Biasing - Headroom

[Littlewyan]

I've always been under the impression that the higher you bias your output stage the less headroom you have. If you bias colder (increase negative grid voltage) then you're increasing the headroom. However I read on Tube Depot's website that its actually the opposite. Which is it?

[martin manning]

I think it would be reasonable to say that in general hotter bias reduces headroom. Hotter bias uses up more of the available voltage swing between the idle point and the onset of grid current flow and the possibility of blocking distortion. It may be that the power tubes are not the determining factor (the clipping begins elsewhere), in which case it wouldn't seem to make any difference until they are. However, increasing the power stage idle current will reduce voltages across the board, and therefore reduce headroom in all preamp stages. Can you provide a link to where this claim is made?

[Littlewyan]

I always thought of the bias like a car throttle idle. The higher the idle throttle is set, the closer you are to the limit so much less room for...........throttle swing?

Anyway here is the link:

https://tubedepot.com/pages/bias-point

[martin manning]

The description there is written from the point of view of a fixed voltage, i.e. for fixed plate, screen, and bias voltages the idle current will be higher or lower for a given tube. The ones that produce a higher current will need a lower (more negative) bias voltage to bring the current down to a given level, and would therefore have more headroom available.

[tubeswell]

All other things being equal centre-biasing gives the most headroom.

[pdf64]

All other things being equal centre-biasing gives the most headroom

As the topic is regarding the output stage, doesn't the above statement require the qualifiction "For Class A"?
As I don't think it applies to other classes?

[matt h]

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[pdf64]

Ha, that's an interesting perspective on it, matt!
Though wouldn't that logic put centre biasing as being straight class B?

[matt h]

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[Littlewyan]

so basically if you lower the current draw until you get to a point just before you start getting crossover distortion then you'll have more headroom than if you increased the current draw until just before the valves red plate. Is that right?

[matt h]

(deleted)

[Merlinb]

so basically if you lower the current draw until you get to a point just before you start getting crossover distortion then you'll have more headroom than if you increased the current draw until just before the valves red plate. Is that right?

If you have a fixed input voltage from the guitar, and a fixed amount of voltage gain in the amp, then yes.

However, it's a matter of perspective. In a power output stage the 'headroom' is relly a matter of power, and the power output doesn't change, whatever the bias. Class A, AB, or B, you still get the same maximum output power. Only changing the HT or OT impedance will change the power.

Changing the bias simply changes the gain of the output stage. The hotter you bias it, the less signal voltage it takes to drive it to full power, which could be interpreted as less headroom. Of course, you could just turn down the volume control a bit, in which case you would have the same headroom as you did before. The amp still delivers the same maximum output power either way, so you still have the same 'loudness' available.

[Littlewyan]

Thanks Merlin, that cleared things up for me.