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Old February 15th, 2013, 11:08 AM   #1 (permalink)
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Questions about power tube distortion

Hi All,
As a newcomer to tube amps, I could use some guidance/ reassurance...
My only valve amp is a 15 watt Laney Cub Head. The amp has a gain control and a master volume.

My understanding is that the "gain" control affects the amount of pre-amp gain/ dirt (pre-amp valves) and the master "volume" affects the volume/ drive from the power amp tubes. Right or wrong?

Why then, if I crack the master volume wide open (10 on the dial) and run the pre-amp "gain" control at 3 or less (on the dial) does the amp stay clean (using a US Std strat with the strat volume up full) - shouldn't I get "power tube distortion"?

Surely, if I set the amp up this way, as I raise the "gain" control,won't I just be hearing pre-amp distortion as the clean sound starts to break up?

Can I only get "power tube" distortion from a non-master volume amp?

As I said, I'm very new to tube amps so feel free to treat me as an idiot - I'd really like to understand this stuff and if I'm missing something, I'd love to learn..

Cheers

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Old February 15th, 2013, 11:18 AM   #2 (permalink)
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In most amps, the master volume does not create any distortion. It only controls the amount of signal that goes into the power output section.

The Gain knob controls how much signal gets to the second stage preamp.

If you turn up the gain really high, the second stage gets a really strong signal, which causes clipping, aka distortion.

the amp will also sound real loud, so you turn down the overall volume with the Master control.

if you want power-tube distortion, you have to turn the Master all the way up. But you will probably find that power-tube distortion is pretty subtle; in fact many of us think it's maybe more hype than fact.

I'm sure several others will soon chime in with more and better explanations.
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Old February 15th, 2013, 11:30 AM   #3 (permalink)
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Volume controls are subtractive. Running the master on full is the 'least amount of volume removal' possible. If this is still clean, it means you're not playing loud enough!
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Old February 15th, 2013, 11:47 AM   #4 (permalink)
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Quote:
Originally Posted by waparker4 View Post
Volume controls are subtractive. Running the master on full is the 'least amount of volume removal' possible. If this is still clean, it means you're not playing loud enough!
I didn't know that. So an amp is basically running full out whenever the power is on irregardless of the power going to the speakers?
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Old February 15th, 2013, 12:32 PM   #5 (permalink)
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the amp is full-on all the time. However, it doesn't amplify everything full-out all the time. It amplifies the signal that it receives.

The first pre-amp stage amplifies whatever it gets from the guitar. You can control that with the guitar's Volume knob. Most guitars don't have a strong enough signal to create any distortion in the first stage. Sometimes we put a stomp-box there to increase the signal to the point that it will over-drive the first tube and cause distortion.

The first stage usually puts out a pretty strong signal - more than enough to cause distortion in following stages. So there is a control - usually called Gain - that allows you to lower that signal if you want less distortion (or NO distortion) on the following stage.

Some amps might have a few extra stages, often with another Gain control, or more likely with the gain set internally through a voltage-divider network.

After the preamp, the signal usually goes through a phase inverter to split it in two; each half of the signal will feed one side of the power amp. After this stage, there is sometimes a Master volume to bring down the signal which is now quite strong.

The power output tubes are sitting there waiting - tuned up to full-blast. If they get a small signal, they will reproduce it quite cleanly. If they get a really huge signal, they will also distort some. The Master volume controls this.

There is a lot of interest in 'power-tube distortion'. But most power tubes were designed to avoid distortion, and most guitar amps don't really go too far into the power-tube distortion area.

I think that a lot of the 'power-tube distortion' talk is not really accurate, and that most people are simply hearing the sonic qualities of the power tubes, NOT necessarily distortion.
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Old February 15th, 2013, 01:53 PM   #6 (permalink)
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I'd agree that a lot of power tube distortion talk in larger amp designs isn't accurate and that a good deal of this mojo is happening at the phase inverter. I'd also say that single ended power tube distortion is a fact of life, there's not a whole lot of other places in a circuit like the 5f1 for that amount of distortion to come from.
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Old February 15th, 2013, 02:09 PM   #7 (permalink)
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"Why then, if I crack the master volume wide open (10 on the dial) and run the pre-amp "gain" control at 3 or less (on the dial) does the amp stay clean (using a US Std strat with the strat volume up full) - shouldn't I get "power tube distortion"? "

You arent going to get power tube distortion out of that situation, as the others have suggested. Real power tube distortion comes from bone-shaking volume situations, particular older style amps with no master volume.

I could be wrong about that, even that might be pre-amp tube distortion as well.
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Old February 15th, 2013, 02:44 PM   #8 (permalink)
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There is a degree of controversy on this topic, but try this.

1. Turn your master volume up all the way and adjust the volume with the gain control.

Be sure to turn your amp up to the point where it's playing loud.

2. Turn the gain control up all the way and then adjust the volume with the master volume.

Again, turn up the volume to where the amp is playing loud.

Do you hear any difference between the two methods? If so and you like one more than the other then use that one. What's important is you get a sound out of your amp that you like.

Distortion tone has many variables and among the things that have an impact are
pre-amp tubes, the phase inverter tube, power tubes, output transformer and the speakers. These things all interact with one another and in most every guitar amp they all contribute to the voice of the amp. One rarely hears distortion out of an amp from just one of these factors, so speaking about either pre-amp tube distortion or power amp distortion is sort of oversimplification of what is actually going on. There is some controversy over which parts of the chain of parts invloved here are responsible for the sound, (IE: just how much is coming from the transformer, power tubes, phase inverter and the speakers) but most people agree that this chain of parts, in total, has a major impact on the sound.

As a generalization, if one relies on the preamp stage for most of the distortion, you end up with a sort of buzzy or fizzy sound. Turn the amp up to where all stages of the amp are working and you hear usually hear a different sort of distortion. The interaction between the parts of the phase inverter-power tube-output transformer-speaker section of your amp starts to become a larger part of the amp's overall sound.

These are all just tools an amp designer uses to shape what comes out of the amp. A good tech can also tweek your amp to change the way it sounds.

As for your situation, the main thing is to experiment. Use the volume pot on your guitar, both the gain and master volume controls on the amp and adjust things ubntil you find out all the different sounds you can make by adjusting these variables and I imagine you'll find some sounds you didn't know your amp could make. Use your ears and not your eyes or expectations about what you should be hearing.

This is a cool analysis of what goes on in a guitar amp, Click on this by Billm. He doesnt talk about the power tubes role in all this but I think he knows what's going on inside a guitar amp as well as about anyone. it's not discussing your particular amp, but this is probably similar to what's going on in yours.
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Old February 15th, 2013, 03:06 PM   #9 (permalink)
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Some people feel that a PI will distort long before it pushes enough current to send a power tube into distortion, especially if the PI is a 12AX7, Y7 or 5751 (which cap out at about 1.5 mA).
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Old February 15th, 2013, 03:21 PM   #10 (permalink)
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Some people feel that a PI will distort long before it pushes enough current to send a power tube into distortion, especially if the PI is a 12AX7, Y7 or 5751 (which cap out at about 1.5 mA).
"Some people" don't know what they are talkinig about.

In a typical cascade of gain stages, it they are designed properly, and you don't have any MV nonsense in the way, the last stage will be the first one to hit the wall and distort.

In a typical RC coupled stage between the PI and the output, the PI does not "push" any current to the outputs. They are voltage controlled, and the grids will clamp the input drive voltage just as soon as the grids go slightly positive with respect to the cathodes.
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Old February 15th, 2013, 03:23 PM   #11 (permalink)
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Do the preceeding stages push current to the PI? Did you read the article? I ask because it's probably my understanding of the article that is in the wrong. I probably misstated something. I'm trying to get my head around these concepts as well.
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Old February 15th, 2013, 03:24 PM   #12 (permalink)
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John's got it, as usual. By the time the power section distorts, everything in front of it has been there awhile.
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Old February 15th, 2013, 05:01 PM   #13 (permalink)
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Do the preceeding stages push current to the PI? Did you read the article? I ask because it's probably my understanding of the article that is in the wrong. I probably misstated something. I'm trying to get my head around these concepts as well.
No. RC coupled voltage amplifiers do not "push current".

I did not read the article.
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Old February 15th, 2013, 05:12 PM   #14 (permalink)
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OK, I just read the article. Mr. Rose has a basic fundamental misunderstanding of how RC coupled voltage amplifiers work.

I see he formerly worked for Groove Tubes.

If you want to read how things work in the real world, I would refer you to any college level EE textbook dealing with discreet RC coupled voltage amplifier stages, which incorporate BJTs or FETs. FETs work more like tubes because they are voltage controlled like a tube, unlike a BJT which is current controlled. Look at the FET examples of RC coupled gain stages and extrapolate to the tubes, since tubes are voltage controlled also and just operate at higher voltages than FETs.

A FET has more of a pentode-like transfer characteristic compared to a 12A?7 which would have triode curves, but for the purpose of understanding how RC coupled voltage amplifiers work, that does not matter.
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