Guidelines and tips for assembling avacuum tube guitar amp

Guideline and tips for assembling a tube guitar amp
3. Ground of tube guitar amp The ground is a critical concept in guitar amps. What is amp ground? Why does an amp need ground? Where should you ground it? How do you deal with ground surfaces? What problems could a bad ground cause? All these questions require deep consideration before you start assembling an amp. 3.1 What is electrical ground of amp? In electronic devices including guitar amp, the term "Ground" refer to a common reference point or potential voltage, which is typically assumed to be "zero potential". The voltages of other points in the circuit are compared with this ground. Certainly, "Ground" is only relative concept, any points in circuit can be defined by zero potential point. Normally, we chose common return point for all power supply and various amp stages as ground point. 3.2 The functions of amp ground The primary function of amp ground is safety.The amp should be grounded to earth for safety reasons. More importantly, the amp should be connected to the same ground potential as all the other audio equipment in the system. A bad ground can lead to personal injury if someone touches the metal parts of amplifier. Improper grounding can also lead to malfunctioning and damage of the amp and its connected audio devices. Secondly, the ground line of amp serves as common curren return path for all circuits within the amp. The ground lines of amp are often idealized as an infinite source or sink for an electric charge, meaning they can absorb an unlimited amount of current without changing their potential. The audio circuits of the amp should be grounded to a zero-potential signal ground to further reduce noise in the signal .This is especially important in amp, as they are designed to amplify the signal and any noise within the signal can be magnified. Improper grounding or loop layout can cause resonance between circuits, leading to buzz or hum. Third, ground provides the "shield" for some the components of amp, such as input and output jacks.A good ground scheme will: .Minimize series impedance in the signal ground. .Avoid ground loops. .Prevent 'noisy' ground currents from flowing in quiet signal grounds. 3.3 Grounds in tube guitar amp Safety Earth(Ground): Physically connecting the chassis to planet Earth via the main earth wire Although many amps are enclosed into the wooden case, most guitar amps chassis is metal, it is still possible for the user to touch the metal via fixing screws or when replacing tubes, etc.So, it is important to make sure the chassis connected with planet Earth by the main earth wire, provided a safe path for current to run to ground. Once the chassis is earthed it will be at the same potential as the person using it, and if any live wire were to touch the chassis it would immediately be shorted to earth and cannot shock the user, whether or not a fuse blows. We use one single point being safety earth, which is at the one of fixing bolts of Power transformer. Chassis ground: the ground points on the metal chassis of amp. the chassis acts as a barrier for the electromagnetic field, both inside and outside the amp. It shields the amp from external radio-frequency and other electromagnetic interference that could cause hum or buzz. On the other hand, it also prevents the internal electromagnetic field of the amp from leaking out, which could interfere with nearby electronic devices such as TVs and mobile phones (EMI). There are three grounding areas in the chassis. First, the safety earth point, which we discussed individually above because it is so important. The areas where the potentiometers (pots) and output sockets are fixed should also be grounded. Ensure the shells of the pots and the grounding of the output jack are connected to the chassis, allowing them to share a common potential. Since the chassis is connected to the earth through the safety earth, the entire metal chassis will be at 'ground potential.' This not only shields the amp circuit from external electromagnetic interference but also prevents internal fields from leaking out.
Tip: All ground areas on the chassis need to be polished and cleaned carefully. The resistance between any ground component and the chassis should be less than 1 ohm (typically 0.5 ohms or less).
Bad grounding connection will make primary AC line frequency or 2x the line frequency inject into into the signal path, cause big hum and hizz , which is the big problem affect sound of amp. Circuit ground: the zero voltage reference point of a circuit. There are many kinds of circuits dealing with different voltage and currents in tube guitar amp: the low-voltage small signal input stage; high-voltage big current the output signals and power supply stage. All of the currents need one common return path, which is ground line of amp circuits. Because the ground line is common return path of different stage of amp.The power supply and output stage which are high voltage and big current, could cause interference with the sensitive, high-gain preamp stages through common ground line, inject hum or buzz into the signal path. In order to obtain a clean signal ground, it need distinct the different current ground line in the circuit of amp, keep power supply and output stage ground currents from flowing in the ground return of the low-level input stages. Reference JCM25 ground below:Ground 1 are main ground of circuit , the ground of the different stages of the amp are connected with it. Ground 2 only provided ground of input jack and first preamp. This way separate the small input signal with the high voltage big current of post-stages of amp, so obtain clean and high signal-to-noise ratio signal.

4. Layout of amp Because the vacuum tubes are voltage amplifier, so there are many wires with different voltages and currents in the amp. If you did not pay attention to layout of amp, bad wire routing would causes a wire with strong signal interfere an adjacent weak current wire,created hiss and hum. Other problem from bad layout is: When there is coupling between the input and output stages, positive feedback between them let the amp to become an oscillator, so cause resonance and squeal.
Tips of layout of tube amp are below:
(1).Move output wires away from input wires. (2).Shorten excessively long wires. Long wires tend to act like antennas so keeping wires short will help to reduce this effect. Wires that are oriented parallel to one another act as if they were coupled together by a capacitor. Magnetic coupling between adjacent wires also occurs. The longer the wires, and the closer together they are, the greater the capacitive and magnetic coupling. Magnetic coupling is dominant in high-current wiring, such as in the filament supply. Capacitive coupling is more significant at higher frequencies.To minimize this coupling effect, wires that carry power and those that carry signals should not be run parallel to one another and should vertical cross if possible. This is called orthogonal wiring. (3).Use shielded wires on the input sockets and other important connections (The shielded wires are indicated on the layout figures of amp). (4).Place AC power as far away from the inputs as possible. (5).Keep the output transformer separated from the power transformer and from the inputs. (6).If squealing occurs, first you can turn around the volume and tone controls to see if it affects the oscillation. This also will estimate if the coupling is occurring before or after a given control. Then squealing is easily corrected by moving the physical location of the wires around; second you can use a wooden stick (old chop stick works great) move the wires(the wires between the soldered board, the tube sockets, and the controls) around. Turn the amp on as if you are going to play (The amp must be connected to a cabinet), turn the controls fully up, and with your wooden probe, move the wires in the amp around, find the locations for these wires that are the quietest (for hum and for potential feedback). You can use a dab of silicone sealant to hold the wires in place if you like. 5. Resonance of amp The squeal caused by resonance is common problem of assembly amp, even if experienced people would gotten into this trouble sometime. Some beginners look down the important of layout of wire and grounding of amp, they think of that it is no problem only the components connecting right. Commonly, after they finish assembly thenturn on the power, except hizz and hum, nothing they hear.The reasons caused resonance include below: (1). Bad grounding. The principle of layout has discussed at 3 "Ground of tube guitar amp". (2). Bad layout of wires. The principle of layout has discussed at 4 "Layout of amp". (3). The bad contact between tube and socket maybe cause resonance (4). Exchange the input wires of OPT P1 and P2 during resonance. (5). When amp oscillates, besides hiss and hum, the static operating point of tube will abnormal, commonly double times than normal. During you look for the point of problem, besides the methods listed on 4.6, you could measure the static operting point of per tubes comparing with the DC voltage map of main board of amp we provide.
Tips: Prevent resonance of amp
1.Select good 12AX7 being first preamp , shield it by cover. 2.Try plug different guitars or wires, try to stand on different directions. 3. Close it into enclouse which has shield film. 6. Bias of power tube amplifier If you want to bias power tubes be aware that there are lethal voltages inside the chassis. If you do not feel absolutely confident that you understand how to perform this procedure and keep yourself safe, please consult a professional. 6.1 The meaning of tube bias Bias voltage is the voltage between a tube's cathode and control grid. Bias current is the current flowing from the cathode to the plate with no audio signal applied to the control grid (idle current). The current through an amplifier tube is controlled by the bias. Current can be stopped totally (cutoff), or it can be at a maximum (saturation), depending on the bias setting and the signal. 6.2 Why bias adjustments are necessary A) Changing the bias can change the tone of amp. B) Different kinds of output tubes need different bias levels. You don't always need to bias power tube amplifiers after a tube change 6.3 What kinds of amp need bias when change power tubes There are two kinds of bias of amp : one is fixed bias, other is cathode bias(self bias). Only fixed bias amp need adjust bias during change tube. These amps include : JCM 25, Bassman25 etc. Cathode bias amp do not need adjust bias . they include : PVA18, UT 18 . 6.4 Meaning of 'hot' and 'cold' bias A)'Hot' bias means that the amp has "more" idle current than the optimum. The result is 'fatter' and warmer tone and shorter valve life. B) "Clod' bias means that amp has "less" than the optimum idle current. The result is a 'thinner' tone and longer tube life. 6.5 The biasing procedure A) Set the tube's idle, or no-signal current flow at a point to keep from overheating the tube's plate. B) Set the idle current. C) Measure the voltage across the cathode Resistor (R24) (refer to the layout and schematic diagrams for your amp to determine the location of R24). The bias adjustment is accomplished with a variable resistor. The location is indicated on the layout. For 6V6 tubes, the voltage across the cathode resistor should be between 20 mV and 30 mV. We recommend 30 mV. For 6L6/EL34/KT77/KT88 tubes, the voltage across the cathode resistor should be between 20 mV and 35 mV. We recommend 35 mV. More about Bias , please watch More discussions about tube biasing

P13 The dots measuring bias voltage
7. Some DO NOTS (1) Never, Never, Never run the amp without a speaker plugged in. This can cause major damage. (2) Do not flip the power switch off and then back on rapidly. This can cause damage to the power supply. (3) Never replace a blown fuse with one that has a higher amperage rating. (4) Never ignore signs of overly high heat. A wisp of smoke or a burning smell is NOT normal. (5) Never ignore a red tube glow other than the small orange ends of the filaments. A red glow over a large part of the internal plates of the output tubes means they are about to melt. If you notice this, shut the amp down immediately and get a technician to help you find out what is wrong. 8. Some DOs Add another speaker into the "external speaker"jack; a mismatched speaker load won't damage the amp, while an open circuit (disconnected speakers) may do so. Overdrive the stuffing out of it. Tubes are very forgiving of massive overdrives, unlike solid-state circuits. As long as the tubes don’t overheat or stay overdriven for long periods, it is not fatal. 9. Services from KLD All of our amp kits undergo strict QC testing before leaving the factory. Assembly is not difficult if you follow the manual carefully and patiently. However, if you encounter any issues during assembly, you can easily reach out to us for assistance. When providing feedback about assembly problems, please include the following details: 1. Order details: Name of the kit and purchase date. 2. Description of your issue: The status of the amp, your observations, and a brief description of the steps you’ve taken. 3. Photos of the amp layout, especially highlighting any areas where you suspect the issue may be.