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'Bias Therory?'
Author:Paul Fawcett (registered user: 3303 posts )
Date: Sun, Mar 19th, 2017 @ 13:06 ( . )

On 03/19/2017 @ 11:41, mcneb wrote :
Now, I have come across a part of the circuit that I may not understand. The Bias of the output tubes in a push pull setup. We create another circuit to give a negative voltage to use for the bias. Is this circuit just to, “lower the floor of our ground” so to speak. Does this circuit give us a larger differential than using chassis ground or “the zero floor” . Using some numbers for example, let's say we have +200v on the plate of the Pi, the bias is at –20v, have we just lowered the floor giving us a differential of 220v is that what we have done there?

As you've said, the key factor to keep in mind here is that the bias is set by the difference between the voltage at the grid and the cathode of the tube. By the nature of thermionic tubes, the cathode must be at a *higher* voltage than the grid to achieve a useful bias. The tube doesn't really care how you achieve this differential. In a fixed-bias setup, the cathode is referenced to ground. Therefore, if the grid must be lower, it must be referenced to some negative voltage below ground, however this is achieved. Similarly, in a cathode bias setup, it is the grid that is referenced to ground, so the cathode voltage is forced to be above ground, usually by virtue of a cathode resistor across which a potential will be developed by virtue of the current through the tube. It is is true that in this setup, the plate to cathode voltage is reduced by the amount "wasted" across the cathode resistor. And unless you bypass the cathode resistor, there will also be significant negative feedback that reduces gain.. This is essentially because the voltage at the cathode will follow the voltage at the grid, which reduces the key differential voltage between grid and cathode. It is for these reasons that fixed bias is usually preferred if the goal is to maximize power.

On 03/19/2017 @ 11:41, mcneb wrote :
Forgive me if this obvious, but I'm thinking of a voltage regulator to use in this setup using Zener diodes, and it did not make sense when I was looking at it for this purpose. I have a understanding of the differential of the grid voltage to cathode for the bias of the tubes. Where we want the tube to be in it’s operational swing. If we have created a lower floor to use, why is it or why not use it for the preamp tubes also?.

While fixed bias can indeed be used with preamp tubes, there are a couple of reasons why the "lower floor" isn't used with preamp tubes too. First of all, there is generally no problem with getting sufficient voltage across the preamp tubes, so there is no special reason to maximize this. Consider that the bias voltage of preamp tubes is only a few volts anyway! Second, the cathode resistor with bypass cap is a primary method for tone shaping in guitar amp. Finally, it would be a huge PITA to establish separate negative voltages for the power tubes and (possibly each) of the preamp tubes.

One final thought. It's probably a bad idea to regulate bias voltages unless the rest of the voltages in the amp are regulated too. Bias voltage should float with screen and plate voltages as voltages from the wall varies. If it doesn't, you will effectively get different bias settings as the wall voltage varies.

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--* Bias Therory?
3/19/2017 @ 13:45--mcneb

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