Sherlock Amps | Tech 2

		Dual Triode

TUBE THEORY
TUBE CONSTRUCTION AND OPERATION A tube is an electronic device consisting of 4 or 5 main sections or elements, a Heater or Filament, a Cathode, Control Grid and Anode or Plate, (and with Pentodes and Tetrodes, a fifth element is added called the Screen grid) which are all meticulously assembled and then encapsulated in a evacuated glass envelope. Simply, with all the correct voltages applied to the elements of the tube, the heater heats the cathode which in tern emits negatively charged electrons. These electrons flow from the cathode, through the control grid by being attracted to the anode or plate which is positively charged by the high voltage applied to it. By manipulating the voltage on the grid or the input of the tube (i.e. your guitars pickup signal applied to the control grid), we can control the flow of electrons from cathode to anode to produce an exact replica of the input signal on the tubes output (Anode) but of a greatly increased size. This phenomenon is called amplification. The amount of the increase of the input signal by the tube is called Gain. By increasing the input signal on the grid to a high negative level we can impede the electron flow and increasing it higher again (very negative) we can cut the flow off completely (Cut Off) or we make the input signal more positive to increase the electron flow, and even more positive again to increase flow to maximum (Saturation). For clean sounds the tube is operated in-between these two extremes which is call the linear portion of the tubes characteristic curve. And where we set the starting point of this increasing and decreasing of tubes electron flow (or current flow) is called the bias point. (More on biasing later) The output signal is not distorted and is very dynamic because the tube is operated well within its cut off/saturation limits or linear range. For dirty sounds we push or drive the tube to operate beyond its linear range, into its nonlinear (Cut off and Saturation) range. The output signal becomes distorted or clips and results in a compressed or overdriven sound due to the tubes limits being reached. The tube also produces harmonic overtones, adding to the richness of the distorted sound, and increased sustain and fattening of the notes or chords played, all the attributes loved by Blues and Rock players. And if the input signal is increase to ridicules levels by adding extra stages (more tubes) we drive the tube even harder into its extreme operating conditions (saturation) producing heavily distorted, compressed and powerful notes with an abundance of complex harmonic overtones. This is the Hard Rock/Metal territory! Lighter levels of saturation can be used for heavy crunch rhythm playing and increased drive for endless singing sustain used for lead solos.

TUBE TYPES AND THERE USE IN AMPLIFIERS There are basically two main sections to a guitar amplifier, the Preamp and the Power amp and both use tubes designed for these sections. Preamp tubes are the smaller tubes and are mainly Dual Triodes such as 12AX7, 12AT7, and 12AU7. Other preamp designs use small signal Pentodes like the EF86. These preamp tubes are voltage amplifying tubes and form the gain stages, and are used to amplify the small signal from the guitars pickups to something more useful. The gain control is usually place after the first gain stage and the signal is then shaped with the tone control circuit, some overdrive may be added with additional tube gain stages. The master volume and the effects loop are usually placed after the gain and tone stages and now the much larger signal is ready for the power amp section or tube output stage. But first the signal must be converted into 2 signals of opposite phase to feed the output tubes in a push pull amp. (note: phase inverter stages are not required for single ended amps) The Phase inverter is also know as the power tube driver and is usually a12AX7 or 12AT7 tube. The power stage consists of much larger tubes and are usually power Pentodes or Tetrodes such as EL34, 6L6, 6550, 5881, EL84. They receive the signal from the phases splitter and amplify it to produce enough power to drive a speaker. The final device in the amplifiers signal chain is the the output transformer and it converts the high voltage, low current into low voltage high current for the speaker. There is also another type of tube called a Rectifier tube used in the power supply of some amps and its used to convert the AC voltage into DC. Common types are 5AR4, 5U4G, 5Y3, GZ34. Modern amps use silicon diodes but the softer sound obtained from tube rectifiers is still preferred by some amp makers and guitarist. To Contents		Electrical symbols
		Tetrode
		Pentode
		Diode

WHAT IS BIASING? Biasing is setting the amps internal voltages up so that the tube operates correctly in the circuit the tube is used in. In particular, biasing is adjusting the voltage on the grid of a tube so that the tube operates in the desired current range. In most cases preamp tubes are self biasing and do not require bias adjustments. Power tubes on the other hand are a bit different and require there bias to be adjusted for correct operation. This is usually done by a technician as it requires knowledge and some test equipment. Other names for biasing are setting the quiescent conditions or the idle state. The following is a mechanical analogy for tube biasing. It's like setting the idle speed on a cars engine. Mechanics set the idle speed to say around 500rpm to 800rpm so that the engine will rev up from this point to full revs and back down to the idle speed again. If the idle speed is set too low the engine will stall. So the bias is set on a tube amp so that its tubes can increase and decrease output power with a minimum of unwanted effects such as over heating, distortion. Incorrectly biased output tubes will either run too hot, which will greatly reduce their life or too cold resulting in distortion at any level. To Contents

CLASS OF OPERATION AND OUTPUT CONFIGURATION Although there are other classes of output stage operation, the two main ones are simply class A and class B and it's basically where we set the bias point of the tube. For class A, its bias voltage is set so that the tube current is half way between full on (saturation) or full off. It's current increase or decrease with an input signal and comes back to half way again with no signal. For class B operation we set the bias just up from the full off state so that a small amount of current flows and from there the current can increase up to saturation and back down to just on again. In practice most amps are biased somewhere between class A and class B and this is call class AB. There are also 2 output configurations for guitar amps, Single Ended and Push Pull. Single Ended or one output tube uses class A operation and operates as described above. (top dia.) Push Pull output configurations usually use class A B bias settings and have 2 output tubes (or more) working in push pull or when ones is ON the other is OFF and visa versa. So a positive going signal will increase the current in one tube higher and turn the other totally off. The opposite tube would increase its current for a negative signal, the other cut off, and both tubes would return to the just on condition when the signal is removed. And finally there are 2 types of bias circuits, Self bias where the tubes Cathode resistor supplies its own bias voltage and Fixed bias is where there is a separate circuit used to supply the bias voltage. The most common configurations are as follows: Class A, Single Ended, Self-bias: mainly low powered amps like fender champs, Goldentones etc. Class AB, Push Pull, Self-bias: Vox AC30, Matchless, uses 4 EL84s in this configuration. Bias is derived from the tubes Cathode resistor. Class AB, Push Pull, Fixed bias, 2 or 4 tubes: Marshall, Laney, Fender, Hi-watt, Boogie etc. the Bias supply is derived from a separate source. To Contents

CLASS AND OUTPUT POWER Class A Single Ended is the least efficient with the most distortion and with one EL34 can produce about only 6-10 watts. More power can be obtained by paralleling additional tubes. This is called Parallel Single-Ended and uses a single primary winding on the output transformer. Two or more tubes can be configured with an output transformer having a center tapped primary winding. This is known as Push pull and the tubes are usually biased between class B and class AB, the most efficient has less distortion and it's power is virtually doubled, but may suffer from crossover distortion if not biased properly. Output power starts at about 10-20 watts for the smaller power tubes like EL 84's. EL34 up to 50-60 watts depending on the voltages they operate at and may go up to 300 watts (Ampeg SVT) depending on the tube type and quantity. For 12AX7s in pre-amp circuits, they usually operate in a class A, self-bias configuration. To Contents
TUBES

ALL TUBES ARE NOT MADE EQUAL A tube is a delicate, intricate, mechanically constructed device. Although made under strict design and manufacturing rules each tube turns out slightly different to the next one, even though they are all the same types. So you end up with the same tube type, an EL34 for example, but with a spread in there characteristic. To compensate for the differences we need to adjust bias voltages to suit the individual tube or select the tube for the bias voltages. To Contents

TUBE GRADING OR RATING As we spoke before of the spread in tube characteristics, (the electrical differences in the same type of tubes) it is possible to use this to our advantage. Tubes at one end of this range or spread, will conduct a current quicker and require a lower bias voltage than tubes at the other end of the range, which are slower to conduct and require higher bias voltages. The ones that require lower bias voltages will sound dirtier with less headroom and more saturation, great for power amp distortion. At the other end of the range with higher bias voltages required, these tubes will sound the cleanest with maximum headroom and min distortion. Some tube suppliers us a scale from 1-12 or colored dots or just the current value marked on the tube. To Contents

BIAS ADJUSTMENT Amps are made with either a fixed bias voltage or an adjustable bias voltage, usually with a trim pot operating in a small range. Amps with a fixed bias voltage, i.e. Mesa Boogie, need the tubes to be selected for the amp. This is done so that the amp sounds the way its maker wants it to sound before and after a re-tube. You have to use the same grading of tube to re-tube this amp properly. You can change the resistors that derives the bias voltage but if you go too far you may change the sound too much. Amps with adjustable bias voltages can use a range of graded tubes without too much change in sound after a re-tube. For a Marshall as an example, you would select a matched set in its bias range, then adjust them for the correct current. If you want the amp to sound the same after a re-tube, use the same grading and bias settings as used with the old set. To Contents

SETTING THE BIAS There are many way to set the bias current after re-tubing. We do the following. Install the tubes with bias probes. Plug the amp into a dummy load monitored by an oscilloscope. Feed a test signal into the amp and adjust the volume of the amp so that the output waveform on the oscilloscope just clips. Adjust the bias control to reduce the crossover distortion to zero. Remove the test signal and then measure the current on the bias probes to see if it's within the specs of the tube. Then, plug a speaker in and play the amp, tweaking the bias for the best sound (also known as setting the bias by ear). Then we leave it on for about a half an hour or so to see if we have any premature tube failures and tweak the bias again if necessary and that's it. This method ensures the amp sound the best it can and prevents the customer from problems because we've tested the amp as hard as he's going to play it. To Contents

TUBE FAULTS
As stated, tubes are made up of a number of mechanical parts vacuumed seal in a class envelope and are fragile by nature. Various factors, such as how often and, how loud the amp is played, vibration from a speaker in combos, road travel, setup and pull down all contribute to the life of your tubes. At any time you notice your amp not sounding quite right it could be a tube. To Contents

TUBE WEAR SYMPTOMS AND LIFE EXPECTANCY Got some bad news, tubes don't last forever! They wear out like tires on you car. The more you play, the louder you play the quicker they wear. There are many symptoms of worn tubes and here are most of them. Power tubes; Power down and fluctuating, Bottom end not tight any more, Loosing bottom end and top end, Inconsistent sound, Amp humming to much, Amp lacking punch, Rattling and microphonic noises. Pre-amp tubes; More hiss, More hum, Lacking sensitivity, High gain channel loosing gain. If this sounds like your amp, and you haven't change your tubes for some time or have never changed them then now's the time before you have a power tube failure which may do some other internal damage costing you for repairs as well as a new set of tubes! Please see our repairs page for our retube service. To Contents
CHECK FOR MICROPHONIC TUBES Over time and even from new, tubes can be microphonic. This can result in high-pitched squealing or other strange noises coming from your amp. Turning the master volume down may stop this squealing if it's a pre-amp tube. To check your amp for microphonic tubes, turn the volumes to a reasonable level (you may also have to plug your guitar in but turn its volume down or off) and gently tap the top of the amp or the pre-amp tubes. If this tapping sound comes through the speakers, and or if it starts to squealing its time to change some pre-amp tubes. Microphonic power tubes will not squeal at a high pitch, but they do make a low-pitched droning sound that may continue when excited by a speaker, or only happen when you tap the amp or the power tube it's self. To Contents
TUBE RATTLES Another problem tubes have which is quite noticeable with combos and amps sitting on speaker boxes, are worn tubes with their internal elements rattling. Heat, vibration and hard work have loosen the elements in the tube over time. This rattling sound which sounds something like a stick with coke bottle caps nailed to it, may come through the speaker, or from the amp when playing certain notes on your guitar. To check for this one, pick the tube up and hold it close to your ear and shake it or tap it gently. If it sounds like a set of Maraca's, it's ready for the bin, same goes for pre amp tubes. If your tubes are OK, check the rest of the amp for something loose that's vibrating. Some times, poor quality and even good quality tubes will rattle from new, especially from the filaments. These tubes are not really suitable for combos, so great care should be take in selecting tubes for combos if you want a rattle/noise free amp. To Contents
A WORD ON FUSES

A fuse forms a simple, but effective protection system, the weakest electrical link to break if something inside the amp goes wrong, disconnecting the mains power from the amps circuitry. If you take away this weak link, then in the event of a fault something else is going to blow. That could be a transformer, which is expensive to replace. So it important to always use the recommended fuses. Tube amp general have 2 fuses, the first being the mains fuse. This fuse protects every thing inside the amp, it's the first thing the electricity hits when you plug the amp into the socket in the wall. The second is the HT fuse, which is basically the power tube fuse. In the event of a power tube failure this fuse blows isolating the damage. So if this fuse keeps blowing, your in need of some new tubes To Contents.

FUSE TYPES FOR TUBE AMPS For the mains fuse in tube amps, a SLOW BLOW is required. This is because when you first turn the amp on, there is an initial in rush of current, which may exceed the rating of the fuse, but not burn it out because its slow to blow, then this rush subsides back to the normal current drawn by the amp. If you put a fast blow in there, it may blow every time you turn the amp on! With the HT fuse, we want it to blow as fast as possible for the fastest protection, so use FAST BLOW or NORMAL BLOW. (Note: Some amps do use slow blow for the HT. Always check the back of the amp or it's manual). How to tell them apart, easy, normal fuses only have a thin straight wire seen through the glass body. Slow Blows have, a spring, or a wire with a blob on it, or is filled with sand, or is marked with its rating the letter "T" for thermal. To Contents
Fast Blow	Slow Blow	Slow Blow spring type

GENERAL FUSE RATINGS
100-watt amp with 4 power tube amps: mains fuse, 2-3 Amp slow blow. HT 1 Amp normal blow.
50-watt amp with 2 power tube amps: mains fuse, 2 Amp slow blow. HT 0.5 Amp normal blow.
30-watt amp with 4 power tube amps: mains fuse, 2 Amp slow blow. HT 0.5 Amp normal blow.
Note: there is no difference between large or small body fuses, it's the rating that counts! All of the problems mentioned in these notes can be remedied by Sherlock Amplifiers. Please call for quote on repairs. To Contents
TRANSFORMERS
POWER TRANSFORMERS The power transformer is the device, which supplies the amp circuitry with its operating voltages and currents. A Primary winding which connects to our domestic power supply of 240VAC induces voltages in secondary windings. For tube amps these voltages are high, in the order of hundreds of volts (400-500VDC after rectification) for the High Tension, (HT) about 6 VAC for the filaments or heaters and some amps have a separate bias winding (50-90VDC after rectification) for the output tube biasing. For solid state amps these secondary voltages are much lower, at around the 100-150 VDC to operate transistors and Mosfet devices, and even lower voltages of about 15-30VDC to operate smaller transistors or op-amps used in pre-amps etc. The solid state amp is a much more efficient amplifier, but to date lacks the performance of high quality tube amps To Contents
OUTPUT TRANSFOMERS Mainly used in tube amps these days, output transformers are devices used to couple or connect certain circuits to each other. For a tube amp it is used to connect a tube output stage to a speaker. The output stage of a tube amp, as stated before, operates with high voltages in the order of hundreds of volts (400-500V) and low currents, and with impedance's or resistance in the thousands of ohms. Speakers have impedance's of 4-8-16 ohms, and require much lower voltages and higher currents. For 100 watts of power, an 8ohm speaker requires up to 30VAC. So we use a transformer to convert the high voltage, low current from the tube circuit to the high current low voltage for the speaker. The transformer is in the direct signal path so for high quality amplification, things like frequency response, distortion, etc. are all important for the overall quality of the amp. To Contents

CHOKES Chokes or Inductors are used in the power supply of tube amps, forming what is called a PIE filter. Connected usually with 2 capacitors its job is to regulate the voltage feeding the screen or anode connections of the output tubes. This produces a smoother, stronger sound with increased sustain. The choke in its PIE filter connection is also used to minimize the hum noise that you may hear coming from you amp. Chokes only have one winding and they act as a resistor, or provide high resistance to any AC current left after rectification (converting AC to DC) in the power supply. To Contents

TRANSFOMER FAILURE An Output transformers can fail for a number of reasons, most common are, operating the amp without a speaker, power tube failure with incorrect HT fuse or no HT fuse at all, internal shorts due to poor quality construction and internal shorts from moisture, etc. Power Transformers and Chokes may also fail due to the above reasons , but some other causes are: fault occurring in amp with incorrect mains fuse or no main fuse at all, shorted capacitor or diode in power supplies, internal shorts due to moisture, etc. To Contents

REPAIRS AND REWINDS Sherlock Amplifiers being a maker of tube amps, can also repair and rewind faulty transformers, restoring old tube amps to there former glory. We can also supply better quality transformers for new solid state and tube amplifiers. Please see our repairs page for these details. To Contents

120v to 240v POWER TRANSFORMER CONVERSIONS Please see our repairs page for these details. To Contents

VINTAGE AMPS For Vintage amps, we would pull the faulty transformer down taking note of the winding techniques and materials used and rewind exactly the same way. We will use the original steel laminations, same winding techniques, same covers, wire color coding and mounting brackets to restore the transformer. This will ensure the transformer sounded and looks the same, and also fits straight back into the holes in the chassis. To Contents

Tube Amp Tech Talk

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