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Old 26th November 2004, 01:28 PM   #1
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Default Biasing a guitar amp for distortion


I wanted to ask for your thoughts to help me reach a conclusion.

If you want a guitar to make a screaming lead sound, you need quite a lot of valve distortion. Eddie Van Halen used to play using Plexi 100 watt amplifiers that where normally quite clean sounding, with not much distortion.

He used to run these from Variacs. He'd never really explain how he was using them or what he was doing to try and keep people from copying him. But I am almost certain he would use these to lower the plate voltages, allowing him to run the valves with much higher cathode currents. With the cathode currents closer to their limits, they would produce more signal distortion as the amplifier was turned up. This gave an excellent sound to his guitar! Really warm, smooth and nasty. All be it, at the expense of the valves.

He used to call it the brown sound, a bit too similar to voltage brown out. He then said he called it that because of something to do with his brother's drum kit.

I am looking to do something similar with a small home made 12AX7 preamplifier.

Looking at the grid curves for a 12AX7, the maximum cathode current is 8mA. The grid curve only goes up to 6mA, so I'll go with 6mA for the rest of the example.

To pull this amount of current through the valve with 0V on the grid requires roughly 280V on the plates. This means that at 0V, the anode would need to dissipate 0.006 * 280 = 1.68W. But each half of a 12AX7 is rated for 1W of dissipation, so I'm .68W over.

However, the dissipation rating is for DC current right? Since I am running an alternating signal into the grid, and it'll only reach 1.68W of dissipation on the peak of the waveforms, I'm effectively running the valve in a pulsed like mode beyond it's limits; not continuously.

If I was to fit a small fan over the valves, and maybe some heatsinks to the cases, I should be able to run them with plate dissipations much higher than they're rated for, so long as I keep the time spent there low, right? I have read about this idea on other sites but in different forms, such as biasing to 70% idle current instead of 50%.

I thought it might be helpful to try playing my guitar into my scope to see how long I actually spend with the output at it's peak. From the rough tests I did a while ago, I believe the output voltage of the pickups actually falls off quite quickly after strumming a note or chord to a much lower value.

Relying on the grid saturating at 0V + for distortion in guitar amps can cause problems, since the grid begins to conduct and causes the gain to start wobbling. A lot of high gain, home built amps suffer from this problem, with the gain going from insane to much less as the grid is saturated. Instead, I want more of my distortion to come from the cathode being overdriven.

I've been so busy it's been months since I last looked at amplifiers, so I need a quick check. Am I going totally off on a tangent with this or does it seem correct?
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Old 26th November 2004, 03:49 PM   #2
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Afaik, you can't "over drive" a cathode. The current through the cathode depends on that current being dissapated by the plate.

Reducing the B+ on the plate of the tube will reduce the swing and permit you to run somewhat higher currents - also changing the Rp at the same time... You can pretty much freely bias the tube anywhere in the range as long as the DC (quiescent) current results in a total POWER dissapation on the plate that is under the max for the tube (disregarding any issues like tube life, which may or may not change depending on what your actual set up is...)

There is nothing I know of that you can do that will drive a capacitively coupled grid past "0", so the tube will look like it is clipping at that point no matter what else you're doing.

So, in brief what you want to do is to *listen* to the various combinations of drive (input level), plate load, B+ and bias points and decide what sounds good to your ear for a given tube...

Other than that I know of no prescription for "magic" tone. You could put the thing in SPICE and try for maxing ur 2nd & 4th harmonics... that's an idea...

What Eddie Van Halen did is probably butt simple - and actually more mystique and myth than anything else. I'd guess that he merely had the Variac to set his line voltage level properly, since it varies all over the map in live venues... some bozo tech probably set that up for him... If he had the variac on the AC mains of any stock amp - one with filaments AND B+ on the same xfrmr, the amount that he could reduce the AC mains and not cause the filaments to drop to the point where they weren't hot enough wouldn't be that far. But maybe that was his entire trick... so put a variac on the filament xfmr of your test set up and see if the sound changes when the fils are barely lit...

_-_-bear -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
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Old 26th November 2004, 04:03 PM   #3
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Another thought...

As you lower the Variac on the mains, here's what happens to Eddie's amp:

Assuming about 10 - 15% line voltage drop being about max...

B+ drops from ~450v to ~<390vdc
Bias "drops" from ~ - 50v to ~<-38v
Tube operation goes from more or less class B/AB to closer to Class A
Total emission from the cathode decreases - so max power decreases there.
Power out is decreased because the B+ is lower and the bias is higher (more into class A).
Max power out is decreased as the B+ is lower - so is the max voltage swing.

All the preamp tubes - in class A - are running on lower B+ but self bias, so they look about the same operation wise... but clip sooner due to the lower B+

If this is how a player wanted to run his amp, it would be better to just have them set up with the requisite voltages in the first place, not use a variac, imho. Which, is why I think it is/was hype and he was really just using it to set the line voltage so that the way the amp *was* set up *was* the way it ran every day...

_-_-bear -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
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Old 26th November 2004, 04:40 PM   #4
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A cathode has a discrete, set, surface area. The surface area can only emit so much current because only so many electrons can boil off the surface of the electrode's coating in a set period of time.

The distortion of his amplifier was particularly distinct, which is why I don't think it's coming just from having the grid run to 0 and cause signal clipping.

My thoughts are that he was perhaps running the valve in such a way that the cathode reached it's maximum current limit before the grid came to 0V, such that the cathode became the clipping element before the grid.

The difference between solid state and valve distortion is the rounding effect that occurs in valves, whereby the valve smooths over the clipped edges as the grid cuts the signal off. I expect the cathode would have an even smoother clipped edge in comparison to the grid cutting off. Which would explain the very smooth distortion of his Plexi.

Not a lot of people, if any to the best of my knowledge, are able to make their Plexi sound similar to Van Halen's, which makes me suspect he was doing something more complicated than a lot of people are happy with.

If he was running the amplifier at a lower voltage through the variac, he'd almost certainly have had a separate winding or transformer for the elements and then rebiased the cathodes for a higher current, or he'd have lost power. In an interview he mentioned something vague about running the valves very hot.

The level of distortion the amps were producing wasn't anywhere near what they would have managed off the shelf. Plexi's are the same amps Jimi used to play through, where as Eddie's sound more like 80's or 90's rock amps.

I don't know, the Variac and numerous mentions of current and hot valves make me wonder is all!
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Old 27th November 2004, 08:17 AM   #5
Enzo is offline Enzo  United States
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I think you are making it overly complex and overly simple at the same time. From time to time they discuss Eddie's sound over at if you want to pursue it.

The distortion in Eddies sound and anyone else's is not just in the output section. I would describe MArshall plexis many ways but clean is not one of them. Cranked inputs yield all sorts of overdrive distortion. This is the kind of distortion that is still there when you turn down a master volume or use a pedal. Then there is output tube distortion that only occurs when the power tubes are fully cranked. Part of the EVH sound was due to the fact that he played at Godawful loud levels. This affects the sound not only from power tube distortion, but also from the acoustics of his speakers and also his strings/pickups playing in the intense sound field.

What you hear recorded won't happen at lower levels.

So get all you can out of the preamp, then work on the power amp. Just keep in mind that preamp tube distortion is a totally separate issue from power amp distortion. We adjust the bias to 70-75% of max dissipation at idle as a matter of course. In his Variac days, Eddies tech was not just setting up the AC mains to "proper" voltage, they were indeed browning out the amps for effect. Get a Variac and dial down the mains voltage and experiment. Don't overanalyze it.
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Old 27th November 2004, 10:21 AM   #6
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You're now part of my sig Enzo.
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Old 1st December 2004, 08:16 AM   #7
teemuk is offline teemuk  Finland
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You may want to check out this page. Especially the part concerning using a variac.
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Old 1st December 2004, 04:11 PM   #8
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Thanks for the link, I think I might have read the page before, but not for a few months at least.

His reasoning, however, simply futhers my belief that the Variacs are being used for attaining higher cathode currents. My answers to his reasoning...

1.) Reducing the AC input will reduce the B+ to be sure, but you will also reduce the heater voltage by the same margin. This can cause 'cathode stripping' of the tubes1 (if the heater voltage is far too low).
A lower filament voltage will mean that it doesn't heat up properly. If the cathode isn't hot, it won't emit electrons anywhere near as quickly. So, by far too low, I take it he means virtually off, since this would cause the tubes to be exposed to the HV more frequently.

Also, if you are in possession of a Variac, I would assume you know what a filament winding is and could easily afford to buy one to heat the filaments independantly.

You are also reducing the bias voltage as an added 'bonus'. Unfortunately, the bias voltage does not track the plate voltage proportionately. And we all know how anal the Internet 'guru' is about the 'proper' bias voltage.
Of the guitarists I know of, more of them know that valves have a bias voltage than know how, or why, the valves are heated in the first place.

This problem could be overcome by... I dunno... rebiasing the amplifier?

Filter capacitors tend to have a 'memory'. Running a 500-volt capacitor for any extended length of time at 400 volts will make the capacitor think it is a 400-volt capacitor. The next time you plug your Marshall straight into the wall AC and put 500VDC on a capacitor that has been brainwashed into believing it is a 400VDC capacitor could be trouble (if the capacitor fails to 'reform'). I have been told this is a remote chance, but I chance I don't want to take.
Remote being the word!

Electrolytes explode when the electrolytic gel boils. Switchmode power supplies put huge amounts of ripple current through their filter electrolytics and seem to work fine.

Also, this guy should consider the implications that statement has for variable power supplies. Or indeed, any application that involves changing the voltage on an electrolytic.

Using an 'ungrounded' Variac can cause serious bodily injury, and even death.
And the mains won't right?

There are two very good reasons not to use a Variac to raise the AC input voltage to your Marshall. You will ruin your tubes2

You accelorate the wear of your valves just turning the amplifier on. Rebiasing your amplifier using a variac, within the valve's operating region, shouldn't accelorate that wear any more than it would be in any other design using the same operating parameters.

and possibly your output transformer.
A good reason. However, very few guitarists take the same care of their gear as audiophiles do. An output transformer is a lump of iron in their amplifier to a guitarist. So long as it's not smoking, it's probably working okay. Unless your transformer is wound to work just on the boundry of the valve's output, it's unlikely it will be damaged rebiasing the amplifier.


These kind of statements just make me think that this guy is either trying to stop people from hurting themselves, or is trying to hide something he's doing. Either way, he's lying to some extent; because he shows that he knows a lot about valve amplifiers elsewhere on his site.

Variacs aren't something you can pick up at the guitar store, and almost no guitarist knows what they're for anyway. Most guitarists are also adults, who can take a 'You could shock yourself' warning. Which leds me to believe my second guess is correct.
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Old 1st December 2004, 04:36 PM   #9
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The guy on the Marshall amp page is slightly confused as well.

The quote he gives from Radiotron refers to *DIRECTLY HEATED* filaments/cathodes, not indirectly heated. Increasing the fils on an indirectly heated tube within reason will have little effect.

On a directely heated cathode, will have an effect.

The issue of the variac... just a bad way to get "tone". Assuming
an enclosed Variac with AC plugs & sockets, knock urself out.

As far as "increasing cathode current" - there is no possibility of change in cathode current without a comensurate change in PLATE current. They go together.

There are only a few ways to get more cathode or plate current, and that includes changing the bias, the load impedance, the plate voltage (up for an increase with all other parameters being the same), and that's about it. There's nothing else there.

Starving the fils will *reduce* emissions, if the fils are starved enough, and the tube will be reduced in terms of what it can do, not increased.

Increasing ur line voltage on a Marshall where the tubes are being run close to their design max (or over) and the caps are at or near their design max is a receipe for problems.

A decrease of ~ 10% is probably actually a decent idea.

The idea of "rebiasing" the tubes back to their original bias point on the curve after a reduction of the line voltage ("brown") is silly, since most of the change in tone is likely to to the effective reduction in negative bias voltage - shifting it back is probably the opposite of the way you want to go.

What this is saying is that you'd like to run the tubes in class A - and someone posted that they are run at 70%, if this meant 70% ON, as in past Class A which is ~50%, sorry no way - you can't. Not at those plate voltages, with those tubes, and not for long. Those tubes will glow and melt in Class A at ~500vdc on the plates.

IF you reduce the plate voltage, you have a shot at a genuine class A type bias, but with the loss of about 1/2 or more of your total output power.

Which brings me back to what I said in the first place. If you *really* want a Class A output stage "sound" the way to get it is to slam a different power tranny in the hole, (lower B+) and rebias ur tubes. No need to drop the fils at all.

If you want to dork your fils, have a tech put in some regulated fil supply components with a pot on the rear or front panel to control the fil voltage at will. You'll get less hum from the amp, worst case.

Modify the amp properly, if you want to mod it. Dorking with an external variac to get "tone" is kinda like putting a valve on the gas line to your car to make it go the right speed... sort of the right idea, but not.

_-_-bear -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
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Old 1st December 2004, 06:22 PM   #10
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Bear, the Variac idea certainly isn't to move the circuit closer to class A, it's to move it futher towards class AB.

There are very few amplifiers running purely in class A. It's not only a problem achieving true class A operation at the volume require to compete with a drum kit using valves, it's probably actually a step back from the sound a lot of people expect an electric guitar to make.

The distortion of an amplifier helps mask some of the mistakes, like fretting inconsistencies and picking differences, that a class A amplifier shows up.

Van Halens Plexi's were usually quite clean, closer to Class A sounding when stock. Even though he was running them incredibly loud, they still had far more distortion than they should have had. Positive distortion, that sounds excellent on a guitar.

Guitar amplifiers and audio amplifiers begin to split apart at distortion. The only similarity is the argument over the induced distortion of a valve Hi-Fi being more pleasant than a SS Hi-Fi by the members of the forum who prefer valves, and vice versa.

Similarly, the different origins of the distortion in a guitar amplifier effects it's sound. For instance, valve distortion is usually much smoother and rounded sounding than solid state distortion, which sounds more like fuzz or grit. Valve rectifiers introduce sag distortion, that rounds the notes together and makes the amplifier sound, in my opinion, too smoothed over. Combined with a lot of valve induced distortion, chords end up turning into mush. I actually prefer solid state rectifiers to valve rectifiers at the moment.

The Variac idea is easier if you think about it in reverse. If you keep the voltage constant and increase the current, you increase plate dissipation, probably beyond the valve's rating. To stop the plates melting, you need to lower the plate voltage in accordance with the cathode current.

When you start doing this, you simply add a second transformer to the arrangement for the filaments, so that you can run them on their own socket, separate to the plates.

Like I say, the distortion of Van Halen's amplifiers was incredibly smoothed, but not mushy. The chords came through crystal clear, but the individual notes didn't bite into your ears. To obtain the level of distortion he did, with those amplifiers, he must have at least had them rebiased. If he rebiased them for higher currents, he will have had to reduce the plate voltages to avoid exceeding the plate dissipation wattages.
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