![]() a 12AX7) needs to be in an optimal range - not too much, or the tube will be damaged and not too little, or you will only be able to amplify relatively small input signals before clipping occurs. So, to recap: The main thing to remember is, the B+ for a particular tube (e.g. The size of the plate resistor also influences linearity via increasing or decreasing harmonic distortion however I am not going to attempt to describe that here. However, the larger the plate resistor, the more B+ is dropped across it so if you want to recover the B+ at idle that you originally had with a smaller plate resistor, you would need to increase the overall B+ by the appropriate amount. Thus in an example 12AX7 circuit (borrowed from Merlin Blencowe), a 100K plate resistor might give you an actual voltage gain of 60 (out of a possible 100, remember) while a 47K plate resistor might give you a voltage gain of only 43 and a 220K plate resistor might give you a voltage gain of 68. If you increase the size of the plate resistor, you shift the voltage divider to increase its output, and thus take more advantage of the tube's amplification factor another way of thinking of this is that the larger plate resistor will allow a wider output voltage swing before the clipping limits are reached. The plate resistor helps create the voltage divider that I mentioned, which allows the varying current on the plate to be converted into a varying output voltage. What increasing the B+ voltage (up to a certain point) really does is increase the ability of a tube circuit to amplify larger signals in as linear a fashion as possible, minimizing the tendency of tubes in general to conduct in a non-linear fashion when plate current is too low, while minimizing clipping of the signal that occurs when the output voltage bumps into either of its two limits (negative down to zero vs. If you put a 1VAC signal into such a circuit, it will be amplified to 60VAC. ![]() Typically, when a 12AX7 is put into a circuit w/ various components including a plate resistor, the tube's maximum amplification factor of 100 is realized only partially, so that the actual gain ends up, say, 60 or so. There are other factors, such as whether the cathode on the tube is bypassed, but never mind those for now. ![]() a 12AX7 has an amplification factor of 100 2) the voltage divider that is created by the plate resistor together with the tube's internal plate resistance and the impedance, or load, of whatever the tube circuit is driving. The amount of voltage amplification (or gain), of a circuit using a triode is determined by 1) the design of the tube itself, as measured by "amplification factor" - e.g. Let's look at the simplest amplifying tube, a triode. In a way this is correct - but it's an indirect rather than direct relationship. My guess is that you are thinking that intuitively, "more B+ voltage should equal more gain" - i.e., that the degree of amplification of a small signal voltage into a big voltage output must surely be controlled by how much voltage is made available for this purpose in other words, how much B+. But even so, maybe it will help if I try to explain what little I know, or once knew, or mabye only imagined I knew, all of it strictly from a novice's POV. Malcolm has already suggested a way to start thinking about tube amplification, and he really knows this stuff backwards & forwards whereas I am a rank novice plodding through books on vacuum tube circuits and since it's been a few months since my last bout of reading, I've already forgotten almost everything I thought I had learned.
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