Power Amp Distortion - Can it be controlled??

[TIMBO]

Hi Guys, Sitting around while the troops are out hunting and gathering for Xmas (merry Xmas to all) and thoughts started gathering in my head and i came up with a few question on how to add bits to a amp via a flick of a switch.

I kinda know how some of these question can be done but i'll ask anyway :-

Bright/Normal Sw. - Cap on Vol pot
Tight/Deep Sw. - Not sure, possible V1 cathode cap/resistor combo
Hot/Clean Sw. - Maybe similar to Tight/Deep switch or plate resistor value
Mid/Normal Sw. - Similar to mid sw. on TOS
Tone Stack Sw. - Sw. to alter slope resistor to emulate fender,marshall and vox three knob ST
Vintage/Modern Sw. - Not sure, possible extra filter cap and choke added to tighten up sag
This is the one i not sure it can be done in any way that will sound any good.

Can power tubes be distorted in anyway via a control pot?
When a cathode bias is used you can alter the resistor for a particular setting that can alter headroom and early breakup?
What i am thinking is if a resistor can set power tubes at say "normal" could a veriable resistor (pot) be added to adjust the bias to shift the tubes into a more "hot" setting
Can distorted power tubes sound any good or be a useful?
Maybe this can be done another way?

Tubenit has posted "bells and whistles" and has been a great help. Merry Xmas  

[jim]

The cathode resistor sets the correct operating enviroment for the power tubes and the window between cold and hot is very narrow and would likely be difficult to control safely with a pot.  You could switch in different bypass caps/cathode resistors combinations safely with a rotory switch. Each selection would then be tested to be totally safe for the output section.  Vintage/modern switching cold be hot/cool bias or perhaps tube rectifier/metal rectifier or maybe grounding out the midrange for a scooped sound.   Power scaling or VVR is the way that comes to mind when I think continuously variable output distortion.  Jim

[HotBluePlates]

I kinda know how some of these question can be done but i'll ask anyway :-

Bright/Normal Sw. - Cap on Vol pot
Tight/Deep Sw. - Not sure, possible V1 cathode cap/resistor combo
Hot/Clean Sw. - Maybe similar to Tight/Deep switch or plate resistor value
Mid/Normal Sw. - Similar to mid sw. on TOS
Tone Stack Sw. - Sw. to alter slope resistor to emulate fender,marshall and vox three knob ST
Vintage/Modern Sw. - Not sure, possible extra filter cap and choke added to tighten up sag

Were you asking for confirmation on these and/or other ways to incorporate them?

Can power tubes be distorted in anyway via a control pot?
... for a particular setting that can alter headroom and early breakup? ...

This is a can of worms. See below.

[LooseChange]

The Power Scaler and VVR circuits were designed to make Power Amp distortion easy to achieve with one simple control.

[birt]

i guess it's called a volume control. more volume to the power tubes = more distortion. life can be easy sometimes. and power tube distortion can be very useful. how loud it can be is something else

[HotBluePlates]

Can power tubes be distorted in anyway via a control pot?
... for a particular setting that can alter headroom and early breakup? ...

So are you looking for a way to make the output tubes distort more or less, while also keeping the same volume level?

That's a tough nut to crack. Evidence are the thousands of distortion pedals out there, as well as the amps with master volume schemes to cause preamp distortion.

How do tubes distort? Any tube can be operated within a given load to give a clean sound. It might take a small signal and reproduce it accurately at the output. The interesting part happens when you feed the tube a big signal.

"Clean" can be defined as a change in input signal which causes an exactly corresponding change in the output. With a big enough signal, three things might cause distortion.

All tubes have portions of their characteristics where the grid lines are spaced closer together than at some other part of the curves. Only those portions that intersect the loadline are a factor. Usually, the gridlines are more closely spaced towards the low current area of operation. So in this case your input signal gets bigger, but because the gridlines are spaced ever more closely the output signal doesn't continue to get "correspondingly big". Because the output doesn't exactly follow the input, this is at least one form of distortion. This cause of distortion tends to affect the portion of the loadline that is "down and to the right" on plate curves, which is the area of plate swing towards low current, high voltage.

Another way for the tube to distort is to swing so far "down and to the right" that plate current is turned off. This is cutoff, and once you reach zero plate current, no matter how big the input signal becomes, the output stays at zero current (the plate can't swing a negative current). So a part of the output signal is lopped off. You could look at this as being the same thing that a rectifier does.

To an extent, both of these two causes of distortion are mitigated when you use push-pull operation. We know there is such a thing as "class AB" where one side of the output section cuts off while the other side keeps working. Two things are happening here... First, the inductance of the output transformer causes the plate voltage of the cut-off tube to soar to a high voltage (above B+) even after the tube cuts off. Essentially, this acts like a flywheel that makes the plate voltage appear to swing as it would if the tube could pull a negative current and keep the plate voltage rising. Second, the output transformer stitches together the opposing tubes signals. If each side distorted in complimentary (same but opposite) ways, the distortion would cancel.

With power triodes, the composite output tube formed by a push-pull pair becomes quite linear due to this effect, up to maximum power output. It still works with pentodes/beam power tubes, but the composite tube is not quite as linear as in the triode case.

What this means is that in push-pull, the first two causes of distortion are not such a big factor.

[HotBluePlates]

The third major cause is when the grid is driven positive (we won't discuss positive grid drive here, because it's a special case and you likely have never seen a class B or positive grid drive amp in person; okay, you might have played a MusicMan amp, but that don't count).

We usually assume the grid represents an infinite input impedance. Once the grid is driven to and beyond 0v absolute (meaning driven to a momentarily positive voltage), the grid impedance is no longer very high and it starts drawing current. This imposes a heavy load on the phase inverter, whose gain (and maybe output) drops as a result. As a reult, the output tubes tend to stop increasing power output, and the output there is tends to become more distorted. That's again because you apply an increasingly large signal input, but don't get a correspondingly increasing output. It would be an interesting experiment to monitor the output stage output signal, the output tube grid signal and the phase inverter output signal to see how the output tube grid current causes the phase inverter to give up and distort.

There is a fourth mode of distortion for pentode/beam power tubes, but you'll see why it's not as important to consider. If you look at the gridlines on the plate characteristics, as you move towards lower plate voltage, all the gridlines converge into essentially a single line. The bend where the lines separate and move horizontally is the "knee" of the curves. If you increased the load impedance enough, the loadline would become more horizontal and intersect the gridlines below the knee. At this point, it might not matter if you have 0v or -20v on the grid, the plate current/plate voltage is the same. This is a hard limit, much like cutoff.

The reason this mode is not so important to consider is that most designers won't run the loadline below the knee, because output power would be limited. While you might create a "bad design" which does run the loadline below the knee, there's probably not a convenient way to greatly increase the load impedance without slapping a big resistor in series with the speaker. Most of the resulting output power would be dissipated in the resistor, leaving very little to push the speaker. Sounds like a great attenuator, right? Except folks have done it, and reported that "it don't sound like a speaker."

[HotBluePlates]

The Power Scaler and VVR circuits were designed to make Power Amp distortion easy to achieve with one simple control.

Exactly.

So carrying on, what does power scaling/VVR do for you?

If you lower the supply voltage to the screen/plate, you reduce the amount of clean power output swing available. At the same time, you have to lessen the amount of bias voltage to keep the tube from being cut off. If you make the bias smaller (say, move from -45v to -20v at idle), the output tube can now only handle a smaller signal before running to the point of driving the grid positive and creating distortion.

One implication you might arrive at is that with a similar output tube, of two amps with different bias voltages, the one with the more negative bias (the -45v in the earlier example) needs a bigger input signal to drive it to full output power, and probably also makes more output power before distortion. You could also guess ahead of time that it has higher screen and plate voltages, necessitating the larger bias voltage (and perhaps moving the output stage from class A to class AB to class B).

An important feature is that the bias voltage oughta track the changes you make in the supply voltage with your VVR/power scale control. A cathode biased amp does this automatically; the lower supply voltage causes tube current to drop, which lessens the voltage drop across the cathode resistor and reduces the bias voltage. A fixed bias amp needs a tracking bias supply, which is a clever bit of engineering. As the supply voltage drops, the bias has to become more positive.

[HotBluePlates]

Final thought:

If we picked a given acoustic volume level, what's the difference between a high-powered amp that is clean at that volume, and a low-powered amp that is distorted at that volume? Pretty much the distortion. So it makes sense that the best way to vary the distortion is to have a method to vary the output section's power capability, as turning down the power is the same as turning up the distortion.

I say that because people ask about how to mod an amp for more "headroom" and they don't want to see that it really requires a "bigger engine" (meaning an output stage that actually makes more output power). Otherwise, they might be lessening the amp's distortion, but they're not getting "louder, cleaner" which is what I think "headroom" translates to.

So all the above is giving you some insight in why the power scale answer. I don't know much about VVR, except that the version I've seen has always been in cathode-biased amps. I don't know if Hall offers a fixed-bias version. I know London Power offers kits for both approaches, and I trust O'Connor's method of deriving the tracking fixed bias supply.

Whether your output stage ought to be fixed or cathode biased likely boils down to the question: "What is the most clean output power you'll ever need/want?" The answer might push you closer to Fender Twin/100w Marshall (fixed bias and high voltage essential), or towards Deluxe Reverb/early tweed Pro (this acoustic volume level can be reached which cathode-biased 6L6/EL34 pretty easily).

[TIMBO]

Man, You really know your STUFF. I can actually understand some of what you said.  The reason i ask the question is in a guitar mag was a new amp "Egnater Tweaker 88" which supports some of Tubenits "bells and whistles".

Single Input - Nobrainer
Channel Sw. - It has two separate channels LEAD and RHYTHM,sounds like the rhythm ch. has same sw. as lead just voiced different.
Gain on both ch. which has sw. :-
Bright/ Normal - Cap on Vol pot
Mid cut/Normal - similar to mid sw. on TOS
Tight/Deep - Can be done by Cathode Cap being sw. in/out
Hot/Clean - Can be done by Cathode Resistor being sw. in/out

Tone Stack - Three settings USA, AC, BRIT via a three position switch
Three knob ST which could have the slope resistor value changed

Master Volume - Possible type #4
Vintage/ Modern Sw. - Could be a few things sw. in/out to alter "SAG"

And as these SW. covers most of the amp it leaves the final SWITCHES which are "BOOST" :-
Clean/Gain - I not sure which part this is connected too so i was thinking and hoping it was something to do with altering the gain of the Power Tubes. So this SW. could be altering the Cathode Cap or something else,Grid Resistor,PI Cathode Resistor?????/
It has a pot with this part of the circuit and this could be the power scaling to promote the power tube "BOOST" but i can't confirm that  this "TWEAK" is connected to the Power Amp but it would be COOL.

I just had another thought that this pot could alter the FEEDBACK, that flicking the CLEAN/GAIN SW. engages the pot to very the feedback resistor value????

I love pulling these amps apart to try to find how they work and at the end of the day they employ a simple "FLICK OF A SWITCH".Thanks

[HotBluePlates]

Well, that makes sense. Egnater has a cool concept that is my idea of modeling amps, with their "module series".

I always wanted to make an amp like that which could hold multiple modules for different sounds, but I don't want to spend the money for the metalwork on a one-off amp.

There are many ways to do all the switched enhancements you noted on an amp.

Bright/ Normal - Cap on Vol pot

That could be. But this could also be a small coupling cap vs a "normal" one, or the addition of a small cap across one leg of a interstage voltage divider, or maybe a smallish cathode bypass cap. All of these things are used to distinguish the bright channel vs the normal channel of a vintage Marshall. It could also be switching in/out a plate load bypass cap.

Tight/Deep - Can be done by Cathode Cap being sw. in/out

Look also at the AA864 Bassman. There is a cap in the tonestack that helps define the range of the bass control, which also runs to a "Deep" switch. The amp was likely voiced for the deep sound, and you can switch the cap out to have "not deep". One way you could do something similar in an AB763-type amp would be to have 0.022uF caps for both mid and bass (or maybe even slightly smaller), and use the deep switch to add 0.047uF or 0.1uF in parallel. The trick is working out the voicing for each position, especially in the face of all the other optional settings available.

Hot/Clean - Can be done by Cathode Resistor being sw. in/out

Lots of ways to get there.

Could be an extra gain stage. Could be a change cathode resistor or added bypass cap (with typical 12AX7 values, the addition of a bypass cap doubles the stage gain). Could be the difference between a split plate load or interstage attenuator and no attenuation (over one or more stages, especially if the panel switch controls a relay).

Master Volume - Possible type #4
Vintage/ Modern Sw. - Could be a few things sw. in/out to alter "SAG"

The master volume type is very likely dependent on the type of phase inverter used.

The vintage/modern switch could be the addition of a smallish cathode resistor for the output tubes in an otherwise fixed-biased amp. Or (more likely), it alters the rectifier type, the addition of a series resistor, the overall B+ level, or similar.

[HotBluePlates]

And as these SW. covers most of the amp it leaves the final SWITCHES which are "BOOST" :-
Clean/Gain - I not sure which part this is connected too so i was thinking and hoping it was something to do with altering the gain of the Power Tubes. So this SW. could be altering the Cathode Cap or something else,Grid Resistor,PI Cathode Resistor?????/
It has a pot with this part of the circuit and this could be the power scaling to promote the power tube "BOOST" but i can't confirm that  this "TWEAK" is connected to the Power Amp but it would be COOL.

They claim a "clean boost" for each channel. I wouldn't be surprised if this circuit is akin to a Fulltone Fatboost or similar. Essentially, those are JFET boost pedals, but there's no reason you have to put the boost at the front of the amp.

But again, there's a lot of possible ways to add boost. One way is to design the gain structure for the "full boost" mode, then find a way to lose signal (such as through a divider). And if the switch then adds a component to variably bypass your loss mechanism (say, a 1M pot wired as a rheostat across a 100-470k resistor forming the upper leg of a voltage divider), then you have a "free boost". You can also look at designing in treble/bass "boosts" the same way, by designing the boosted sound as your basic circuit, then finding a way to get rid of the boost. Then, call the non-boosted mode "normal" and you have the impression of boost.

In any event, I'm not sure I think this is power scaling/VVR. Both those circuit require ~2w pots that are sizeable, and might just barely fit in the front panel space afforded.

I love pulling these amps apart to try to find how they work and at the end of the day they employ a simple "FLICK OF A SWITCH".Thanks

I like finding out how they did it too! The bad part is I might be a little jaded, in that I usually find the means of getting the switched effect wasn't that novel... just that it might have been well executed, and had cool marketing buzz-words to make it seem different than everyone else's trick. The best case is if it adds an extra feature or sound to a classic tone which players always wish the classic amp was able to produce.

[TIMBO]

HBP,I agree with what you said and some of Tubenits builds he has added the "bells and whistles" but ended up settling for one praticular setting because it was the sound/tone required.

Well the "marketing" caught my eye and got me thinking and like you the "TWEAKS" may not be as complicated as we think but a simple value change (clever engineering) with a catchy name and chrome switch. Thanks for your info and hopefully i can use some of this in the future. 

[HotBluePlates]

I like the idea too!

Catch is, I think I'd do what Tubenit has wound up doing. We oughta ask LC if he still uses all the switches and knobs on his Standard amp.

[LooseChange]

We oughta ask LC if he still uses all the switches and knobs on his Standard amp.

Nope. (Except for the volume and sometimes the tones)
But when I build, I will add a switch for some A/B testing before I go hard wire.