Bias Pot Question

[Kramer]

When you turn the bias trim pot up (clockwise), you increase resistance, thus lowering the bias/current of the power tubes, and when you turn it down (counter clockwise) you lower the resistance and therefore increase the bias/current, correct?

Thanks

Alex

[flemingmras]

When you turn the bias trim pot up (clockwise), you increase resistance, thus lowering the bias/current of the power tubes, and when you turn it down (counter clockwise) you lower the resistance and therefore increase the bias/current, correct?

Thanks

Alex

Sort of.

Rotating the pot one way increases resistance in the bias supply, which increases the voltage drop across the 47K resistor and the bias pot, making the bias voltage itself more negative. Rotating it the other way decreases bias supply resistance, which decreases the voltage drop across the pot and the 47K resistor, making the bias voltage less negative. But...the bias supply resistance only has to do with bias voltage itself. Bias voltage itself controls the power tube's effective plate resistance regardless of the amount of resistance in the bias supply. So...

To better state it, the voltage drop across the pot and the 47K/15K resistor connection point increases when the pot increases the total resistance to ground across the pot and the 47K/15K resistor connection point, and vice versa.

The bias supply circuit resistance in and of itself does not affect the tube at all. It is the resulting negative voltage across this resistance that controls the tubes' effective plate resistance, which controls plate current.

The cathode thermally emits electrons once it reaches proper operating temperature. Electrons are negatively charged. They flow to the positively charged plate (opposite charges attract). The control grid (pin 5) is placed between the cathode and the plate. By applying a negative voltage to the control grid, the negative voltage repels some of the negative electrons from the cathode (like charges repel), thereby creating a resistance, which is the tube's effective plate resistance.

The more negative this voltage = more electrons repelled from the cathode = higher effective plate resistance = less electron current flow, and vice versa.

Too much negative voltage can push the tube into "cutoff". This is where the negative voltage is high enough that the positive plate voltage cannot overcome the repelling force of the negative voltage from the grid, hence all plate current flow is "cut off".

Not enough negative voltage and the tube will overheat and burn up, possibly taking other parts with it.

You want the tube pulling no more current than what the 70% plate dissipation rule tells you, which is dependant on plate voltage. For an EL34, this formula is -

17,500/Plate Voltage at Pin 3 = Max Bias Current in mA (mV with the 1 ohm resistor method)

Just to explain where I got the 17,500 from..

17.5 Watts is 70% of the max rated plate dissipation of an EL34 (25 watts is max). If you plug that value into the equation, it will give you bias current in Amps. However, moving the decimal in 17.5 three spots to the right (which makes it 17500) will give you bias current in milliamps (mA) (the numbers will be the same, with the end result being a whole number in mA rather than a decimal number that you have to convert from A to mA by moving the decimal yourself). The 1 ohm resistor through Ohm's Law converts this mA into millivolts (mV) (any amount of current flow through 1 ohm of resistance equals itself in voltage due to Current X Resistance = Voltage, and anything multiplied by 1 equals itself).

That is your absolute maximum for Class AB (most tube guitar amplifiers). You can set it lower than this to your ears liking, but you don't want to go higher than this value.

Now for a little lesson.

Current flow through your output tubes is known as "plate current". The amount of plate current that your output tubes draw with no signal applied is what you're adjusting when you set the bias. This current flow is also known as "steady state" current. This is because when no input signal is applied, current flow through the power tubes is steady with minimal variance, hence the name "steady state current".

The input signal makes the plate current fluctuate above and below the steady state current, at the input signal frequency. It does this by making the bias voltage fluctuate between more or less negative at the input signal frequency. This makes the plate current fluctuate above and below the set steady state current at the input signal frequency. Because a small change in bias voltage causes a large change in effective plate resistance, and therefore a large change in plate current, you get amplification.

And how does it do this?

Electrons possess a negative charge. The cathode thermally emits electrons, which flow from the cathode, through the grids, to the positively charged plate (opposite charges attract). By applying the negative voltage to the control grid, the negative voltage repels some of the electrons flowing from the cathode because electrons being negatively charged, like charges repel, so the negative voltage on the control grid is able to repel the negatively charged electrons from the cathode, thereby making a more resistive path for electrons to flow. This is your tube's effective plate resistance and it limits electron flow from the cathode to the plate.

By making this voltage fluctuate at the input signal frequency, the effective resistance of the tube also fluctuates above and below what it's biased to with no signal applied, which also causes plate current to fluctuate above and below what it's biased to. Example, if your amp is biased to 35mA plate current draw with no input signal, the plate current will fluctuate above and below 35mA once the input signal is applied.

Here's an article in the tech Wiki about series circuits. I wrote it using a Marshall bias supply since a Marshall bias supply is basically nothing more than an adjustable series circuit.

http://metroamp.com/wiki/index.php/Series_Circuits" onclick="window.open(this.href);return false;

[Kramer]

Thanks for the detailed response. This really helps put in perspective.