Tube Amp myths...

[Cynical]

The PI is a part of the power section; if the PI distorts, you've got power section distortion. Even on an amp like a Plexi, the power section distortion is nearly all from the PI, not the big-bottle tubes (which is why you can get good sounding distortion out of those amps with KT88s; anyone who has ever heard serious clipping of a KT88 knows it's not a sound you want, but most classic era Marshalls that were sold in the U.S. came with KT88s, and they sounded fine).

3 on an SLO isn't far off from about 2.2 on a 5150. Remember, Soldano knobs go to 11 (and are mounted slightly crooked, so you can't trust "'o clocks" either, because the half-way point is slightly past noon), so it's 3/11 (0.27) vs 2.2/10 (0.22). Very minor difference.

Saying "distortion is distortion" is a big oversimplification. There's no one on this forum that can't hear the difference between an almost square-wave style hard clipping or soft clipping, or we'd all just be using Boss MT-2s instead of tube amps. Most people here could tell the difference between symmetrical vs. asymmetrical clipping, even if they didn't know which was which. The "purr" of a Recto or SLO is immediately distinguishable from the "angrier" distortion of a boosted Marshall -- the texture is just obviously different because of how the waveform is distorted, in ways that aren't just the relative EQ of the amps. Then there's the spectre of blocking distortion, which *always* sounds bad in every case.

[NOGE]

I'm still going to use the standby switch on my JMP . I just don't like the idea of 550V across a 400V capacitor until the cathodes warm up.

[Ewizard]

The reason I say that the speaker & cab is a fixed variable is that its impedance curve et all is set. The impedance curve doesn't change when you plug a different amp into it. The NFL as you say does in fact help even out that interaction and also does reduce distortion in the PI, again as you say. But if you plug an amp that doesn't have a feedback loop, the speaker/cab will do what the speaker/cab will do; the resultant sound is the some of all parts. In either case, one scenario has what is essentially a late circuit tone control, while the other doesn't. The speaker/cab doesn't suddenly change characteristics when a different amp was plugged into it.

The SLO MV video doesn't show where that distortion is created. The SLO is still a pre-phase MV, so that extra grit is very possibly coming from the PI. When he went to 7 on the dial there was a shift in tone, which could be the point where the power tubes actually start to distort breaking down the feedback loop. Without scoping the signal coming out of the PI and the signal coming out of the power tubes, we cannot definitively say where that extra grit is coming from.

As to cone breakup and making the cone move, if you present the speaker with enough low end it will move regardless of how loud it really appears to be. As I mentioned before, most guitar cabs are not made with any real Theil Small parameters being considered. When 4x12 cabs were originally designed, it was sized to what could be gotten out of a single 4x8' sheet of plywood. Don't even get me started with open back cabs. A speaker will break up sooner in an open back design, closed-back designs do produce more bass due to their sealed design and will have less excursion. Most guitar speakers have very little excursion or X-max limits, and most cabs they are placed within are too big to control the X-Max of the speaker fully. The resonance of the cab is mostly part of its design. There are in most guitar cabs 4 surfaces that are adjacent to one another making them standing wave generating machines! Getting the speaker loud enough to where you can hear those standing waves over the speaker is not hard to do. This is not necessarily cone breakup yet. A term you may or may not have heard of is cone cry. It is a VERY evident distortion that is independent of the amp that sounds more like a buzz. The only real way to find out if you truly have cone breakup is to use measurement software and see where the speaker goes non-linear. Although you will know when you have it, it sounds more like a crunch that occurs over the top of the core sound. You have to be surprisingly loud to get any of those artifacts to show up. Which one happens first is anyone's guess. While I have not A/B'd an SLO to a 5150, if the so-called point of perfection is around 2.2 on the MV of a 5150, then the MV level of 3 on the SLO is probably of little use then. I think what these videos show is that guitarists are too loud.

I think distortion is distortion, objectively speaking anyway. If the signal out is non-linear to what went in, that is the absolute description of distortion. How you get it is what makes a clean amp tuned up to 11 and a gain banger like the SLO with the MV at 1 different. They are both distorting in one way or another. I have a different outlook on sound though. I am a sound engineer by trade and have been playing guitar and performing for the past 27 or so years. I started on guitar and after becoming a sound engineer 17 years ago I learned real quick how problematic musicians were to themselves. My idea of sound is more about linearity, naturalness, and realism. I do what I can to portray a musician's character and sound to the rest of the world by augmenting it with my talent and skills so that the whole band is heard as a single organism.

[Cynical]

You were right in saying the presence knob is a low pass filter. It lets low frequencies pass while dumping high frequencies to ground, so that those high frequencies don't cancel out frequencies to one side of the phase inverter. Remember, whatever frequencies are present in the NFL are the frequencies that won't be in the final sound, so everything is reversed from what's intuitive. The NFL frequency response shouldn't be too interactive with volume on an amp with a traditional MV knob (which comes before the PI), because as you turn volume down, you cancel a smaller waveform going into the PI out with a smaller waveform from the power amp (the same is NOT true on an amp with a PPIMV, which is one of the major drawbacks of that design; with the PPIMV down, you have a very midrangey sound, precisely because the NFL isn't being effective in linearizing the sound, so the frequencies are overwhelmingly those that a guitar naturally produces, which are all in the midrange).

The reason a dual recto sounds so different with the MV up or down in modern mode is precisely because of its lack of an NFL. One of the side effects of an NFL is that it makes it harder to overdrive the PI because as you apply more volume, you get more negative feedback. If you've got access to a Marshall 2203 or 2204 and a place to play it loud, you can see this in action easily -- play it with the MV dimed, preamp gain at about noon, and presence knob all the way off, and see how little distortion you get. Then dime the presence knob, and try playing it. You'll find that you get a ton more distortion and saturation. By removing the NFL in modern mode, the recto gets a bunch of overdrive on that stage, giving it a lot more distortion (and volume!).

There's a lot more going on with cabs and volume than just speaker distortion. As you turn up more, long before the point where the speakers themselves distort, the acoustics of the cab will produce more and more low end, especially if it's a closed back. The lack of low-end at low volumes you describe is largely due to this, not the amp-head itself; you can use an attenuator to control your volume, and you'll still notice the same lacking low-end as if you had turned the amp's volume down (this is why a lot of attenuators have bass-boost switches). Also, speaker cone excursion happens sooner than you might think -- on a 5150/6505 into a cab loaded with V30s, it happens on about 2 on the MV knob (you can easily verify this by shining a light through the grille cloth, a good video of this can be found here: https://www.youtube.com/watch?v=ko_EIHinv-o ). Furthermore, cabs aren't really just a passive box -- because their impedance is variable, they're interactive with the output section of the amp itself, meaning that you can't just say that it's a fixed variable. One of the purposes of an NFL is to isolate the amp's power section from the cabinet's impedance curve to a degree; an amp with stronger NFB will react to different cabs differently than an amp with weaker (or no) NFB.

Re: the importance of power distortion even on "modern high gain amps", here's a useful video: https://www.youtube.com/watch?v=AcWOf3DEvYs Notice how the SLO100, with its supposedly "clean power section", is already adding a ton of distortion when he turns it up to 3, a.k.a. "roughly band rehearsal volume".

Finally, the idea that "distortion is distortion" is waaaaaaaay wrong. You'll never get the sound of The Kink's "You Really Got Me" with the distortion from any tube head. Power distortion and pre-amp distortion have a different feel and sound -- again, you can verify this easily on a 2203 or 2204.

[Ewizard]

There is nothing particularly special about the design of the Dual Rec's power section. It is pretty much like 90% of other amps except for the feature of disconnecting the NFB in modern mode ( the amp does have an NFB that is engaged for the other modes ), which does still leave some components in the PI circuit. Not having messed with one enough to know how it acts, I can't or won't say why it does what you say it does. Obviously, not 100% of amps will react the same way, depending mostly on their topology. The Mesa DR does have stuff left in the PI circuit, so that could be a factor as to why it does what it does?

If you read what I said about how the feedback loop works, you will see I said exactly what you have. I did err in saying low pass filter, where what I meant was Hi-pass filter to allow the highs into the PI. I did iterate the function correctly with the exception of that. That was an oopsie.

The FM curves can cause the perception of an amps sound to be different at different volumes, but would work in the opposite direction to what most would complain about an amp. I would argue that most people with a 50 to 100-watt amp complain that at bedroom levels, it sounds thin, bright, and anemic. The " loudness " button that comes on 99% of consumer electronics is an FM curve eq implementation that boosts the highs and the lows to create an equal perception of sound at lower volumes. The DJ smile if you will. So if your amps is too bright at bedroom levels, then it is because the amp is that bright, or you could say that it has very little low-frequency energy. Another consideration to make for the FM curves is that the fundamental frequencies for the guitar are just about equal to the higher frequencies of the guitar's harmonics. Meaning that if the amplifier was fairly linear, you would perceive the highs and lows about equal at bedroom levels. The equal loudness contours show us how much actual SPL at X frequency is required to hear them equally at X target SPL. This seems to support my theory that the feedback loop in guitar amps is there because, without it, they would sound less pleasant. Negative feedback loops are used in audio amplifiers for the purpose of making the power amp more linear. It helps even out the frequency response. Since the feedback loop requires forward gain to work effectively, it explains why at bedroom levels ( where you are dissipating 10ths of a watt perhaps ) the feedback loop is not as effective.

I do agree that the cabinet does have an effect on the whole of things, but that is a completely different subject. The reason I wouldn't clump it into tube amp myths is because it is a fixed variable. If you only have one cabinet and 5 amps, that cabinet is going to react the way it does regardless of which amp you plug into it. I feel that cone breakup occurs later than most people think it does. Of course, only a scientific test would weed that out. Despite the result, you can't really change that outcome. If you operate the amp loud enough to start creating PI distortion, Power tube distortion, and speaker distortion, then all you can truly say is that you have a LOT of distortion. If that is the sound you are going for, then how you got there is irrelevant.

[Cynical]

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.

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.

[Ewizard]

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 **** 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.

[Gold star]

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.

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...

[Ewizard]

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.

[Gold star]

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....

[Demanic]

Well, I guess that explains why my Picovalve, which I believe is cathode biased, never sounds quite as tight as I would like for modern metal. Though it does do a great classic and old school hard rock sound.

Sent from my SM-A115A using Tapatalk

[Ewizard]

I was saying that it was a myth.

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.

[Gold star]

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.

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.....

[Demanic]

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.

I was saying that it was a myth.

Sent from my SM-A115A using Tapatalk

[Ewizard]

Too deep into the thread to check, but has anyone mentioned this: They always sound better than solid state.?

Sent from my SM-A115A using Tapatalk

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.