WARNING: Much of the details here are generalised and designed for assistance in simple user identification and interest in a valve amplifier. We do not recommend DIY repairs, opening of amplifiers and prodding around inside. You could potentially expose yourself to deadly voltages. Likewise this guide and information are to be taken as information only, it does not explore the technical aspects in detail, some points may be simplified to give a general understanding and may not be technically fully correct .
If you are one of those people who wants to correct us, you are honestly reading the wrong information as it is not designed for your reading. For technical theory I would recommend the author Merlin Blencowe who provides detailed information with the mathematic complex theory.
Electronics are building blocks.
Most faults can be isolated to, preamp, FX, output and power supply issues. Power supplies and discussion are out of bounds of this exploration as it would expose you to potentially high voltages and opening the amplifier, whereas this guide is more for keeping the amp in it’s chassis and identifying the stages for simple repairs.
Understanding amplifier stages.
When attempting to diagnose an issue with a guitar amplifier on the face it can look quite an intimidating beast with lots of knobs, valves an “things” where it can be daunting to know where to start. This guide will help you to understand a typical layout of a simple 1 channel guitar amplifier.
Now every guitar amp is slightly different and there are some general variances, however in 95% of cases this is a fairly standard model of amplification and stages if you strip away all the rest of the fluff.
In looking at the above simplified diagram you will notice the front panel controls are at the top, while the bottom functions are generally found on the back of the amplifier. When reading the block diagram the most general flow direction is left to right, from input to output. When diagnosing it is important to break down the stages as it can help you establish where the issue is.
Preamp side
At the heart of every guitar amplifier ever in the world and ever existed is a first gain stage. In essence the concept is to match the impedance of the pickup to attain absolute power transfer and provide as clean gain amplification with as little noise, this then makes the signal useful for the rest of the amplifier.
1st Gain Stage
This takes the tiny electrical signals from a guitar pickup, (we’re talking like fog size water droplets in comparison to a heavy rain storm) and amplifies it through a valve stage usually a 12AX7 or EEC83 (same difference) to a much larger signal, often into the tens of volts.
EQ
The EQ section can take many forms, from the traditional single control of bass one side and treble the other, through to the more recognised Treble, Mid, Bass controls with various bands and scooping functions to boost and cut, or even a 6 stage graphic EQ to coincide with each string frequency. The EQ stage has a simple buffer and gain with several passive components namely capacitors and occasionally inductors
FX
The FX stage can take many forms, it can be a simple send and return, it can have non bias vibrato, tremolo, reverb and any combination of the above. In this instance we will keep things simple and just see it as a send and return stage. Often this is where you put in your pedals such as wah, reverbs or other effects.
Send
The send on the back of an amplifier will carry the amplified and often EQ coloured signal. A tip for operation is that you can take a line out at this stage generally without causing any issue to the amplifier operation. You can run this send into another amplifier for another parallel output.
Return
The return socket is where it gets a little more exiting, without a plug in the return there is a little switch internally which will in effect bypass and link the signal chain along. Think of this switch is like taking a short patch lead and putting it in the send and return, it’s just linking the two.
If however you put a plug into the return, the signal chain of the amplifier breaks at this point, you disconnect the preamp side. It is here you may want to use just the power amplifier section of the amplifier if you are feeding a line level into the return. If you plug anything in externally into this stage and you are not using the guitar’s preamp section, the EQ will have no effect on the sound at this stage. Remember at the top I said the signal flows from left to right, because we now see the return as the input, everything pre this relatively irrelevant.
Most FX stages will have a buffer amplifier stage which is usually less of a gain, and more of an impedance matcher stage to prepare the signal for the next stage.
Power Amplifier stage
The muscle of the amplifier is found in the power amplifier stage, here the small signals get conditioned into the right format for the output valves, resulting in a high current output to drive speakers
Master Volume
The master volume can generally be found in one of two positions (on amps that have master volume, unlike Fender Twins and other such amps) either inline after the FX buffer feeding into the phase invert, or in the phase invert section using a stereo potentiometer. There is a popular mod which can add a master volume to a non master volume amp known as Post Phase Invert Master Volume (PPIMV) however it can also be found native on many amps.
Phase invert section
A very clever yet simple stage where the signal enters the stage, and two outputs occur 180 degrees out of phase between the two. This stage will often have the presence control which is a simple roll off filter. The section is preparing the signals to be in the right format to drive the output stage which requires out of phase signals to drive the output valves.
Output stage
The two out of phase signals are required by the amplifier stage. To try and explain how a standard Class A/B stage works you need to imagine two people, sat across the table from each other. There is a line in the middle of the table, a ball is being passed between the two sides. The person sat in the positive half can only deal with the ball when it enters their half of the table, the same applies to the negative side. Each person is like the power valves sat in the amp, either as a single valve for a pair of valves, a pair of valves for a quad, three valves for a sextant, and four for an octant of valves. The more the valves on each side the more current can be transferred.
Transformer Stage
A transformer is a pretty amazing device being essentially just loops of wire around an Iron former. As the name suggests it’s job is to transform, in essence it is a ratio converter. It converts voltage and current from one side to the other of an Alternating Current (AC) only signal. For an output transformer we have two sides, the primary High Tension (HT) side ( high voltage, low current) which is known as a centre tap. On the secondary side will be multi tap, most often set up for 4,8, 16 ohm to match the speaker load. To obtain maximum power transfer and reduce losses in a transformer it is important that the impedance for both in and out feeding, and being fed to the transformer as as closely matched as possible. In general operator terms it’s selecting the 8 ohm selector when running with an 8 ohm load.
The high voltage low current of the valve output stage is transformed through the transformer (this is too technical to explain here) into a low voltage, high current designed to move the air and drive your speaker.
Valve Layouts in an amplifier
Like the rest of this guide, there are exceptions, more complicated and more simple amps. This guide however is a general concept and valve layout is fairly inherant, where there are some fairly nominal layout patterns.
Valve numbering (most amps) The lowest number is the closest to the input jack. I.e. You would start V1 from the right hand side working leftwards. In some amps this isn’t correct but in 90% it is.
Power amp side is often closest to the mains lead or socket. Keeping all noisy and high current parts of the circuit together are important for a low noise amplifier. The smaller preamp looking valve closest to the output valves is almost certainly the phase invert valve too.
Pre-amp side of the amp is generally closest to the guitar input jack socket. As such in the picture below the very right hand most valve is generally V1, you may read forums where they say change V1 for more gain or tone, this is the one. The valves going left are generally over drive / 2nd channel, EQ and or reverb drive valves.
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Beginning to diagnose an amplifier
Hoorah you’ve made it this far! With the knowledge of the different blocks we can now start to look at how to find the fault!
The first step in identifying a fault is to try and isolate it to a particular area. Below I give some fairly common occurrences and issues and then explore how to look for a fault and the process involved.
No Lights or glowing of valves
If the lights are off and no glow from the valves you can likely assume there is a power supply issue, past replacing the fuses on the back of the amp or accessible without opening this is where we would say to stop. Manufacturers design user replaceable fuses to be accessible. You can usually just replace these without too much worry of doing any more damage. Fuses inside the amp however are designed to protect more sensitive parts, replacing them without due care and attention i.e. monitoring voltages and looking for other faults which caused the issue could cause you to do more damage and we do not recommend this for a DIY repair.
Crackling and popping.
More often than not a pop or crackle is caused by a valve, it’s the first logical path to choose given you can just pull them out and swap over without too much issue or attention, other times they can be caused by power supply issues, capacitors or noisy resistors, and bad pots.
Here we are, a crackle and pop but no breakfast cereal (I’m a 90’s kid it’s ingrained from the tv adverts!!), suspected valve but there about eight or nine of them, where do we start?
Firstly we need to isolate as best what stages are affected by the crackle, the simplest place to start is the master volume. Turn the master volume down.
Sounding like woodland fire? – chances are the noise is being generated in the output section, could be a phase invert valve or output valves. You can generally swap over the phase invert valve with one of the other small valves to see, keep the master volume down, if the noise is no more, it was that valve. If you’ve still got issues its quite likely the output valves. Remember to replace these as matched sets and always bias / check the bias to ensure the amp is working correct to to the valves.
Crackle / pop stops when master volume is down. – great we know the issue is somewhere in the preamp and FX blocks. Now work back, turn the master volume back up and enjoy the crackles of the woodland fire and move to the next step. Plug a cable into the return, this will isolate the preamp section. If it stops you need to look at the EQ section.
Try adjusting the EQ settings if the crackles get more bassy or trebly when adjusting the tone controls you have now identified that the crackles are coming before the EQ, and this section is doing what it should be by adjusting the tone of the crackles. Try changing the first gain stage valve.
Feedback, high pitch noises and hiss.
Customers often describe humming and buzzing as feedback, or any noise except their guitar noise is feedback. It is super important to establish and differentiate a hum, from a buzz, from feedback, from hissing.
Hum – often 50 or 100hz (most countries running at 240V) it sounds fairly soft and uniform, the noise a desktop fan makes or a transformer if you put your ear to one.
Buzzing, similar to hum but sounds sharper or more jagged, can vary in frequency
Feedback sounds the same as when you put a microphone too close to a speaker, the high pitch shrill of a clean sinewave, but can also happen at low frequencies.
Hissing is the static like sound when the gain is set to 11 on an amplifier without anything plugged in, the white noise sound of old TV sets.
Hum on an amplifier.
Past checking your pickups and jack cable for a bad earth, most humming is caused by failing storage capacitors in an amplifier or other power supply stage component. Sometimes it can be caused by a valve red plating and drawing too much current but this is a short lived event and the hum will be the least of your worries at this point. It’s best to see a technician.
Feedback / Microphonic Valves
More often seen in a combo amp where the speaker is close to the chassis of the amp. Feedback can occur in a mechanically compromised valve, the internals of the valve can rattle and the vibrations from the speaker transfer to the valve who’s internals vibrate altering the gain of the associated section and introduce the feedback – this is known as a microphonic valve. Because of the super high gain, this is generally more noticeable in the first gain stage valve, however can affect any high gain stage such as overdrive. A way to test for microphonic valves is to have the amp on, using a chopstick or a pencil lightly tap on the side of the glass envelope. If you can distinctively hear the sound of the tapping quite sharply or with a after ring it’s best to replace the valve. If you can hear a dull thud, it’s generally not an issue.
Hissing
A hissing gain stage is generally caused by a noisy valve and replacement in most cases is the cure, in more troubling cases it can be a noisy resistor which can occur with age, but from a user point the valve would be the easiest thing to try.
Breaking it down
Though brief and lacking much technical detail this guide was created to help give players and oversight of how breaking down stages can help identify issues and isolate to areas. When we get enquiries some of the more confident players will be able to do these self simple diagnostics, in many cases they don’t need our help and we are happy that they’ve managed to solve the issue. Other times players may have carried out a reasonable user level diagnostics however the fault remains, that is where we can offer a quick resolution looking more at the electronics and fundamental workings to get to the issue and resolve the problem.