are ' grid stopper ' resistors. They help stabilize the amplifier by removing much of the audio signal above human hearing. The "68K" written on the resistor refers to its resistance value of 68, ohms or 68 kilohms.
Sometimes you will see resistor values written as 1K5 which simply means 1. Signal from resistor R3 travels down the wire to tube V1A's grid pin 2 then out the plate to coupling capacitor C1. After going through grid stopper resistor R3 the audio signal flows down the wire to the preamp tube 's control grid, which is the entry to the 'A' half of the preamp tube V1A.
It's called V1A because tubes were called 'Valves' and this is tube number 1 and we're using the 'A' half of the tube. I recommend you now read How Tubes Work and come back here when finished. The preamp tube amplifies the guitar audio signal then sends it out pin 1 plate up the yellow wire to capacitor C1 , which is a 'coupling capacitor' or 'cap.
The 0. In some old documents you'll see "micro-micro" or "uu" which means pico Farad. Use this chart to help convert capacitor size such as:.
Capacitors are made of two conductive plates separated by an insulator or dielectric. Common dielectrics are mica, polypropylene, ceramic and even paper and oil.
How capacitors block DC but let AC pass : Caps are made with sandwiched but separated conductive plates. The separated plates cannot flow DC current but AC fluctuates between positive and negative voltage.
When the AC guitar signal negative voltage excess electrons is applied to the input plate the electrons repel electrons on the output plate so they move off the plate and flow out of the capacitor.
When a positive voltage is applied scarcity of electrons to the input plate the output plate attracts electrons so electrons flow into the capacitor. That's how capacitors really work but I like to visualize them as having a stretchable rubber membrane inside that blocks the flow of electricity. When voltage is applied to a capacitor the 'rubber membrane' stretches and bulges as electrons try to flow through it. The higher the voltage the more the membrane bulges.
If you quickly reverse the capacitor's voltage polarity it will go from bulging one way to bulging the other way. This is what a small AC signal does--it stretches the 'membrane' back and forth as the voltage alternates which allows electrons on both sides of the capacitor to move back and forth alternate but a constant DC voltage that is trying to flow in one direction will be blocked by the membrane. If you are familiar with hydraulics a coupling capacitor is like a piston in a hydraulic line.
Small alternating pressure changes will make the piston move back and forth so fluid is moved on both sides of the piston -- this is how small alternating current signals move through a capacitor. High voltage DC direct current power used by the tube is brought in through resistor R5 , which is a ' load resistor. The wire between tube pin 1 plate and R5 carries up to volts DC.
That wire carries both the AC audio signal out and the high voltage DC power the tube needs in. Coupling capacitor C1 allows the AC audio signal to pass through but blocks the DC on the wire and keeps it out of the volume pot. Signal flows from capacitor C1 to the Volume pot then down the orange wire to tube V1B's grid pin 7 then out the plate pin 6 to capacitor C2, then to a fork in the road--resistor R9 one way and the other way down the yellow wire to the power tube V2.
After going through capacitor C1 the audio signal flows up the yellow wire to the volume potentiometer pot which acts as a variable voltage divider. The signal then flows from the volume pot down the orange wire all the way to tube V1B 's pin 7 grid.
V1B is the second half of the preamp tube. This second gain stage is called the output stage driver because it boosts the signal to the level needed by the power tube. The audio signal leaves tube V1B via pin 6 plate and flows up the yellow wire to capacitor C2 , another coupling cap that blocks DC.
High voltage DC is fed to the tube via load resistor R7. After C2 the signal flows down the yellow wire to the power tube's pin 5 grid. Resistor R9 has a dual function. It adds input impedance to the power tube amplifier circuit and acts as the tube's 'grid leak' resistor which keeps the grid at 0 volts DC.
Signal leaves V2's pin 3 and flows out the blue wire to the Output Transformer's primary winding then out the secondary winding to the speaker jack. The power tube , V2 is sometimes referred to as the output tube. V2 is the final stage of amplification and its purpose is to amplify for power voltage x current where V1A and V1B were focused on voltage amplification.
The signal enters the power tube at pin 5 grid and leaves via pin 3 plate. It then goes to the output transformer OT which is mounted on the backside of the chassis and is not shown on the layout diagram.
Like we saw with the guitar's pickup, magnetism can be used to generate electricity in a coil. You can also do the reverse and pass electricity through a coil and generate magnetism.
The amplifier's output transformer uses both of these principles to pass alternating current AC from its primary input winding to the iron core as magnetic flux and on to the secondary output winding as alternating current.
The output transformer's windings are really just two wire coils wrapped around an iron core. The input, or primary winding uses electric current flowing through it to generate a magnetic field or flux. This magnetic field fluctuates with the guitar AC signal voltage and is captured by the transformer's iron core.
The captured magnetic flux flowing through the core generates a voltage and current in the secondary winding. You can alter the voltage and current from primary to secondary by changing the ratio of coil wraps from primary coil to secondary. Current flowing into the primary winding above left induces magnetic flux flow around the transformer iron core which in turn induces an electric voltage and current in the secondary winding.
Put fewer wire wraps on the secondary output winding and its voltage will decrease step down but its current will increase. Most guitar amp transformers are of the 'double window' type bottom of left diagram and made with laminated iron magnetic cores. The Power Transformer is lying on its side while the Output Transformer is standing vertically. Example: The primary winding has wraps of wire in its coil and the secondary has wraps. The current will change proportionally in the opposite direction.
If 1 ampere of AC current is applied to the primary the secondary will generate 2 amps. This is what an amplifier's output transformer does, it steps down the signal's voltage but steps up the current because the speaker's voice coil needs current to generate a magnetic field to move the speaker cone.
The output transformer's primary takes in a high voltage, low current signal high impedance signal and puts out a low voltage, high current signal low impedance signal through the green wire to the speaker jack and on to the speaker. For you mechanical types you can think of the output transformer as a gearbox that alters speed voltage and torque current. The power tubes send large voltage swing into the output transformer and you can think of this as high speed from an "engine".
The alternating current audio signal flows through the speaker's voice coil which generates a magnetic field. The voice coil is simply a single wire wrapped into a coil as shown below. The magnetic field created by the voice coil is either attracted to or repelled by the speaker's magnet.
Positive voltage generates a repulsive magnetic force and the speaker coil and cone moves outward away from the speaker magnet, negative voltage generates an attractive magnetic force and pulls the speaker cone inward. The speaker cone alternates between moving outward and inward as the signal voltage alternates between positive and negative. For every guitar string movement there is a corresponding speaker cone movemen t.
Electric current flowing through the speaker's voice coil generates a magnetic field. When the electric current in the voice coil reverses, the magnetic field also reverses causing attraction and repulsion to the speaker magnet.
This magnetic attraction and repulsion moves the voice coil and speaker cone back and forth to create air pressure waves that our ears perceive as sound--the sweet sound of electric guitar.
When the speaker cone moves outward a positive air pressure wave is created and when the cone moves inward a negative low pressure wave trough is generated. These air pressure waves move our ear drums in and out. Bonus Info : You can determine the ohm rating of a guitar speaker by measuring the DC ohms resistance between the speaker terminals while disconnected from the amp and then multiply by 1. Example: You measure 6.
The 'voice coil' is an electromagnet that interacts with the speaker magnet. The 'spider' supports the voice coil but allows it to move in and out freely. This excellent DIY speaker recone video shows speaker parts and function in detail. Dynamic microphones work exactly in reverse of how a speaker works.
They have a diaphragm like a speaker cone that gets moved by sound air pressure waves. The diaphragm moves a coil of wire wrapped around a magnet. The coil moving through the magnetic field creates electricity in the coil wire--an alternating current signal voltage.
When the microphone diaphragm moves the coil, electricity is generated. When a singer sings a note her vocal chords vibrate like a guitar string. The movement of the vocal chords create air pressure waves that strike the microphone diaphragm and cause it to move. Speaking of diaphragms, our ear drums are diaphragms that when moved by sound waves cause neurons to fire to communicate with our brain.
Yea, our ears are biological dynamic microphones. When a positive, high pressure sound wave hits the microphone diaphragm it is pushed inward and a positive electrical current and voltage are created. When the low pressure wave trough hits the diaphragm it is pulled outward and a negative current and voltage are created in the coil.
The microphone creates an alternating current voltage signal similar to an electric guitar AC voltage signal. The 5F1 amplifier uses negative feedback NFB to reduce distortion, increase headroom, decrease damping factor and improve stability but a drawback is it also reduces overall amplifier gain.
Negative feedback works by taking the speaker output voltage and feeding it back into the amp's signal stream before the driver or phase inverter circuit.
A feedback resistor reduces the voltage to a suitable level before it joins the amp's signal stream. It's negative feedback because the signal is out of phase so when it's injected into the amp's signal stream it reduces the amp's signal voltage.
A green wire running from the 5F1's speaker jack carries the amplified audio signal through resistor R13 and injects the feedback at V1B's pin 8 cathode. Resistor R13 is the Feedback Resistor and controls the level of feedback voltage passed to the cathode.
Adding a switch to the NFB circuit is a common modification. Removing feedback makes an amp more aggressive with earlier break up and distortion at lower volume levels.
So the main purpose of a guitar amplifier is to take the tiny AC electrical signal generated by the guitar's pickup coil and make it strong enough to push and pull a speaker cone. The guitar amp is also used to shape the tone and control signal distortion giving us the clean, mellow sound of jazz guitar or the animal growl of hard rock. Distortion is an important part of guitar amplifier design and this is the primary difference between guitar and audio amplifiers. Audio amps are usually designed for absolute minimum distortion.
Now that we've covered the signal flow I'll go back and cover the other amplifier components that I didn't mention. The fuse is a volt, 2 amp slow blow fuse. Slow blow means it won't blow instantaneously when the turn-on power surge runs through it.
Sustained current greater than 2 amps is required to blow the fuse. Next the power flows to the power switch S1, which is located on the volume pot.
Multimeters show voltage in RMS. You can convert peak voltage to RMS by multiplying by 0. You can convert peak-to-peak to RMS by multiplying by 0. This is why AC electrical noise picked up by guitar amps is often described as a 60 Hertz hum. Why is our wall power 60 cycles per second?
Because power company electrical generators in the US turn at 60 revolutions per second revolutions per minute or RPM. One way to visualize how AC electricity flows is to think of the amplifier's power system as a rope and pulley system. Think of the wall power plug and the amplifier's power transformer as pulleys. A loop of rope representing the hot and neutral wires would be wrapped tightly around the wall power and transformer pulleys.
The power company's AC generator is like a hand grabbing the power rope hot wire and pushing it forward a few feet then stopping the rope movement and pulling the rope back, then pushing the rope again, then pulling it in this alternating pattern doing one push-pull cycle 60 times per second. Electrons actually alternate their movement forward and backward, reversing course through AC wires and circuits like this rope movement. Bonus Bonus Bonus Info: volt circuits in the United States use two volt hot wires instead of volt's single hot wire and ground called neutral.
Alright, back to the amplifier. The white Neutral wire is a ground wire and is connected to the same ground as the Safety Ground wire at the building's electrical service entrance.
The 5F1's power transformer high voltage winding is rated at v. Yes, that's high voltage that can kill you. At this very high voltage the 5F1 power transformer only needs to be rated for a paltry 70 milliamps 0.
The power transformer has three secondary windings. Two other small secondary windings step the v AC down to 6. Notice all voltages in transformer secondaries are always AC because a transformer can't pass DC from primary to secondary.
The 6. The 5 volts are used to directly heat the rectifier tube's cathode. Bonus Info: When I first learned that the power transformer primary coil was made up of one long wire that directly connects the v hot wire to the neutral ground wire I wondered why it didn't short out. The reason is the primary and secondary coils are coupled together by the transformer's iron core. Alternating current in the primary coil creates a magnetic field or flux that is captured by the core. That flux in the core creates an AC voltage in the secondary coil.
The load impedance placed on the secondary winding by the amplifier is transferred through the core to the primary coil. That impedance keeps the primary coil from "shorting out. Modern U. Power cord wire colors are sometimes non-standard so use a multimeter to identify Hot and Neutral. Europeans sometimes use the letters E: Earth safety ground , L: Line hot and N: Neutral to describe the three plug wires. The volts AC power from the power transformer is fed directly into V3 , the rectifier tube.
V3 is a full wave dual plate rectifier tube that converts alternating current AC into direct current DC. The amplifier's electronics need DC to amplify. The amp is powered by DC but the guitar signal moving through the amp is AC. The flow of power starts at the power transformer at far left.
V3 puts out V of DC. This diagram shows "conventional" current flow but the actual electrons flow in the opposite direction. Tube rectifiers are popular in guitar amps due to their dynamic power sag which adds to the amp's playing dynamics and note "bloom".
Audio stereo tube amps usually use solid state rectifiers to reduce voltage sag that would be seen as distortion to the Hi Fi listener.
These resistors and capacitors form RC resistance capacitance low pass filters that take the lumpy, pulsing DC output of the rectifier tube and smooth it out--the smoother the better. Any waves or ripples left over in the DC power would be added to our audio signal and heard as hum in the preamp and power tubes. These big capacitors also function as current reservoirs that help feed the amp during high demand.
The hydraulic equivalent of a filter capacitor is a hydraulic accumulator. The '10K 2W' written on resistor R10 is its rating of 10, ohms and 2 watts. With no guitar signal present the 'idle' voltage at the V1 preamp tube's plate pins 1 and 6 will be around volts DC after flowing through the Load Resistors R5 and R7. Although the Champ does not use a choke many amps do use them to filter the power supply. Typically the choke is placed between the power tube plate and power tube screen power nodes.
This is done as a cost savings measure. A choke would have to be very large and expensive to filter the entire power supply for a 50 or watt amplifier. When electrical current flows through a wire it creates a magnetic field around the wire. Chokes are inductors that use this magnetic field to reduce changes in voltage and current. When no current flows through a wire you can have voltage but no current there is no magnetic field generated around the wire.
When current increases some of the current will be used to grow a magnetic field around the wire. When current decreases the magnetic field shrinks and the magnetic energy is converted into electrical current.
These inductor properties "fight" current changes. A choke is simply one long wire wound in many loops. A choke will have two leads which are simply the two ends of the one long wire. The wire is usually looped around an iron core which makes it work better.
Looping the wire increases the effect of the induced magnetic field "inductor" comes from the word "induced". When power supply voltage ripple flows through a choke, the choke "fights" the ripple. As ripple voltage increases, ripple current increases through the choke. The choke will convert some of the current increase into a magnetic field.
The choke's magnetic field is stored energy. As the power supply ripple voltage decreases the choke's magnetic field collapses and converts into current. So increasing voltage and current are cut, and decreasing voltage and current are reinforced which reduces the amplitude of voltage ripple.
Inductors in AC circuits work the same way. Inductance "fights" changes in current and AC audio signals are made up of voltage and current changes. A small inductor can remove high frequencies. A larger value inductor can remove medium and high frequencies.
A very high value inductor, like the choke in power supplies, can remove all audio frequencies. How is it that the 5F1 Champ uses 0. It's due to power conversion. The 6V6GT power tube datasheet shows it 0. The 12AX7 datasheet shows it uses 0. We have a 1. If our mains voltage is volts then Well that's it for the 5F1 Champ. It's a great sounding but simple guitar amp. The signal flow is very similar to most other Fender amps, they just have more parts.
Really understanding the 5F1 will help you understand other more complex amps. The 5E3 Deluxe is the most common tube amp kit available.
Click the image to view the full size readable annotated schematic. Notice how convoluted the signal path is compared to the schematic. Input jacks are at top right and the speaker jack is bottom center. Click the image to view the full size readable annotated layout.
My 5F6-A Bassman amp is the sweetest amp I have ever heard. See this for an explanation of How the Bassman Works. Bonus Info: How Fender multiple guitar input jacks and channel jumpering works. The more gain an amplifier offers up the more likely it is to oscillate and hum.
That's why many high gain amps have "extra" high frequency filtering plate load bypass caps, DC preamp heater voltage, shielded signal cable and "stability" caps across the phase inverter plates. Component placement, lead dress wire length and placement and power filtering all become more critical. Higher gain amps can take what would be an acceptable level of noise and amplify it to the point the amp is unusable. If you build a high gain kit amp you can expect to spend some time troubleshooting hum and noise issues until you get the kinks out--you've really got to pay attention to lead dress, especially around the first couple of gain stages.
The 5F1's V1 and V2 use "common cathode biasing", also referred to as "self biasing" or just "cathode biased. V1B's bias is set by R6.
V2's bias is set by R8. Capacitor C6 is a cathode bypass cap that helps decrease local feedback and increase V2's gain. Although not shown on the original 5F1 Champ Fender schematic and layout, most Champs came from the factory with the C7 cathode bypass capacitor shown at extreme right.
The bypass cap boosted the amp's gain. For Champs without the C7 bypass cap adding one is a common and recommended modification.
For the tube's control grid to control the flow of electrons between the cathode and plate there must be a voltage difference between the cathode and control grid. The voltage difference is what repels the electrons to control their flow.
The cathode is 'boiling off' negatively charged electrons and a more negatively charged control grid can keep them in place because like charges repel. This voltage difference between the cathode and control grid is called tube 'bias. The much more powerful 5E3P amplifier shown below uses an " adjustable fixed bias " system to supply the bias voltage.
It is called "fixed bias" because a steady bias voltage is applied to the tube control grid. A cathode biased amp's bias voltage will fluctuate with the input signal it's not fixed. A fixed bias amp applies a negative voltage usually between to volts DC to the power tubes' control grids and the cathodes are connected directly to ground at zero volts there is no cathode resistor.
Power tubes have a maximum heat dissipation rating given in watts. Exceed this limit and you can melt the tube.
The power tube grid voltage is always negative on fixed bias amps and a hotter bias will have the grid voltage closer to zero. See this for info on How to Measure and Adjust Bias. The AC power flows into the diode's negative terminal cathode so 50 volts of pulsing negative DC flows out.
The 27K resistor sets the maximum hot bias, reduce it for hotter max bias, increase it for cooler. The most you should do, even to an unplugged amp, is change the tubes. If you have any doubts about anything consult an amp tech. This information should serve as general advice only and act to narrow down potential issues. Amp are complex so what you think is an obvious solution may not be the case. Many of the problems listed below can can be confirmed simply be placing an old set of tubes you know work into your amp.
For the more serious problems below amp not turning on they can still be a useful tool in helping your diagnosis. This is one of the least serious, and perfectly normal signs. If your tubes have had a good run several years they usually last depending on use then they may just be past their best and need replacing.
There may be nothing wrong with them from a technical point of view, but if it drops below what you think is an acceptable level, then a change is needed. The old tubes are good to keep as a test pair for diagnosing potential future problems. If your tubes have been in for a while it may be time to change them.
Generally, power tubes wear out faster than preamp tubes — so look at them first. If you want to be a nerd about it, try to keep a note of the approximate age of a tube.
This will help you narrow down the problem tube. In a way this is similar to above. Your amp may be quieter than it once was. Look below for visual signs that a tube has failed. This one may be straightforward. The problem is usually a connection breaking somewhere in the tube. Read below for identifying the bad tube or try one of your old stock tubes in each position until you find the culprit.
Your amp could be working fine but then have a sudden drop in volume, or even turn off completely. This may be the first sign of a tube slowly giving up. However, it could also be the sign of something else. Try testing with an old stock tube and see if that solves the problem. The first thing you should do is check the fuse. These exist for safety reasons and to protect your amp.
If too much current flows through the device the wire in the fuse melts and breaks the circuit. If they have both blown then it could indicate a problem with your amp.
It may be an idea to get an amp tech to look at your amp. If you find that is is just one tube, then replacing the fuse and blown tube could sort your problem easily. So you think a tube may be bad, but how do you know for sure? And which tube is it? After reading how a tube works above, it becomes obvious why. But if you replace with another tube, and it starts to glow, you can be pretty certain it was the tube.
This one is an easy diagnosis. Red plating is when the plate in addition to the filament glows red and looks kind of sinister. This means too much power is flowing through the tube and is a bad sign.
Very soon the tube will die. Red plating is the result of a bad bias adjustment or a bad tube. If you notice this after a bias adjustment, or on more than one tube, then take it back to the amp tech.
Certain tube brands just deal better than others with a bad bias adjustment, so may not red plate. Microphonic tubes are annoying because they add noise into your tone from small vibrations which are created in your amplifier. Sometimes the sound from the amp vibrates them enough to hear the microphonics.
All you need to do is take a pen and gently tap the tube while in use. Sometimes tubes can arrive microphonic usually damaged through transportation. Or they can develop the fault over time. It could be something else.
So, a part or component will need to be replaced. Although, most of the time it will be a biasing issue and the fuse should stop damage in most cases.
Tube amps are pretty complex and it could be any number of things for an experienced amp tech to diagnose. Any potential contaminants on the surface of the internal tube are removed during the burn in phase. JJ tubes do come pre burnt in, as do many other brands, but they should improve within the first several hours of using the tube. Tube rolling - Tube rolling is where you try out different tubes in the same slot.
This means you could end up with a range of brands in your amp. With experience, you'll get to know the brands. NOTE - when rolling power tubes, it's best to replace a whole set at a time, and not just replace an individual one. This is well done and easy to follow. I took an electronics class in college so we studied and experimited with some of these devices.
Your article put it in perspective for music. Thank you. Well done! Thanks for the info :.
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