nonsense...
Tube Amp myths...
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nonsense...
Jacew
New member
e
Correct me if I'm wrong, but it's about halogen lights, right?
They're not that specialized really: Used in all cars not using leds, and very often in house lighting too. At least here.
Easy to see how that knowledge would be mixed with tubes.
Correct me if I'm wrong, but it's about halogen lights, right?
They're not that specialized really: Used in all cars not using leds, and very often in house lighting too. At least here.
Easy to see how that knowledge would be mixed with tubes.
JB_From_Hell
Jomo's Nimions
You are correct.
St_Genesius
New member
Obviously, you should have repaired it and then replaced the badging with RICHARD GERE in the Peavey font.
Then just wait for people to notice.
Then just wait for people to notice.
Pretty much all the " things " you have heard about tube amps are myths, with the exception of a few very important ones.
1. The tube amp does warm up per se and it just takes a moment for the cathode to get hot enough to boil off electrons. The tubes don't care if there is a signal or not. In fact, I would strive to make it not try and conduct a signal while the tubes heat up. If you ask the tubes to do something they are not ready to yet, that may be worse than not asking them to do anything at all.
2. The standby switch isn't needed at all and it doesn't matter if you use it or not. In fact, using the standby switch is more likely to cause pops and damage than anything else you do. More in a bit.
The only actual, 100% important thing that must have care taken in assuring is that the amplifier is plugged into a speaker while it is on and running. In standby, this doesn't matter. While in standby, the HT or high voltage that feeds the tubes is switched off. With no HT, there is no way for any of the tubes or the output transformer to conduct, so you cannot hurt the amp if nothing is disconnected if and only if the amp is in standby mode. However, it is paramount to the life of your amp that it is connected to a speaker at any time the amp is able to fully operate!
The whole MUST have the SAME ohm rating of speaker cab as the tap set on your amp is literally only 1/2 true. The ohms tap on a transformer is like a gear in a car's transmission. Now let's imagine that the speaker is actually the tire size of the car. If you put a giant tire on the car and don't change the gearing in the transmission, the car will be hesitant to move and you could burn out the transmission's clutch or even the engine. Conversely, if you put a teeny tiny tire on the car, the transmission and engine will be spinning like crazy to make the car move at an acceptable speed. Same sort of thing for an output transformer and a speaker load. The output tap is the gear and the speaker is the tire. It is generally ok at nominal volume levels to be one impedance tap off from the speaker load. I.E. An 8ohm speaker into the 16ohm or 4ohm tap. Things change when trying to run the amplifier full tilt. A 16ohm speaker is a large tire and a 4ohm speaker is a small tire. A small tire ( 4ohm ) on the gearing for a big tire ( 16ohm tap ) will cause the OT to overheat because the amps engine ( tubes ) will be trying to conduct all the power that they can through the transformer with no resistance. Now if you have a 16ohm speaker on the 4ohm tap, the speaker is presenting a very high resistance to movement ( due to electrical conversion ), which loads up the transmission and engine ( amp ) if you will. This causes electrons to build up on the plates of the tubes, and excess energy is stored in the OT. If enough of this energy gets stored, the build-up of electrons in the winding of the transformer can break through the wire's insulation and cause arcing, or the tubes plates will simply melt. This takes a fair amount of effort though, the build-up of electrons on the plates and in the transformer takes time to build up, and if you stop playing, this unloads things and the charge will dissipate. So going from a 4ohm tap into a 16ohm speaker will greatly reduce output, and could eventually cause the tubes and transformer to burn up, but it takes a fair amount of work for that to occur. BUT going from a 16ohm tap into a 4ohm speaker means the amp will be trying to produce all the power it can all the time unimpeded, this is a problem as it will eventually cause the OT to heat up and melt. Another way to look at it is the amount of potential conduction. The less impedance the OT see's, the more it will try and conduct, this causes more heat and may eventually burn up. Loading the OT up with a higher impedance reduces the amount of conduction, but causes a build-up of electrons that can eventually cause arcing or melting of the tubes plates. You can be off either way by one tap safely, for example, a 4ohm speaker into the 8ohm tap, or a 16ohm speaker into the 8ohm tap, or an 8phm speaker into either a 4 or 16ohm tap. So long as you are within one tap difference from your speaker load, you should have no problems at nominal levels. I do suggest though, that if you plan on diming the amp, that you use the same speaker load as the tap that you lug into.
The good old standby switch. It does serve a purpose, but it is more for servicing than for playing under normal use. It is nothing more than a mute switch. It just so happens that instead of shunting the input signal to ground, it cuts the HT voltage from the tubes. If there is no HT voltage on the tubes, they cannot conduct, but there is still 300v to as much as 600v waiting to instantly turn the tubes on when the switch is flipped. Depending on how your standby switch is wired, it can allow a pop as you suddenly snap the HT voltage into all the tubes. The idea that having the standby switch on when you turn the amp on allows all the HT voltage to sit on the plate of the tube which can cause arcing is not very true except in amps that have cathode follower stages. The tubes can't conduct if the cathode is not heated, so there is little to worry about with voltage on the plates. In a cathode follower stage, the HT is actually placed on the grid of that tube! This can allow arcing to occur but is generally of little fear with healthy tubes. As tubes get older and worn, they become gassy or have a breakdown of the cathode element, which can allow paths for arcing to occur, once it happens once, it becomes more and more prevalent. By that point in time, it is necessary to replace the tube anyway. If you have ANY amp with a tube rectifier, you truly have no need for a standby switch. A tube rectifier takes time to " warm-up " and it won't conduct until it is ready, which means that by then, the other tubes will also be ready. In a solid-state rectified amp, the HT voltage is pretty much instantly ready to go, but there is generally a bleed resistor that discharges the capacitors while not in use. So when you turn on a solid-state rectified amp, the capacitors eat up that initial shock of voltage as they charge up. The inline resistors for the different power nodes also slow the shock of that sudden inrush of current. So while leaving the standby switch on when you turn on a solid-state rectified amp will allow the HT to go to the tubes before they are able to conduct, it is a soft start at least. For all intents and purposes, the standby switch is nothing more than a mute switch.
The standby switch is also part of the cathode poisoning myth. Which is only partially true again. The idea is that if you leave the standby off, with the amp on for prolonged periods of time, the cathode will build a high resistance layer on the cathode limiting its ability to boil off electrons. A tube is like a light bulb, it has a life expectancy. The more the tube is on, the closer to death that tube will be. A NOS tube, even used, is only still good because it hasn't gone gassy and has so little use it still has some life left in it. So if you leave an amp on whether the standby switch is on or not, the tubes are then on, slowly but surely sucking the life out of them. you cannot expect a tube to last 1 million hours just because you NEVER had the standby switch off while the amp was on. A tube's life is counted in hours. The more it is on, the closer you are to its last hour. It is certainly not suggested to leave an amp on for prolonged periods of time with or without the standby on. This is one very good reason that a standby switch is destructive. You remembered to hit the standby switch, but not the power switch. Imagine if there was only just a power switch to turn off and no extra step required to " shut down " the amp... The standby switch doesn't help with anything other than muting the amp. A Boss TU3 does that too.
1. The tube amp does warm up per se and it just takes a moment for the cathode to get hot enough to boil off electrons. The tubes don't care if there is a signal or not. In fact, I would strive to make it not try and conduct a signal while the tubes heat up. If you ask the tubes to do something they are not ready to yet, that may be worse than not asking them to do anything at all.
2. The standby switch isn't needed at all and it doesn't matter if you use it or not. In fact, using the standby switch is more likely to cause pops and damage than anything else you do. More in a bit.
The only actual, 100% important thing that must have care taken in assuring is that the amplifier is plugged into a speaker while it is on and running. In standby, this doesn't matter. While in standby, the HT or high voltage that feeds the tubes is switched off. With no HT, there is no way for any of the tubes or the output transformer to conduct, so you cannot hurt the amp if nothing is disconnected if and only if the amp is in standby mode. However, it is paramount to the life of your amp that it is connected to a speaker at any time the amp is able to fully operate!
The whole MUST have the SAME ohm rating of speaker cab as the tap set on your amp is literally only 1/2 true. The ohms tap on a transformer is like a gear in a car's transmission. Now let's imagine that the speaker is actually the tire size of the car. If you put a giant tire on the car and don't change the gearing in the transmission, the car will be hesitant to move and you could burn out the transmission's clutch or even the engine. Conversely, if you put a teeny tiny tire on the car, the transmission and engine will be spinning like crazy to make the car move at an acceptable speed. Same sort of thing for an output transformer and a speaker load. The output tap is the gear and the speaker is the tire. It is generally ok at nominal volume levels to be one impedance tap off from the speaker load. I.E. An 8ohm speaker into the 16ohm or 4ohm tap. Things change when trying to run the amplifier full tilt. A 16ohm speaker is a large tire and a 4ohm speaker is a small tire. A small tire ( 4ohm ) on the gearing for a big tire ( 16ohm tap ) will cause the OT to overheat because the amps engine ( tubes ) will be trying to conduct all the power that they can through the transformer with no resistance. Now if you have a 16ohm speaker on the 4ohm tap, the speaker is presenting a very high resistance to movement ( due to electrical conversion ), which loads up the transmission and engine ( amp ) if you will. This causes electrons to build up on the plates of the tubes, and excess energy is stored in the OT. If enough of this energy gets stored, the build-up of electrons in the winding of the transformer can break through the wire's insulation and cause arcing, or the tubes plates will simply melt. This takes a fair amount of effort though, the build-up of electrons on the plates and in the transformer takes time to build up, and if you stop playing, this unloads things and the charge will dissipate. So going from a 4ohm tap into a 16ohm speaker will greatly reduce output, and could eventually cause the tubes and transformer to burn up, but it takes a fair amount of work for that to occur. BUT going from a 16ohm tap into a 4ohm speaker means the amp will be trying to produce all the power it can all the time unimpeded, this is a problem as it will eventually cause the OT to heat up and melt. Another way to look at it is the amount of potential conduction. The less impedance the OT see's, the more it will try and conduct, this causes more heat and may eventually burn up. Loading the OT up with a higher impedance reduces the amount of conduction, but causes a build-up of electrons that can eventually cause arcing or melting of the tubes plates. You can be off either way by one tap safely, for example, a 4ohm speaker into the 8ohm tap, or a 16ohm speaker into the 8ohm tap, or an 8phm speaker into either a 4 or 16ohm tap. So long as you are within one tap difference from your speaker load, you should have no problems at nominal levels. I do suggest though, that if you plan on diming the amp, that you use the same speaker load as the tap that you lug into.
The good old standby switch. It does serve a purpose, but it is more for servicing than for playing under normal use. It is nothing more than a mute switch. It just so happens that instead of shunting the input signal to ground, it cuts the HT voltage from the tubes. If there is no HT voltage on the tubes, they cannot conduct, but there is still 300v to as much as 600v waiting to instantly turn the tubes on when the switch is flipped. Depending on how your standby switch is wired, it can allow a pop as you suddenly snap the HT voltage into all the tubes. The idea that having the standby switch on when you turn the amp on allows all the HT voltage to sit on the plate of the tube which can cause arcing is not very true except in amps that have cathode follower stages. The tubes can't conduct if the cathode is not heated, so there is little to worry about with voltage on the plates. In a cathode follower stage, the HT is actually placed on the grid of that tube! This can allow arcing to occur but is generally of little fear with healthy tubes. As tubes get older and worn, they become gassy or have a breakdown of the cathode element, which can allow paths for arcing to occur, once it happens once, it becomes more and more prevalent. By that point in time, it is necessary to replace the tube anyway. If you have ANY amp with a tube rectifier, you truly have no need for a standby switch. A tube rectifier takes time to " warm-up " and it won't conduct until it is ready, which means that by then, the other tubes will also be ready. In a solid-state rectified amp, the HT voltage is pretty much instantly ready to go, but there is generally a bleed resistor that discharges the capacitors while not in use. So when you turn on a solid-state rectified amp, the capacitors eat up that initial shock of voltage as they charge up. The inline resistors for the different power nodes also slow the shock of that sudden inrush of current. So while leaving the standby switch on when you turn on a solid-state rectified amp will allow the HT to go to the tubes before they are able to conduct, it is a soft start at least. For all intents and purposes, the standby switch is nothing more than a mute switch.
The standby switch is also part of the cathode poisoning myth. Which is only partially true again. The idea is that if you leave the standby off, with the amp on for prolonged periods of time, the cathode will build a high resistance layer on the cathode limiting its ability to boil off electrons. A tube is like a light bulb, it has a life expectancy. The more the tube is on, the closer to death that tube will be. A NOS tube, even used, is only still good because it hasn't gone gassy and has so little use it still has some life left in it. So if you leave an amp on whether the standby switch is on or not, the tubes are then on, slowly but surely sucking the life out of them. you cannot expect a tube to last 1 million hours just because you NEVER had the standby switch off while the amp was on. A tube's life is counted in hours. The more it is on, the closer you are to its last hour. It is certainly not suggested to leave an amp on for prolonged periods of time with or without the standby on. This is one very good reason that a standby switch is destructive. You remembered to hit the standby switch, but not the power switch. Imagine if there was only just a power switch to turn off and no extra step required to " shut down " the amp... The standby switch doesn't help with anything other than muting the amp. A Boss TU3 does that too.
Another issue is BIAS. For example changing the power tubes means having someone check or regulate the voltage to the tubes-but many amps are self biasing..in my case a Vox AC10 C1 ...you just change the tubes, However people still advise taking it to a tech
NOGE
New member
Yep. Always use the standby switch on old Marshalls. I bet the coupling capacitors in front of the output tubes have gone leaky. These see too much voltage as the amp warms up if the standby switch is not used.
Then the amp tech was not worth his salt. It was not because of the standby switch ( used or unused ) that parts failed. Parts go bad over time, period. The use of the standby switch is really only there to use for maintenance.
A capacitor has a voltage rating and provided it isn't exceeded, should last several thousands of hours in use. In fact, using it more often than not is better than not using it at all. Most all of the popular Marshall amps use SS rectifiers. All this means is that when you hit the power switch, there is no warm-up time before the HT ( high voltage ) is ready to power things. The standby switch is located usually between the rectifier and the first node of the power supply. So if the standby switch is off when you start the amp NONE of the power rail is actually energized. This means when you bring the amp out of standby the first time, you are still shocking all the capacitors and subsequent parts with a voltage spike. Many modern amps use a power supply bleeder circuit to bleed the power supply rail of its voltage while not in use so that the amp is not sitting there with 400 volts ready to zap you when you open it up ( if you need to do so ). When placed in standby, this circuit will bleed voltage off the power supply even while the amp is on. So every time you hit the standby switch, you are zapping the power supply with a sudden voltage spike. The standby switch is nothing more than a mute switch, that is it.
The capacitors can handle a very short over-voltage, and the likely spike from the amp being turned on will probably not exceed that voltage by much if at all. The number of capacitors and resistors will easily manage the spike. And of course, we are also assuming that there is even a spike to begin with. Again, the standby switch was a thing that Fender popularized and was really only for amp troubleshooting.
My best guess with that amp is that the screen grid resistors took a dump. These resistors are used to help control the flow of electrons in the power tube. These resistors are designed to protect the tube from literal meltdown if the tube for any reason shorts out. If the tube shorts out, it will often take the screen grid resistor with it. The hope is that the amp's fuse will blow before the screen grid resistor fails completely allowing the tube to actually go into full meltdown, taking the output transformer and other things with it in glorious form. Most people just replace the fuse and think all is good again. Not so if the screen grid resistor is bad. If the resistor is shorted or way off of spec, it will cause the fuse to blow over and over again. The issue is that the screen grid resistors don't often look burned, mainly because they did their job and caused a situation where the fuse of the amp should blow. You have to actually measure the screen grid resistor to be sure. Secondly, you have to find out why the screen grid resistor failed? If the tube went bad and shorted, that will do it, but the tube may not have shorted permanently, so it will appear fine; until it's not again. This will then take out another screen grid resistor and fuse.
Most things about tube amplifiers are myth, legend, and old wives' tales. When you break them down to the bare essentials, they are very simple, very safe, and very reliable to just about any type of crap we can do to them. Like a TV that suddenly doesn't have a working display anymore, things just go bad. Older amps with lots of use are just that much closer to death every day. A 20+ year old amplifier is pretty common these days, so it is not surprising that one will fail every now and again. Most tube amps are pretty easy to repair if the tech has half a brain. But many techs just don't have EE degrees and rely on the super obvious and easily seen issues to guide them. Then there is the money..... If it isn't quick and easy enough, it is sometimes easier to take your bench fee and say it's a boat anchor.
I wouldn't say that
Otherwise, there wouldn't be such a market for overpriced effect pedals and whatnot. The Roland JC120 is a good example of an amp that honestly does its thing and does it very well. The Ibanez Tone Blaster amp is actually pretty decent sounding as well considering its price and lineage. And let's not forget the success of Randall. These are a couple of options that I think dollar for dollar would beat a tube amp at the one thing they do well.
Demanic
PenultimateTone Member
I was saying that it was a myth.I wouldn't say that
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A lot of detail there !; you seem to know your electronics On the issue of tubes & power tubes in particular , people talk of matching tubes....if a pair of power tubes , which will be of the same type and if they are also the same brand , then all those tubes will be identical..why the need to 'match'. Voltage bias is another one. Many amps don't need any voltage regulation when fitting new output tubes..(self bias ) just change them but some suppliers will still say this needs to be done' to avoid early failure ' The main thing is that the quality of new tubes is maintained...some suggest today's tubes fade sooner than was the case in the past.....
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That's why I put the wink. I got you.
As to matched tube sets, there are a couple of different mindsets about it. I don't think they need to be matched to a high degree, just close or within reason. You don't want one tube that is hot near the verge of red plating while the other is as cold as Antarctica. Spending the extra money for a matched set is kind of pointless when you consider one reason why the tubes no matter how well-matched they are will still not bias exactly the same.
The Output Transformer ( OT ) has a brown and a blue primary that connects to the tubes. If you measure each one's impedance relative to the center tap, you will almost always have a mismatched impedance. Albeit small, that mismatch in impedance does affect the bias of the tubes. If you have tubes that are slightly off in bias, one thing you can try doing to even them out a little more is to swap the positions of the tubes. A cold tube on a hotter OT primary side will draw the dissipation of that tube/s up. This can help even the biasing out.
Then there is whether or not a perfectly matched biasing of the tubes actually results in a better sound? Some say yes, others say no. One camp believes the perfect biasing of all tubes so that they all perform the same will result in the most accurate and pleasing sound. This is probably VERY true in Hi-Fi and music reproduction situations where having as linear of a power amp is ideal. For guitarists who desire the harmonics and distortion, having a slight mismatch is perhaps more ideal. If they are within a few milliamps of each other, it should be of no worry. When they are 10 milliamps or more apart, then you should really consider getting tubes that are closer in performance to one another.
Cathode bias and fixed bias are two different topologies that result in different power amp operations that suit certain types of music and or output ( wattage ) needs better. Cathode bias is great for a more loose, mushy, or blooming type of amp sound, it is also good for biasing tubes closer to class A operation because the tubes will automatically bias colder as crank the amp up. Its downside is that it doesn't produce as high a wattage as fixed bias does and for some musical genres, the instability ( moving bias range ) makes the amp hard to control. Cathode biased amps are not self-biasing the way many think they are. The bias is just set so that a large range of tube performances can be used and they will work. If you place a set of 6L6's in an amp biased for 6V6's, you don't suddenly get the added output power of the 6L6 tubes. The large cathode resistor ( that biases the tubes ) will regulate the total dissipation in conjunction with the properties of the tube ( internal resistances and characteristics ). If you place a tube that is really hot in the amp, it could actually red plate and be over biased. Cathode biasing doesn't self adjust as much as it is set to be within a safe range of a multitude of tubes. Because the amp will reduce bias as it is played louder, this allows a slightly hotter idle bias setting.
Fixed bias amps require a more sophisticated biasing circuit that controls the bias of the tube much better ( more stable ). It also allows for a higher wattage to be dissipated. One of its other features is that it has less sag, is more articulate, and due to its stability, allows you to control the amp a little better at higher volumes ( transition between distortion and clean with your playing in conjunction with the negative feedback presence loop ). The downside to fixed bias is that each set of tubes NEEDS to be biased to ensure they are biased optimally for tone and performance. Another downside is that as you work the amp hard, the sag in the power supply will pull the bias voltage up ( less negative) increasing the bias of the tubes. How you ask does it go less negative? The negative voltage is created by tapping off a common power supply. The cathode of the tube is connected to the ground and the grid ( which is connected to the negative voltage supply ) is set to be negative in relation to the cathode. If the power supply sags, the negative voltage goes less negative ( as its relationship to ground is reduced ) which drags the bias up hotter. Read it like this: The negative voltage is relative to ground and is created from the power supply. If the power supply reduces in voltage ( sags ), then the negative voltage of the bias circuit is then reduced as well, becoming less negative in relation to ground. Less negative means that the bias supply drifts closer to ground potential or 0 volts. I.E. if you were set at -45volts at idle, when the amp is played hard enough, that voltage may drift less negative to say -40volts. This is why you have to set the bias to within an " ideal " range, usually around 70% of total dissipation. Playing the amp will draw the bias up hotter and potentially into red plating. This is why you want to play the amp at performance level for a while after setting the bias, it allows you to see if you are in a safe operating range. No red plating while playing at full blast, all is good.
As you can see, if the tubes are grossly mismatched, it can cause biasing issues at performance level, but if they are relatively close, those differences are of little worry and may actually increase the harmonic character of the amp. I do believe it is prudent to buy tubes in sets that are sold as relative performance matches. Most all sets these days are. If you buy a set of 4 EH 6L6's, even if they don't say they are a matched set, they will be sold as a relatively close performing set of tubes. If they sold you sets that were of grossly different performing characteristics, no one would buy them anymore. The only time I would worry about gross mismatch is if you have two different brands and the data info of the tubes shows they are of obviously different performance ranges. A 6L6G and a 6L6GC are not the same tubes and while they will both work, they may be different enough to have issues. If you replace one tube, you may as well replace them all. Old and worn tubes do perform differently than new, fresh tubes. I think modern tubes of today are made just as well as they were 50+ years ago. What makes a 50+ year old tube so great today? Well, you could say that the fact it has sat around for 50+ years and didn't die, is a good indication that it is a good tube, regardless of how it was made. Not all the tubes that sat in warehouses since then have faired as well. It's the same for modern tubes. Not all of them are gems, but the majority of them are perfectly fine and perform to the same if not better standards of tubes made 50+ years ago.
Hi...Just wanted to pick up on one thing you said about Cathode bias ..you say the output to the tubes actually diminishes slightly as you play through the amp ? in other words tubes get slightly les hot. I know of people (online) who change the value of the cathode resistor in class A amps -(higher value) in order to reduce the voltage input to the tubes as according to them, the bias is often set too hot at the factory to impress people when they try an amp out in a guitar store !! not sure whether this is needed and in the case of single channel amps may mean losing the amount of distortion -gain, you can get from the amp....
Not exactly. Let me explain. Cathode bias allows you to bias the tubes closer to class-A operation at idle ( amp not producing sound ) because as you crank it up louder and louder, the bias will drift downward deeper into class A/B operation as you play. As soon as you stop playing, the bias will drift back up. It is kind of like compression. The drift in bias is nearly instant and is dependent upon how much current the tubes are drawing. This is why cathode bias amps are harder to get into that magic spot when playing at higher volumes, the bias will drift when you play, making the sweet spot a moving target. Cathode bias amps are really good at two things, low volume playing where the bias doesn't shift, or full bore where the squish, compression, and mushiness of the bias shift is part of the sound you are going for. The bias in cathode bias amps is still set to a safe range, perhaps 90% of full dissipation so that replacement of tubes doesn't require a re-bias. Again, different grades of tubes, may or may not work with a bias set to make a low grade ( read as a colder tube ) tubes bias at 90+%. If you put in a higher grade ( hotter tube ) it could be biased too hot in that case? Cathode bias amps have no requirement to be biased near class-A though, you can still bias them to standard class A/B operation, you just have to bias them hotter than perhaps 70% of dissipation because it could drift downward into class-C at full volume.
Most manufacturers actually bias the amps pretty cold from the factory. This is to increase tube life and since most people don't know how to bias an amp, if you replace tubes with a similar grade to the factory tube ( like Mesa's system ) you are likely to still be in a safe range without having to adjust anything. This is also a better practice than trying to run the bias as hot as possible. As the tube degrades over time it will be more and more prone to red plating. If it red plates and you don't catch it in time, you literally can melt the tube, and subsequently the output transformers and more with it!
Furthermore, bias setting hot vs cold and quality of tone is a HUGE myth. Every tube set has a bias point where its tone will shift, whether that point is too hot for safety or really cold is the question mark? With fixed bias amps, you have to be very careful with how hot you bias them. If you bias them really hot at idle ( not producing sound ) they will shift bias hotter when you have it cranked up. Just like a cathode biased amp, it does shift, it is just not as dramatic because the bias is more stable and tracks with the rest of the amp, masking it a bit. The negative feedback ( presence ) loop that exists in most fixed bias amps also helps to mask that bias shift a little. You should set the bias for the best sound at the coldest bias setting you can get away with. A hot biased set of tubes is not guaranteed to sound good. Some people actually like the sound and fizz that biasing tubes cold produces. When biased too cold, you get crossover distortion which is actually nonlinear distortion from the amplifier in relation to the preamp. Beauty is in the ear of the listener you could say. I bias for the best tone at the lowest bias. This tends to land right around the 70% of dissipation mark, not always, but close.
I guess we can attack another myth while we are at it. Most people believe running their amp at high volumes rounds them out because you are getting power tube distortion and making the power tubes work. NOT TRUE. In 90% of amps, the power tubes are the last tubes to start breaking up, and even then, you have to be at ear blistering volume. The change in tone when you start cranking the amp up is the feedback loop actually doing what it was meant to do. At lower volumes, the feedback loop isn't sending enough signal back to be very effective at nulling the high-frequency content. If your amp sounds like absolute dookey at bedroom levels, but sounds like a monster at stage volume, it is because of the feedback loop. If you were to eliminate the feedback loop altogether, your amp would sound like dookey at ALL volume levels. The sound of your amp at bedroom levels ( the thin, ball-less, wirey, anemic sound ) is actually the sound of the amp, the feedback loop is what makes that design work and sound the way it does at higher volume levels. Feedback loops are not a requirement for class A/B fixed bias amps, it is just the design 99% of vendors use. The presence feedback loop is designed to allow high-frequency signal to go back into the phase inverter. The loop is fed into the inverting side of the PI ( phase inverter ) where it will be out of phase, hence canceling out or nulling those frequencies to some degree from the output of the PI. What you hear is the result of the low pass filter this circuit creates when it has enough forward gain from the feedback loop to function. As you increase the presence, you are actually decreasing the signal fed into the PI. As you turn the amp up, it feeds more voltage into the feedback loop, increasing its effectiveness, so as you turn the volume up, you generally have to reduce this feedback voltage to allow more highs. This is why at lower volumes you turn the presence down to reduce the highs, you are actually allowing more voltage into the PI in order to make the presence loop function. This is why you tend to need to adjust the presence knob depending on the volume of the amp.
The negative feedback loop is really more designed to even out the frequency response of the power amp section. There is no requirement to have a low pass or high pass in the loop. The resonance control is precisely the same thing as a presence control except instead of being a low pass circuit, it is instead a high pass circuit. The circuit is used to impede certain frequencies from going into the PI. Since those frequencies are not fed into the PI, it will not be canceled out or reduced. The feedback loop is a great way to have tone shaping very late in the circuit. As we now know, it is not without its downsides. It is almost always either a low pass or high pass circuit and its effectiveness is dependent upon the volume of the amp. Another interesting tidbit is that the feedback loop completely falls apart when the power tubes distort.
When the power tubes distort, the feedback loop is sending back a signal that doesn't correlate at all with what the PI is sending. I.E. it is nonlinear. When there is nonlinearity, the effectiveness of the feedback circuit pretty much fails completely. The resultant sound is still warm and super crunchy because you are getting ALL KINDS of distortion and harmonics from the full frequency spectrum of the entire amp, and the compression from the distortion helps to round things out a little more. The feedback loop is still doing something, but it is not doing it linearly ( it can only reduce what is near a 1:1 ratio of what went out vs. what came in ). I would will a pretty good bet that most people running even 50-watt amps never get the amp loud enough to achieve significant power-tube distortion. A way to test this theory is to run your amp at the level you think it is about to blow up and adjust the presence knob. If there is little or no significant change, you may be getting significant power tube breakup, if the presence knob still seems to do what it supposed to do, then you don't have power tube breakup. Also helping to warm up the sound at higher volumes is the tone changing from the bias shift. As you bias hotter, typically, the sound will round out. So as you get near power tube saturation, you are probably also starting to shift the bias. This will warm things up a little if you initially had a colder bias that was outside of this range.
Going back to factory biased amps, you can see how that last sentence above can be real. Most factories bias their amps on the cold side. So as you crank them up, the shift in bias can warm them up a little. As you can imagine, if the tube is already in its sweet spot, this phenomenon won't be as apparent. You may also notice that in other posts and early in this post I mentioned closer to class-A operation for cathode biased amps. Many manufacturers try and sell class-A as being something. The problem is that most people don't know what class-A, class- A/B or class-C is. The only amps truly capable of running in true class-A operation are single-ended amps such as a Fender Champ, or the THD Bivalve. 99% of other amps that have multiples of power tubes such as the Vox AC30 will say they are class-A but are absolutely not. They can be biased closer to class-A operation at idle conditions, but they are not class-A amps. As with most things in the music industry, it revolves around myth, legend, and snake oil.
Most manufacturers actually bias the amps pretty cold from the factory. This is to increase tube life and since most people don't know how to bias an amp, if you replace tubes with a similar grade to the factory tube ( like Mesa's system ) you are likely to still be in a safe range without having to adjust anything. This is also a better practice than trying to run the bias as hot as possible. As the tube degrades over time it will be more and more prone to red plating. If it red plates and you don't catch it in time, you literally can melt the tube, and subsequently the output transformers and more with it!
Furthermore, bias setting hot vs cold and quality of tone is a HUGE myth. Every tube set has a bias point where its tone will shift, whether that point is too hot for safety or really cold is the question mark? With fixed bias amps, you have to be very careful with how hot you bias them. If you bias them really hot at idle ( not producing sound ) they will shift bias hotter when you have it cranked up. Just like a cathode biased amp, it does shift, it is just not as dramatic because the bias is more stable and tracks with the rest of the amp, masking it a bit. The negative feedback ( presence ) loop that exists in most fixed bias amps also helps to mask that bias shift a little. You should set the bias for the best sound at the coldest bias setting you can get away with. A hot biased set of tubes is not guaranteed to sound good. Some people actually like the sound and fizz that biasing tubes cold produces. When biased too cold, you get crossover distortion which is actually nonlinear distortion from the amplifier in relation to the preamp. Beauty is in the ear of the listener you could say. I bias for the best tone at the lowest bias. This tends to land right around the 70% of dissipation mark, not always, but close.
I guess we can attack another myth while we are at it. Most people believe running their amp at high volumes rounds them out because you are getting power tube distortion and making the power tubes work. NOT TRUE. In 90% of amps, the power tubes are the last tubes to start breaking up, and even then, you have to be at ear blistering volume. The change in tone when you start cranking the amp up is the feedback loop actually doing what it was meant to do. At lower volumes, the feedback loop isn't sending enough signal back to be very effective at nulling the high-frequency content. If your amp sounds like absolute dookey at bedroom levels, but sounds like a monster at stage volume, it is because of the feedback loop. If you were to eliminate the feedback loop altogether, your amp would sound like dookey at ALL volume levels. The sound of your amp at bedroom levels ( the thin, ball-less, wirey, anemic sound ) is actually the sound of the amp, the feedback loop is what makes that design work and sound the way it does at higher volume levels. Feedback loops are not a requirement for class A/B fixed bias amps, it is just the design 99% of vendors use. The presence feedback loop is designed to allow high-frequency signal to go back into the phase inverter. The loop is fed into the inverting side of the PI ( phase inverter ) where it will be out of phase, hence canceling out or nulling those frequencies to some degree from the output of the PI. What you hear is the result of the low pass filter this circuit creates when it has enough forward gain from the feedback loop to function. As you increase the presence, you are actually decreasing the signal fed into the PI. As you turn the amp up, it feeds more voltage into the feedback loop, increasing its effectiveness, so as you turn the volume up, you generally have to reduce this feedback voltage to allow more highs. This is why at lower volumes you turn the presence down to reduce the highs, you are actually allowing more voltage into the PI in order to make the presence loop function. This is why you tend to need to adjust the presence knob depending on the volume of the amp.
The negative feedback loop is really more designed to even out the frequency response of the power amp section. There is no requirement to have a low pass or high pass in the loop. The resonance control is precisely the same thing as a presence control except instead of being a low pass circuit, it is instead a high pass circuit. The circuit is used to impede certain frequencies from going into the PI. Since those frequencies are not fed into the PI, it will not be canceled out or reduced. The feedback loop is a great way to have tone shaping very late in the circuit. As we now know, it is not without its downsides. It is almost always either a low pass or high pass circuit and its effectiveness is dependent upon the volume of the amp. Another interesting tidbit is that the feedback loop completely falls apart when the power tubes distort.
When the power tubes distort, the feedback loop is sending back a signal that doesn't correlate at all with what the PI is sending. I.E. it is nonlinear. When there is nonlinearity, the effectiveness of the feedback circuit pretty much fails completely. The resultant sound is still warm and super crunchy because you are getting ALL KINDS of distortion and harmonics from the full frequency spectrum of the entire amp, and the compression from the distortion helps to round things out a little more. The feedback loop is still doing something, but it is not doing it linearly ( it can only reduce what is near a 1:1 ratio of what went out vs. what came in ). I would will a pretty good bet that most people running even 50-watt amps never get the amp loud enough to achieve significant power-tube distortion. A way to test this theory is to run your amp at the level you think it is about to blow up and adjust the presence knob. If there is little or no significant change, you may be getting significant power tube breakup, if the presence knob still seems to do what it supposed to do, then you don't have power tube breakup. Also helping to warm up the sound at higher volumes is the tone changing from the bias shift. As you bias hotter, typically, the sound will round out. So as you get near power tube saturation, you are probably also starting to shift the bias. This will warm things up a little if you initially had a colder bias that was outside of this range.
Going back to factory biased amps, you can see how that last sentence above can be real. Most factories bias their amps on the cold side. So as you crank them up, the shift in bias can warm them up a little. As you can imagine, if the tube is already in its sweet spot, this phenomenon won't be as apparent. You may also notice that in other posts and early in this post I mentioned closer to class-A operation for cathode biased amps. Many manufacturers try and sell class-A as being something. The problem is that most people don't know what class-A, class- A/B or class-C is. The only amps truly capable of running in true class-A operation are single-ended amps such as a Fender Champ, or the THD Bivalve. 99% of other amps that have multiples of power tubes such as the Vox AC30 will say they are class-A but are absolutely not. They can be biased closer to class-A operation at idle conditions, but they are not class-A amps. As with most things in the music industry, it revolves around myth, legend, and snake oil.
Interesting...my tube amp happens to be a Vox AC10 C1 which is single channel , gain & master volume. two preamp ; two power. I've modified the back panel so the output tubes are exposed. the tubes certainly feel very warm (no orange glow though ) -but then I cant compare it to anything else as It's my first tube amp. You suggest most factories bias amps on the cold side, which would make sense as if set too hot at idle , too much voltage going to the valves ,then that would lead to reliability issues ... I could at some some get a tech to check it over, but the sound and tone have remained very stable on this...
A very good and safe mantra to live by is that if it isn't broke, don't try and fix it. I encourage people to learn about their tools and toys, but the need and drive to try and always improve what you have is a very expensive and possibly dangerous task. Biasing an amp for tone purposes yields very little return on the trouble of doing it. Cathode biased amps are more difficult to bias for tone because you MUST remove and install a large resistor. These resistors only come in increments and dialing it in to be between two values is difficult and very impractical ( they are big and tying multiples together will be very ugly and eat a lot of space ). With cathode bias, you sort of have to choose how hot you want to bias it, and if the resistors you have, get you close, that is it.
Adjusting a fixed bias amp for tone will be more dramatic ( read that as noticeable ), but it will NEVER take the amp from MEH to fuck yeah. It will do something, that is about it. Ideally, you want to get to the point where it just starts to change for the better, check your dissipation, and evaluate from there if you can go hotter or not. While I do not agree with the 70% of dissipation as a hard rule, I do agree that going much beyond that is dancing with trouble; It is very much " do so at your own risk ".
As to learn how to bias an amp, Rob Robinette: https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm has a great intro on a few ways to do it. I use the method that he prescribes ( the Output Transformer Resistance Method ) because it is both safer and more accurate than any other method that requires you to open the amp. If you have the bias probes that allow you to read the dissipation, that is great, it is close enough. However, that is only really good for fixed bias amps that have a dial you can turn to adjust the bias with. With a cathode bias amp, the only way to adjust bias is to remove resistors. So you have to be in the amp anyway. Since the OT Resistance Method is more accurate, it is just easier to learn that method and stick with it for all your biasing. Knowing the other methods doesn't hurt, but find one and stick to it. It needs to be second nature, not second-guessing.
With your AC10, it would seem to me that it is running fine. If it sounds good, it is good. While it may be on the cold side in biasing, heating that bias up may or may not improve tone? There is only one way to find out. Be careful if you do though. Being that sound and operation is stable, it would seem the amp is happy where it's at.
I feel that those looking to get a different sound from their amp should look at speakers instead. The speakers you have are probably 50%, perhaps even 75% of the core sound your amp has. If you change to a different speaker, the resultant sound WILL be different. And not just a little bit, a lot a bit. Now if you are happy with the sound you have, you just wish is it was a little more this, or a little more that, then and only then is opening the amp up to do some component swapping a reasonable idea. If you know the circuit of your amp well, there are usually a couple of different ways to achieve some tonal task. Especially if the need is slight or reasonable. You cannot make an AC30 sound like a Mesa Dual Rec, but you may be able to make your AC30 sound slightly less like an AC30. Read Rob Robinette's pages and you should be able to grasp pretty quickly what you can and can't do.
Adjusting a fixed bias amp for tone will be more dramatic ( read that as noticeable ), but it will NEVER take the amp from MEH to fuck yeah. It will do something, that is about it. Ideally, you want to get to the point where it just starts to change for the better, check your dissipation, and evaluate from there if you can go hotter or not. While I do not agree with the 70% of dissipation as a hard rule, I do agree that going much beyond that is dancing with trouble; It is very much " do so at your own risk ".
As to learn how to bias an amp, Rob Robinette: https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm has a great intro on a few ways to do it. I use the method that he prescribes ( the Output Transformer Resistance Method ) because it is both safer and more accurate than any other method that requires you to open the amp. If you have the bias probes that allow you to read the dissipation, that is great, it is close enough. However, that is only really good for fixed bias amps that have a dial you can turn to adjust the bias with. With a cathode bias amp, the only way to adjust bias is to remove resistors. So you have to be in the amp anyway. Since the OT Resistance Method is more accurate, it is just easier to learn that method and stick with it for all your biasing. Knowing the other methods doesn't hurt, but find one and stick to it. It needs to be second nature, not second-guessing.
With your AC10, it would seem to me that it is running fine. If it sounds good, it is good. While it may be on the cold side in biasing, heating that bias up may or may not improve tone? There is only one way to find out. Be careful if you do though. Being that sound and operation is stable, it would seem the amp is happy where it's at.
I feel that those looking to get a different sound from their amp should look at speakers instead. The speakers you have are probably 50%, perhaps even 75% of the core sound your amp has. If you change to a different speaker, the resultant sound WILL be different. And not just a little bit, a lot a bit. Now if you are happy with the sound you have, you just wish is it was a little more this, or a little more that, then and only then is opening the amp up to do some component swapping a reasonable idea. If you know the circuit of your amp well, there are usually a couple of different ways to achieve some tonal task. Especially if the need is slight or reasonable. You cannot make an AC30 sound like a Mesa Dual Rec, but you may be able to make your AC30 sound slightly less like an AC30. Read Rob Robinette's pages and you should be able to grasp pretty quickly what you can and can't do.
This isn't really correct -- one of the high-gain amps that's most notorious for sounding very different depending on the MV is the Dual Rectifier in Modern mode, which has no NFB at all. Also, you have the role of the presence knob backwards -- the presence knob is how much of the LPF'd negative feedback is being dumped to ground, meaning as you turn it up, less high-end gets nulled and thus the PI is hit with more treble (and more signal in general).
Amps sounding different at high volume has a lot of causes -- sometimes it's the design of the amp itself (even if you're not distorting the big bottles, PI distortion is a critical part of a lot of tones, hence the popularity of PPIMV mods in the '80s), cabinet involvement is a *huge* factor, and also Fletcher Munsen curves mean that even if it sounded exactly the same except quieter or louder, you'd hear it differently.
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