TIPS FOR TUBE GUITAR AMPS
To keep your amp in top condition Sherlock Amplifiers recommends the following tips:
ALWAYS OPERATE YOUR AMP WITH THE CORRECT FUSES. Do not use higher rated fuse, nails or tinfoil as fuses! If your amp keeps blowing fuses, its for a good reason, get it checked out.
TUBES AND BIASING
TUBE CONSTRUCTION AND OPERATION
A tube is an electronic device consisting of 4 main sections or elements, a Heater a Cathode, Grid and Anode or Plate, which are all, sealed in an evacuated glass envelope. Simply, with all the correct voltages applied to the elements of the tube, the filament heats the cathode which. In tern emits electrons. These electrons flow from the cathode, through the grid to the anode or plate. By varying the voltage on the grid we can control this flow of electrons from either full on (saturated) to full off (cut off) or anywhere in between. We use the range in-between these two extremes for amplification with minimal distortion.
TUBE TYPES
Smaller tubes used in guitar amps
are mainly Dual Triodes such as 12AX7, 12AT7, and 12AU7. These are pre-amp tubes or the input and amplifing tubes, and are used to amplify the small signal from your guitar to something more usefull. The gain control
is usually place around this spot. the signal is then shaped with the tone control circuit, some overdrive may be added The Master and the Effects loop are place around this area and now its read for the power amp section or tube
output stage. Large tubes are the Pentodes or Tetrodes such as EL34, 6L6, 6550, 5881, EL84. They receive the signal from the pre-amp tubes and amplify it to produce enough power to drive a 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.
Biasing is setting the amps internal voltages up so that the tube operates correctly. It's like setting the idle speed on a car 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 to 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.
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 a small amount of current (about 20-30ma)
flows and from there the current can increase up to saturation and back down to just on again. 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 as above. Push pull output configurations can use either class A or B bias setting but has 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 its own 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 A, push pull, self-bias: Vox AC30 uses 4 EL84s in this configuration |
Class B, push pull, fixed-bias, 4 EL84: Peavey classic 30 (similar to the Voxbut with fixed bias) |
Class B, push pull, fixed bias, 2 or 4 tubes: Marshall, Laney, Fender, Hi-watt |
CLASS AND OUTPUT POWER
Class A is the less efficient with the most distortion and with one EL34 can produce only about 6-10 watts. Class B, the most efficient has less distortion but its power is virtually doubled, but may suffer crossover distortion if not biased properly. Output power starts at about 20 watts 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.
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.
TUBE MATCHING
Because of this spread in characteristics in the same tube type we have a problem installing them into an amp. If all our EL34 for example are different, and we only have one bias voltage for output tubes biasing, which tube do we set the bias voltage to? If one EL34 is biased correctly, then the others must be biased incorrectly. And if the spread of there characteristics is great for the tubes you have, then you could have a situation were one El34 is biased ok, another is virtually biased off, another could be hard on. The amp would sound terrible with this biasing/tube arrangement. We could set the bias voltage to the average of their characteristics, if the spread is not to fare apart. This would be ok, but the best solution is to use matched tubes. Only a matched set of tubes will give the maximum output power with minimum unwanted distortion and noise, i.e. give the best tone.
As we spoke before of the spread in tube characteristics, (the 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.
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 derive 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.
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. 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. Turn the volume to zero 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.
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, how hard 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.
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. Their are many symptoms of worn tubes and hear are most of them. For 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 microphones noises. Pre-amp tubes; More hiss, more hum, lacking sensitivity, high gain stage lost gain and most of the above. If this sounds like your amp, and you haven't change your tube for some time them 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!
CHECK FOR TUBES NOT WORKING OR BIAS PROBLEMS
First thing to do is checked that all the filaments in the tubes light up. Turn the amp on in a dark area. For 12AX7's there should be 2 filaments per tube, for power tubes, only one (different tube makers have varying amounts of the filament wire exposed from the cathode so some look brighter than others). If they're all ok, check the condition of the power tubes. Look closely at them and check to see if the gray body is not glowing red-hot indicating a bias problem, and check the heat coming from them also. If there extremely hot or stone cold, you have a problem. With all the volumes turned down, listen to the level of hum coming from the amp. It should be low, if it's loud this may also indicate a faulty tube. Properly working and biased power tubes will some times have a faint blue glow inside the elements of the tube and this glow increases its intensity when you play through the amp. Also look for tubes with a white powder inside the glass envelope. This usually indicates the air has got in, because its glass is cracked.
Over time and even from new, tubes can 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.
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 loosed the elements in the tube over time. This rattling sound which sounds something like a stick with coke bottle lids 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 you tubes are OK, check the rest of the amp for something loose that 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 rattle free amp.
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.
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 everytime 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.
FUSE RATINGS are generally as follows:
100-watt, 4 power tube amps: mains fuse, 2-3 Amp slow blow. HT 1 Amp normal blow. |
50-watt, 2 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. |
TRANSFOMERS
WHAT DO TRANSFOMERS DO?
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
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 resistances 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.
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.
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.
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.
Sherlock Amplifiers can make any transformer, either for tube or solid state amps. All that is required is that you tell us what you want or what you want it for and we will design a transformer for you. Any transformer, for any amp can be easily made!
STANDARD REPLACEMENT TRANSFOMERS
We can source transformers for all the common amp i.e. Marshall's and Fenders Crate Laney, Boogie etc. We also can fit them and go over you amp to ensure its in top working order after the installation. Most modern day transformers are mass-produced and some fail prematurely so you can up grade to a hand made transformer from us.
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 fail due to the above reasons also, 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.
Prices are for REWINDS and HAND WOUND custom jobs. Prices vary due to amount of work, number of windings, end covers, etc. for each transformer. The following is a guide.
100watt guitar output transformer $120-$170 |
50 watt guitar output transformer $90- $140 |
30 watt guitar output transformer $70- $100 |
15 watt guitar output transformer $40 -$80 |
100watt guitar power transformer $150-$190 |
50 watt guitar power transformer $120- $160 |
30 watt guitar power transformer $100- $130 |
15 watt guitar power transformer $80 -$100 |
Chokes $40- $70 |