Power Tube Biasing and Headroom

[dave g]

Yes, I think we are arguing semantics. Although I think you mean 75vpp, not rms. It's also a bit more complicated than biasing Vg at -37.5v for the reasons I already gave (namely that such a bias voltage may bias you past cutoff in which case you will introduce large amount of crossover distortion), but it's all besides the point.

I think that there simply is no simple answer concerning headroom (by the guitarist's definition), other than if you want more, get a bigger output section

[Structo]

Yes, I was coming from the guitarist angle.

Usable clean tone before breakup is the type of headroom I was referring to.
Sorry if that is not technically correct but it is the way most guitarists think.

In other words, where in the bias range between say 50% and the 70% that most feel is the upper limit of the bias in regards to power output is the greatest "headroom" or clean range before breakup?

Maybe there isn't a simple answer.

I was just curious if more headroom was attained at a lower setting vs a higher setting.
I think I understand where David was coming from.
The bias that allows the widest voltage swing before distortion occurs.

[Andy Le Blanc]

But.....Isn't the amp supposed to make the tone?

It's the color produced by the harmonic character of the the amp....
especially the clean tone, because of the lack of head room.
Every instrument has a spectrum of harmonic derivatives around the fundamental,
get rid of that and you've rid yourself of the tone your looking for in the first place.

It'll sound like a duck fart in a bath tub.

[jjman]

.............In other words, where in the bias range between say 50% and the 70% that most feel is the upper limit of the bias in regards to power output is the greatest "headroom" or clean range before breakup?

Maybe there isn't a simple answer.

I was just curious if more headroom was attained at a lower setting vs a higher setting.
I think I understand where David was coming from.
The bias that allows the widest voltage swing before distortion occurs.

The "70%" figure is wattage. So I would think that different plate voltages would entail different max-headroom bias point wattages. For example, with a plate voltage of 200, biasing a EL34 at 70% (18watts) would likely have the grid voltage pretty close to the cathode voltage. Driving it with some signal would bring the grid’s net voltage up to/over zero volts pretty soon on the volume, ending the headroom quickly.

I once posted a question about headroom between different brands of tube X. A replier said that the tube will clip/saturate when the grid voltage reaches the cathode voltage (e.g. zero volts in fixed bias.) This was a revelation for me since I always assumed clipping of the top of the current wave was merely a function of the tube's "capacity." Rather the grid gets “clamped” around zero volts ending the upward swing of it and the output (saturation clipping.)

Bias points, headroom, cutoff, and clipping are all triggered by the voltage on the grid (relative to cathode.) The wattage is important of course and needs to be considered, but I don't think it is a starting point regarding a headroom equation.

[Merlinb]

In other words, where in the bias range between say 50% and the 70% that most feel is the upper limit of the bias in regards to power output is the greatest "headroom" or clean range before breakup?

It's very simple: The colder the bias, the more headroom you get (assuming we're not biasing so cold that crossover distortion becomes significant.) The basic tone of the amp will change with bias of course, but if we're playing clean then this isn't so noticeable (you only asked about headroom, anyway).

There's really nothing else to it! Of course, if you want lots of headroom then the more logical approach is simply to turn down the volume a bit. After all, once the amp has been driven to full power, there's no more headroom left, so it's just a question of how hard you want to hit your strings before you hit max power. You can either bias cooler, or stop the signal getting so big that it overdrives the tubes! A 100W amp made with 6L6GCs running at 500V will have exactly as much headroom as a 100W amp made with 6V6GTs running at 250V, provided you feed them each with suitably sized input signals.