# Trying to make a more accurate bias chart

Discussion in 'Amp Tech Center' started by peteb, Jul 17, 2017.

1. ### petebFriend of Leo's

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Clint, thank you for the link but I am not sure what it is meant for.

You read my posts right? I just measured the actual operating point of the tube by measuring the relationship between the signal voltage and bias voltage . If you have any comments relating to that, I would love to hear them.

Thank you

2. ### petebFriend of Leo's

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RLee, I think in class A the power level IS constant.

When idle is the max and it drops down after that, that sounds like cathode biased class AB, which I have no personal xperience with.

3. ### petebFriend of Leo's

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Old tele man, good write up there, thank you.

I thinak screen V adds a level of complexity that complicates the situation, (too much)

The question of the thread is can we look closer at tube operation to evaluate bias points than just go by the 50-60-70 rule.

If I understand what you and Rob are saying, that is different than me, is that you will use the 50-60-70 guideline, AND raise your bias as high as you can within the guideline.

Your overall bias philosophy is. "Mo is betta"

My bias philosophy is more like I want the tube to be fully working, but at the same time, as long as the tube is fully working, I am accepting of a low bias, I think they sound good, and I like lower currents for lower heat, particularly in the PT.

4. ### BendyhaFriend of Leo'sSilver Supporter

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Well..... amps are made up of a circuit with all the parts interreacting between varying AC signal input and fairly stable AC Mains supply, with a degree of spongy DC as the springboard for the signal to bounce higher.

Choosing as you are to try and find a reliable method for whatever it is your aiming for, yet not grasping a hold of all the fundimental major variables, is kind of like trying to get scientific about swimming style, and saying you are not yet ready to think about kicking your legs.

Yes...I am building a guitar amp. It is simple, make sure your phase inverter can put out far more (up to double ) the voltage swing than you need to drive the output tubes to maximum clean level.
Sometimes I turn the guitar right down, and the amp volume right up, and need lots of excess drive.
Sometimes I turn the guitar rigt up, and have to turn the amp right down to reduce distortion.

Most indepth scientific analasis are about obtaining maximum CLEAN level. This doesn't interest me.
Hi-Fi components have pretty much a universaly set maximum output voltage level -line level- it is clean, the amp may have a volume control, but it is desinged to deliver a maximum level that the output can reproduce, cleanly......for this you need to know ever stage of the gain structure. But even then, the amp usually delivers at maximum a bit more than the output can handle.

Guitar amplifiers are there to create tone.....not faithfully to reproduce input signals.

Many of the best sounding guitar amp circuits are absoluty terrible examples of audio engineering praxis,.

If you would to be setting the phase-inverter (asuming it is the output tube driver) to have a maximum output that is just getting the output tubes to the grid limiting stage,..are you designing it to do this when all the guitar and amp volume, tone and gain stages are at maximum as well ? Are you not intending to overdrive anything? If you knobs on your amp that do something, then everything needs to be able to do more than it needs to, when everything upstream is on max, it has to be able to bring the levels back up again.
This is why you need to have lots of reserve to be able to push the output tubes against the wall, pull there screens down, and pump them full of electrons. Not that you will be doing this all the time, but if something upstream is attenuating, you might still want to play loudly.
The input signal voltage starts at your fingertips...the only way to achieve what you propose, is to have a limiter/clipper right at the control-grid, some people do this, but then again, this is what a master volume does...set by ear. ( most use of limiters seems to be for the downwards swing to limit the input after the tube has gone into cut-off, not the swing upwards that is hitting the control-grid. This is for reasons I won't go into for now.)

Any quest to find numbers to tell my ears what sounds good, are not likely to be very helpfull.

Yes I am aware of much "correct audio praxis", and it does interest me, I've read all the books, and muse over theories. But I bias amps within a safe operating range, with my guitar and ears.

Last edited: Jul 22, 2017
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5. ### Old Tele manFriend of Leo's

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Yes, that is, fundamentally, the subscript "1" criteria: that the control grid nevers goes 'positive' in normal operation because the bias voltage (-Vg(dc)) is always greater than (more negative) or equal to the signal voltage (Vg(ac)), ie: Vg(ac) ≤ |Vg(dc)|.

That's a "design" criteria that's bounded by another "design" criteria of having 5% or less THD occur at maximum output power. The "player", however, does not think about nor stay within these criteria when operating the amplifier.

Last edited: Jul 22, 2017
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6. ### RLee77Friend of Leo's

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I think, as others have suggested already, that it would be beneficial for you to fully understand class A power stage design before tackling push pull AB. And how power is dissipated in a linear device.

Instead of focusing entirely on bias adjustments, it would be helpful to acquire a solid working understanding of the other principles involved in setting the operating parameters of an output stage, which Bendyha's posts detailed very well. Bias is a small piece of the big picture.
And comparing bias power at idle (a static D.C. power reading) to amp max output power (a dynamic rms power reading) is not a useful exercise for drawing conclusions about tube operating parameters; at best it gives you an approximation of efficiency.

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7. ### petebFriend of Leo's

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Thanks, but I understand class A plenty well.

As far as the comment on the comment on the comment, I was just trying clarify the clarification the jazz guitar wa making in your comment. Jazz guitar knows his stuff. And class A IS full power all the time, I know, I've measured it.

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9. ### petebFriend of Leo's

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The bias evaluation method I am using gives a closer look at tube operation than the standard bias evaluation method.

The method offers improvement in two important ways:

1. The valve is checked to make sure it is fully opening and closing under playing conditions

This is advantageous in two ways, not only is the valve action measured, the second plus is that it is done during playing conditions as opposed to only being checked at idle.

10. ### petebFriend of Leo's

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This is what I mean by connecting the dots and filling in the gaps,

I was able to map the amp volume control position to the measure of how far open the valve is Open, it also can tie the tone in for an easy bias check. This is somewhat rough but at the same time it fairly accurately matches my samples and should fairly accurately match any fender amp designed in the 60s.

Volume---------percent that-------
Control---------valve is-------------
Setting---------open----------------tone

1-----------------0
2
3
4-------------------------------------onset of overdrive
5-----------------25
6
7
8-------------------------------------full overdrive
9
10----------------50

In this chart above, it shows how far the valve is open at max signal input. (Power chords), but I am also have a comprehensive chart that shows how far the valve is open depending on volume control position, AND signal level, one line for the little e string, one line for the BIG E sting, a line for max chords, and I would like to add a line for medium chords. The relationship between the volume knob position and the opening of the valve is linear, mostly, making my picture of the mechanical equivalent to the valve change from a butterfly valve as shown in the picture above, to a rectangular window that slides open and shut.

I forgot, back to biasing by ear, just like Rob said, you want to have your amp begin to overdrive before the volume is half way up. Just before, is fairly standard. You also want full overdrive to kick in, if it happens about 3/4 of the way up the volume, thus is fairly standard. If you adjust the bias you will be affecting where on the volume adjustment these sounds occur.

Bias hotter and they will happen earlier on the dial, bias colder and will occur later on the volume dial, but biasing will only move these spots a very little.

The main point of biasing on a working amp is to avoid replate, I find there is a wide range of bias that is in the working range.

Last edited: Jul 23, 2017
11. ### petebFriend of Leo's

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I have learned a heck of a lot about valve operation this week, this thread and the input of others has really advanced my knowledge. To show that it has been a group effort, when I started the thread I began by attempting to tweak the standard bias rules by including the amp power out in the calculation as well as the max plate dissipipation. It was the input of others which led to to look closer at the operation of the tube by paying attention to the relationship between the signal voltage to the bias voltage. I guess we can thank Bendyha for first stating that tube operation depends on these measures. My opinion is that these two values alone, provide the clearest picture of what is happening inside the valve as the music is playing thru it.

So on to my latest picture representing and making sense of idle, bias, and signal, it is a rectangular window that slides open and closed.

12. ### petebFriend of Leo's

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How do idle, bias, and signal interact to make a tube work?

The tube is a window, when setting the bias and the idle, part of the window needs to be open, and part of the window needs to be shut. At idle the window can't be completely closed and it can't be completely open, or else red plate will occur. The negative bias voltgage forms a gate or a closed window. At idle the open part of the window represents the idle, and the closed part of the window represents the bias. The positive voltage of the signal opens up the closed part of the window that the bias voltage closed. The absolute limits to the usable range of this signal voltage begins at zero and maxes out when the signal voltage matches the magnitude of the bias voltage. Once this happens, the window that the bias voltage closed, is now fully open and cannot open any more.

So in valve operation, the measure of how open the valve is, is how open is this bias portion of the window is, it isn't the whole window, it is only part of the window, but it is this part of the window that you are opening and closing as the volume dial is moved, also as you increase the guitar input signal by going from single notes to chords, and the AC swing of the signal is rapidly opening and closing the window.

I have found that fender amps typically will only open this window half way at most. and with a hotter bias, hotter bias, a fender amp could be set up to open more than half of this window, perhaps 75%?

Last edited: Jul 23, 2017
13. ### petebFriend of Leo's

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Here is a model showing the interaction between idle, bias, and signal.

The idle is the open, lower half of the window.

The bias is the closed upper half of the window. See how it's negative voltage works its way down the left side, zero bias would be a completely open window.

The signal voltage, being positive, opens the window, it's starting point, no voltage, begins at the bias voltage, and goes up from there, as shown on the far right.

On a typical 60s designed era fender amp, as you sweep the volume from 1-10, with full signal applied, you are opening and closing half,( or a little more)of the bias window portion of the entire window.

In addition, this appears to be true across all amp classes, middle range biases, and amp sizes, excluding cathode biased class AB since I have none to test.

When setting bias point, you are setting how open the window is. A hotter bias is going to start with the window further open, a colder bias is going to start with the window more closed. I am unsure of the relative size of the bias window to the to the entire window or to the idle window but for simplicity I chose to make them equal, maybe being the middle bias spot, and if the window is more open than closed is hot, more closed than open, is cold? I actually chose to make them the same size because idle and bias are equally important and both have to be present, and it reminds me of the look of the older style Windows found in older Homes. To make it accurate mechanically, the upper window is a pocket window where the pane slides above the window opening. Is that how those older Windows were done? With a pocket? I digress.

14. ### petebFriend of Leo's

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And then here's the all comprehensive bias chart, in rough first version form

The volume setting runs along the bottom there.

The vertical scale is how open the valve is. From 0%, no signal, to 50-60% at the top.

The rising lines are the AC signal opening up the valve. The lowest line is measured from the small e string. The second rising line is from the measured BIG E string, the the steepest line is from max chording. Regular chording would lie a bit lower.

I've added color coding. A little blue along the bottom is the valve only open from zero to 5-10%. The next region is lime, looks yellow in the picture, the valve is opened up From 10-20, to 10-25 percent, this is the region prior to overdrive. The big e string, by itself can never get above this region, but you see that chords can go the next region when the volume is set at halfway, and maybe a little bit before. The next region is heaven, it is yellow but looks orange in the picture. The lower edge is where (the tube begins to saturate?) the onset of overdrive occurs. The upper level is full overdrive, this is where your amp stands up and does the boogaloo! If the amp has this full region it is delivering the goods. Thus range is when the tube is approximately 20 percent open to 40 percent open. Typical fenders will reach full overdrive when the volume is around 3/4. The last quarter is a little tone buffer that fender provides for those who want a little more. It is orange which looks red in the picture, there is not a whole lot of tonal variety in this region, as there is quite a lot in the yellow, middle range. The yellow range is the center piece of the amp, and by center, I don't mean central, I mean it is what the amp was made to do. In The orange range, as there is not a lot of tonal variety, the amount of range is not critical, only that this range exists. In the Orange range, the valve is open from 40-50 to 40-60 percent and beyond in really hotly biased Amps. I am not sure how high the range goes, but I don't think it's tonally important, the good stuff has all been covered.

Lastly red, looks purple, is above the orange, I don't know where it is relative to the valve open percentage, but the 50-60-70 rule based on max plate dissipation provides that.

Last edited: Jul 23, 2017
15. ### petebFriend of Leo's

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Thank you Bendyha, that's the kind of stuff I would like to hear, You are very in tune with what the tube is doing.

Do you have any voltages that you can share?

16. ### Old Tele manFriend of Leo's

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Last edited: Jul 23, 2017
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17. ### BendyhaFriend of Leo'sSilver Supporter

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Pete.....It seems to me you have literally rectified the concept of AC amplification.

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18. ### petebFriend of Leo's

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Thank you Bendyha.

In that one day of testing and analysis, I learned more about tube operation than i did in the last 18 years.

Now that class AB has been thoroughly covered, I may look more into class A.

It's better to burn out, than it is to fade away

My my,

Hey hey.

Peace out all

19. ### petebFriend of Leo's

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I looked for documented recommendations for the ratio of the signal voltage to the bias voltage, the best measurement for how open a tube is, and I could not find much.

I was really starting to wonder if this is the best measurement of tube openness, because if it is, nobody has any information about it?

Here is what I found:

As posters have said above, it is an involved task to find the bias point, you got the charts, and the intersection and so on and so fourth. So the amp itself sort of dictates what the stable idle bias is that will provide full power. (That right there is worth noting, tube specs are written about amps trying to get full power out of their tubes, when that is not the case, the tube specs need reinterpreting, which is one point of this thread). The OT affects the bias point, which makes sense, and if you want a class A amp, this is one of the main places to depart from AB design, not only do you use only one power tube, and eliminate the Need for the phase inverter, you also place a bigger load on the tube with the larger OT impedance, this will cause the amp to find its bias in the class A range. You can't easily take a class A amp and rebias it to class AB, or the other way around, you might be able to do it, but it probably isn't maximizing the available power from the tubes.

Back to the ratio of the signal voltage to the bias voltage, which is what opens the tube up.

In the RCA receiving tube manual, in the text inbetween fig 25, a graph showing grid volts, and fig 26, a schem of a cathode-drive circuit, it says:

", the peak signal voltage is greater than the DC bias voltage,"

And in the 6L6 spec it says:

Peak AF grid #1 to grid #1 voltage: 45 V at plate voltages of 250 and 350 in fixed bias

For 6v6, it is 30 and 38 volts at plate voltages of 250 and 285.

AF = audio frequency

Angelfire Amps has an example and they go thru this and helped me understand this spec.

For the 6L6 the 45 volts is the peak volts between the two grids, so the peak volts on one grid is half of this, 22.5 volts, and the (max) RMS volts are 22.5/1.4 =15.9 volts.

Angelfire then goes on to note that the bias voltage equals the max RMS volts, implying that the tube could open 100% of the way, when the signal voltage matches the bias voltage, but my measurements showed that the tube was opening up sufficiently when the tube opens up halfway or a little more????

How can I reconcile these differences?

One thing I noticed is that Angelfire's bias voltage is way lower than mine, way lower than is shown on Fender schematics. ANgelfire's bias voltage is in the teens, where Fender put it up to 40 for a pair of 6L6s. My bias voltage, 52, is much higher than angle fire's so to open up the tube, my signal voltages need to be quite a bit bigger. The RCA spec is 15.9 volts, but at lower plate voltages, and my signal measured as high as 30 volts. My max signal voltage is almost double the spec, but that is not too surprising for a fender amp.

What I get out of this:

On the high side, you would like to see the peak signal voltage equal the bias voltage, which places the max signal voltage RMS at around 71% of the bias voltage, opening up the tube 71%, which agrees with what I measured and generalized. I was saying you want to be able to open up the tube at least halfway, 50-60 is what looked like the upper range, none of my class AB amps are biased hot, so I also estimated that a hot biased amp might open the tube as much as 75%

This gives me a new way of looking at fender schematics. DC voltages are shown, no currents, and no AC voltages except for the input wall voltage. Bias currents can only be calculated on cathode biased amps, by dividing the cathode voltage by the cathode resistor.

Using what I have learned. If you see a 2 volt bias on a pre amp tube, a good guesstimate for a max signal voltage on that stage is 1 volt.

Interestingly, this gives some insight to the operation of the cathodyne phase inverter. Looking at a 6g2 schematic one notices that the bias on the PI tube (50ish) is higher than the bias on the power tube, 35 v.

Using my observations and reasoning, the max signal going in to the PI is 25ish reducing down to a max signal of 17.5 at the power tube, supporting the idea that the cathodyne PI induces a loss

Thanks

Last edited: Jul 25, 2017
20. ### robrobPoster ExtraordinaireAd Free Member

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The typical cathodyne phase inverter bias voltage is 1.5v

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