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Ilya-V

Bias, Bias X, SAG - technical questions

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Hi, first post.

 

I would like to know what Bias and Bias X controls do in terms of component emulation, beyond the general 'feel' descriptions I can already find.

I will elaborate the questions further;

 

BIAS:

My question is whether this control changes the cathode resistance and changes the negative voltage on the grid of the power tube?

If so, what range does this control have in terms of effective bias idle wattage?

ie;

If the emulated tube wattage is 30W (6L6 for that matter),, does changing BIAS between 0 to 10 changes the idle current from 0W to 30W at idle (full class B to full Class A)?

So BIAS value of 6-7 where most emulated class AB amps are set will result in 60%-70% of Tube wattage at idle, like real tube amps biased at?

 

BIAS X: (Excursion I assume?)

If that indeed excursion, my question is what component does this control emulate exactly?

Does this change the voltage swing of the phase inverter, maybe the coupling capacitors size, or grid resistors size?

https://www.ampbooks.com/mobile/amplifier-calculators/bias-excursion/

 

SAG

Does this emulate the HV++, HV+ voltage drop with a resistors in series with the power supply when a large current is drawn?

In a tube amp the secondary winding of the power transformer has significant resistance, so when the amp draws current, all the anode voltages of the amp drop or 'sag'.

 

 

I've bought a Helix recently and these controls imo are very important part of the power amp emulation.

I would appreciate the Helix much more if I actually knew what these controls do in terms of component emulation.

I've also built and modded a few tube amps, hence the technical questions.

 

Thank you.

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I wondered what this was all about also - check out Jasons video on subject

 

 

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I've seen that video, but I'm looking for a more technical description of what the parameters change in terms of the emulated components.

I've measured my amp (into a dummy load) with an oscilloscope and I can clearly see the same crossover distortion and bias excursion behavior in the Helix emulated amps as in the real amp.

 

With sag control at maximum I can hear additional distortion besides the obvious volume drop, probably from the voltage drop on the anodes of the 12ax7's.

Truly remarkable that Line 6 actually emulated that.

Sag also behaves differently between amps and I can hear the speed of voltage drop differs between amps, so maybe it's not a series resistor but a fully emulated resistance+inductance of the secondary windings?

 

On the spectrum analyzer I can see that Hum and Ripple behave harmonically (even harmonics of 60Hz) like an under-filtered (low capacitance) rectifier bridge.

Again, I am amazed that it is emulated that accurately here.

 

There is no doubt that the Helix simulation is component based, but the advanced amp parameters need to be clearly explained for they tech savvy folk too.

I am interested to know what Bias, Bias-X, and Sag do component and voltage wise in the emulation, since they behave remarkably similar to a real tube amp I've measured.

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The L6 people who may know or at least have a clue about this stuff don't hang out here. 

You might try asking over at The Gear Page in the Digital and Modeling Forum where a number of pretty knowledgeable L6 folks check in when they aren't busy working on the updates we're all anxiously awaiting.

 

You might also search here:

 

https://line6.ideascale.com/

 

to see if it's (detailed explanations) already been requested and vote, or, if not already requested, post a request.

 

I wonder - you seem to know what these things do in real amps, and you say that it SEEMS to be the same in the Helix amps, so maybe it is?

 

Thanks, though, for answering the crucial existential question - "Who cares?" :-)

 

^^^^^^^^^^^ Don't get mad! I couldn't help myself!

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On 3/16/2020 at 1:33 AM, Ilya-V said:

Hi, first post.

 

I would like to know what Bias and Bias X controls do in terms of component emulation, beyond the general 'feel' descriptions I can already find.

I will elaborate the questions further;

 

BIAS:

My question is whether this control changes the cathode resistance and changes the negative voltage on the grid of the power tube?

If so, what range does this control have in terms of effective bias idle wattage?

ie;

If the emulated tube wattage is 30W (6L6 for that matter),, does changing BIAS between 0 to 10 changes the idle current from 0W to 30W at idle (full class B to full Class A)?

So BIAS value of 6-7 where most emulated class AB amps are set will result in 60%-70% of Tube wattage at idle, like real tube amps biased at?

 

BIAS X: (Excursion I assume?)

If that indeed excursion, my question is what component does this control emulate exactly?

Does this change the voltage swing of the phase inverter, maybe the coupling capacitors size, or grid resistors size?

https://www.ampbooks.com/mobile/amplifier-calculators/bias-excursion/

 

SAG

Does this emulate the HV++, HV+ voltage drop with a resistors in series with the power supply when a large current is drawn?

In a tube amp the secondary winding of the power transformer has significant resistance, so when the amp draws current, all the anode voltages of the amp drop or 'sag'.

 

 

I've bought a Helix recently and these controls imo are very important part of the power amp emulation.

I would appreciate the Helix much more if I actually knew what these controls do in terms of component emulation.

I've also built and modded a few tube amps, hence the technical questions.

 

Thank you.

 

Don't know the answers to your questions but the way you asked them gives some interesting technical detail on what those controls might do on a real amp. The theoretical doesn't translate in my head to what difference to expect in the actual sound though. If you spliced your technical details to a layman's explanation of the tonal changes they create it might be helpful.  I'm glad you asked though 'cos it was somewhat educational.

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I can't answer the detailed technical questions but can maybe provide some useful guidelines.

 

Biased hot: Stronger overall output, more lows, not as much clean headroom, more saturation and less cutoff distortion, more even order harmonics, tubes wear out faster.

 

Biased cold: Weaker, thin, harsh output, Lows aren't as strong, more clean headroom overall, more cutoff and less saturation distortion, more odd-order harmonics, and the tubes don't breakup as fast.

 

I don't know what the range of the Bias control is, but I would expect 5 to be where the tubes are normally biassed, 0 is colder, 10 is hotter - but how hot/cold is unknown.

 

The above applies to typical Fender, Vox and Marshall amps where the majority of the distortion is coming from the power tubes, and where negative feedback is lost (because the power tubes are clipping and have no more gain), and therefore damping factor be lower.

 

For modern high gain amps where most to the distortion is coming from the preamp section, power tube bias can have a very different impact. A slightly colder bias might tighten up the amp, and improve the transient response. Hotter bias might result in less controlled tone and too much muddy bass. 

 

So biasing hotter for blues in small amps and biasing colder for metal in high-gain amps might be something to try.

 

Bias excursion is caused by distorting power tubes causing increased grid current which charges the coupling capacitor between the phase inverter and power tube, which changes the bias of the power tubes. When the input stops, the coupling capacitor slowly discharges allowing the power tube bias to recover. This could make an amp that is biased hotter, become colder biased as it is pushed further into distortion, changing the color of the distortion. Maybe when the hotter biased power tubes are pushed to the edge of breakup there might be warmer even order harmonics, and better bass. As the tube is pushed harder the bias gets colder, there might be more odd order harmonics for more aggressive sounding distortion, and the bass might be reduced.

 

 

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It occurs to me that a lot of readers of this forum might not know what bias actually is and why they should care. Changing the Bias and Bias X controls on a Helix amp block might not do that much to the tone, depending on how hard the amp is pushed. Understanding what bias is might help you understand what these controls do. Here's a brief description of bias that might provide a context for understanding what it does in Helix.

 

Tubes amplify by heating up a cathode that boils off free electrons. These electrons pass through a grid and are attracted to a positive plate, creating a current in the tube. A small change in voltage on the grid can effect a much larger change in voltage on the plate which is an amplifier.

 

Now imagine an alternating input voltage on the grid. On the negative half of the input voltage, electrons will be repelled by the grid back to the cathode, reducing the current through the tube. On the positive half of the input voltage, electrons will be accelerated past the grid to the plate, increasing the current through the tube. This is how a tube amplifies an input signal.

 

Now a tube has operating limits based on how it is designed - how hot the cathode can get, how big the plate is, how far away from the cathode it is, etc. If you keep increasing the negative voltage on the grid, eventually the electron flow will stop, and at that point increasing the input further results in no corresponding further decrease in the output. This is called cutoff. Its very sharp, and results in a lot of odd order harmonics. If you increase the positive voltage on the grid, it will attempt to push electrons faster and faster to the plate. But eventually the plate won't be able to consume any more of the electrons and the output won't continue to increase with the input. This is called saturation. Saturation distortion is more gradual, less sharp and generates even order harmonics.

 

Bias is a voltage difference between the cathode and grid that establishes the operating point of the tube between saturation and cutoff. Normal bias sets the voltage so that the tube is operating over its linear region. Hot bias reduces the bias voltage, allowing more current to pass through the tube when there is no input - the quiescent current. This makes the tube tend to saturate earlier. Colder bias increases the bias voltage allowing less current to pass through the tube when there is no input, giving the tube more headroom, but increasing the chance that it will reach cutoff.

 

 

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Thank you amsdenj, many people will be reading this thread with more accurate explanations than could be found before.

 

Here is a link with what I think the best technical explanation of Bias Excursion, Grid Clipping, Blocking Distortion on the internet.

https://robrobinette.com/Tube_Guitar_Amp_Overdrive.htm

 

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On 3/15/2020 at 10:33 PM, Ilya-V said:

Hi, first post.

 

I would like to know what Bias and Bias X controls do in terms of component emulation, beyond the general 'feel' descriptions I can already find.

I will elaborate the questions further;

 

BIAS:

My question is whether this control changes the cathode resistance and changes the negative voltage on the grid of the power tube?

If so, what range does this control have in terms of effective bias idle wattage?

ie;

If the emulated tube wattage is 30W (6L6 for that matter),, does changing BIAS between 0 to 10 changes the idle current from 0W to 30W at idle (full class B to full Class A)?

So BIAS value of 6-7 where most emulated class AB amps are set will result in 60%-70% of Tube wattage at idle, like real tube amps biased at?

 

BIAS X: (Excursion I assume?)

If that indeed excursion, my question is what component does this control emulate exactly?

Does this change the voltage swing of the phase inverter, maybe the coupling capacitors size, or grid resistors size?

https://www.ampbooks.com/mobile/amplifier-calculators/bias-excursion/

 

SAG

Does this emulate the HV++, HV+ voltage drop with a resistors in series with the power supply when a large current is drawn?

In a tube amp the secondary winding of the power transformer has significant resistance, so when the amp draws current, all the anode voltages of the amp drop or 'sag'.

 

 

I've bought a Helix recently and these controls imo are very important part of the power amp emulation.

I would appreciate the Helix much more if I actually knew what these controls do in terms of component emulation.

I've also built and modded a few tube amps, hence the technical questions.

 

Thank you.

 

Holy smokes dude. May I just say I wish I knew what you know. 

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