Bias question?

[bluesguitar]

If an amp is negative biased will increasing the negative number run the tube hotter or colder? Is -45 hotter or colder than -30? Fender used to recommend bias around -30mA. Yet my '74 bassman with only a bias balance pot is factory set very near 70% max plate dissipation. This is around -45mA and supposedly optimum for a clean tone. If going to a higher negative voltage will eventually achieve 100% max plate dissipation, isn't that running the tubes "hotter" because of danger of red plating? If this is correct then "breakup" is NOT achieved by driving the tube harder, and therefore hotter, but is achieved by a different phenomenon?

[Reeltarded]

More negative is more.

If you are using a fixed bias amp starting out at the specs vs your plate voltage is a good place and then you have someone play the amp while you try slightly hotter and slightly colder to see where it really sounds best to the ear. Colder bias will start to clip in a dryer kind of way, and hotter bias will make the amp brighter tighter and cleaner to an extent. If you like output breakup you will be better off with slightly less in many cases. If you are playing an amp at or under RMS you might like slightly higher. If you aren't like me it might be exactly the opposite.

[cbass]

-55 will be cooler than -45 I think that's what you are asking

[vibratoking]

For fixed bias, the bias current decreases as you make the grid more negative relative to the cathode. This is colder bias because there is less current to create heat. Bias current is electrons flowing from the lower potential cathode to the higher potentail anode. The grid is between the two. As you make the grid more negative it repels more and more of those electrons and decreases the current.

If going to a higher negative voltage will eventually achieve 100% max plate dissipation, isn't that running the tubes "hotter" because of danger of red plating?

So your comment is backwards.

[bluesguitar]

For fixed bias, the bias current decreases as you make the grid more negative relative to the cathode. This is colder bias because there is less current to create heat. Bias current is electrons flowing from the lower potential cathode to the higher potentail anode. The grid is between the two. As you make the grid more negative it repels more and more of those electrons and decreases the current.

If going to a higher negative voltage will eventually achieve 100% max plate dissipation, isn't that running the tubes "hotter" because of danger of red plating?

So your comment is backwards.

I pretty much grasp what you are saying. The higher the negative voltage on the grid, which is what is being adjusted at the bias pot, the less current is flowing thru the tube. So the tube is cooler. I'm still working on why too much negative voltage achieves 100% max plate dissipation and will burn up the plates (redplate?) Still new to voltage vs current and how they interact. If, in fact, that is the issue.

[vibratoking]

I'm still working on why too much negative voltage achieves 100% max plate dissipation and will burn up the plates (redplate?)

As long as you understand that too much negative voltage does not result in redplating. Too much current does. Try to understand the first plot on the upper right of this link. It shows that Ia increases rapidly as Vg approaches 0V. Ia is the plate current that will cause redplating if it gets to high.

http://www.valvewizard.co.uk/pp.html

[Firestorm]

For fixed bias, the bias current decreases as you make the grid more negative relative to the cathode. This is colder bias because there is less current to create heat. Bias current is electrons flowing from the lower potential cathode to the higher potentail anode. The grid is between the two. As you make the grid more negative it repels more and more of those electrons and decreases the current.

If going to a higher negative voltage will eventually achieve 100% max plate dissipation, isn't that running the tubes "hotter" because of danger of red plating?

So your comment is backwards.

I pretty much grasp what you are saying. The higher the negative voltage on the grid, which is what is being adjusted at the bias pot, the less current is flowing thru the tube. So the tube is cooler. I'm still working on why too much negative voltage achieves 100% max plate dissipation and will burn up the plates (redplate?) Still new to voltage vs current and how they interact. If, in fact, that is the issue.

That's still backwards. More negative voltage does not push the tube towards max dissipation. Too much negative voltage can bias the tube to 0% (that shuts it off, it would take a very negative number).

Your signal voltage is AC; it swings around a zero point. In fixed bias, the idea is to make that zero point -30 volts, or -35 volts or whatever, so the positive signal swing can't push the tube to red plating.

I don't think I've ever seen a Fender current recommendation (mA). Only a negative voltage spec on a schematic. Idle current at 70% of max doesn't seem very Fendery. They were more 50% guys (in fixed bias).

[martin manning]

Making the bias voltage more negative wrt the cathode reduces idle current and the percentage of maximum plate dissipation. Algebraically, this is decreasing the bias voltage. Doing the opposite (algebraically increasing the voltage) will increase plate dissipation.

[bluesguitar]

Thanks to everyone for your wisdom and patience. My confusion was in interpreting what my DMM was telling me. For my intent and purpose the reading is telling me how much current is being applied to the plate. Not what amount of negative voltage is being applied to the grid. If I think of it that way it makes sense why a higher number would run the tube hotter and get you closer to red plating.

[NickC]

Thanks to everyone for your wisdom and patience. My confusion was in interpreting what my DMM was telling me. For my intent and purpose the reading is telling me how much current is being applied to the plate. Not what amount of negative voltage is being applied to the grid. If I think of it that way it makes sense why a higher number would run the tube hotter and get you closer to red plating.

Another (extremely non-technical) analogy:

Imagine a hot water pipe and a cold water pipe both connected to a "Y" and then flowing into the vacuum tube. The hot water is the current, the "go juice". But if it gets too hot, you open the spigot on the cold-water tap, to prevent the hot water (excessive current) from scalding (red-plating) the tube. The cold water is your negative Bias Voltage. It brings the overall water temp (i.e. current) down. The more you open the cold water spigot the more negative bias voltage cools down the mixture.

I understand this is technically flawed (flowed?) way of explaining bias voltage ..... but maybe it'll help grok the general concept.

[Structo]

It's the negative number that always throws people.

More negative is like saying more better.

Picture a number line with 0v in the middle.

-------------- 0 ++++++++++

To the left is negative and to the right is positive.

As that number goes to the left (negative) on the grid , it restricts the current flow between the cathode and plate.

[vibratoking]

With all due respect, I don't think it's the negative number in this case. It often is in other cases. I think the OP needs to understand the difference between current and voltage. There is a confusion at the basic level IMO. To the OP - keep at it. Try to understand Ohm's law and the basics of how current flows. I have confidence you'll get up the learning curve based on the good questions that you have been asking. Keep asking and you'll get a lot of good responses from the guys here.