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Signals and Voltage in a Tube Amp (for Tube Noobs)

Discussion in 'Tube Audio' started by linuxslate, Nov 9, 2018 at 8:54 PM.

  1. linuxslate

    linuxslate Member

    Central Florida, USA
    OK, I am way less experienced than some of the "Pros" (literally and colloquially) here on AK; I'm probably poorer and uglier than some too, but I still feel I may be able help with a basic concept that is both not intuitive, and crucial to even basic work on a tube amp, like "re-capping", or setting the bias.

    The concept is that a single connection point in an tube (such as the plate, for example) have both a DC voltage and an AC signal at the same time.

    Wait -- What?! AC and DC at the same time?!! That's impossible!! The band is not called "AC at some DC offset" It's called "AC/DC" -- One or the other. House sockets are AC, and cars are DC. That doesn't mean motor-homes are both. My Multi-meter has AC settings, and DC settings -- it's not variable in between, and there is no setting for both.

    If you learned basic electricity, you probably learned it like I outlined above. House sockets are AC, and cars are DC. You may know that a rectifier (and filter) can convert AC to DC, or you may have one of those boxes that let you power AC gadgets from your car's DC cigarette lighter (Inverter), but that is converting between them, not both at the same place at the same time.

    But yes, there is AC and DC together at the same point in a circuit. In fact, there is always both.

    You actually have probably already seen this, but perhaps not really understood it. A noise filter in a Car stereo installation, or a "power injector" for a home TV antenna.

    First, a basic analogy: Consider a large tank of water. First, assume the water is perfectly still. How high the water is in the tank is analogous to the DC voltage. That's pretty basic, and you get that. You also get that ripples in the tank are like an AC signal. I can splash it a little with my hand and make small waves, or splash more, and make larger waves. That's AC voltage, or a signal.

    But of course the tank can be any combination of how full, and how big a wave I can make. -- up to a point – well -- 2 points, actually.

    At some point, if the tank is nearly empty, and I make big waves, the bottom of the tank will be exposed, and the waves won't look the same. Similarly, if the tank is too near the top, and I make big waves, water will splash out, and again the waves will be distorted.

    This is basically bias. Biasing means filling the tank someplace in the middle, so that I can make big waves (within reason) without water splashing out, and without exposing the bottom of the tank.

    The plate of a tube has a very large (and dangerous) DC voltage on it, but it also, and simultaneously, carries our audio, which may be only a few volts riding along on top of hundreds (or in, for example, RF transmitters -- thousands) of volts.

    So how do we get both DC and AC at the same time? Well, there are basically 2 devices I can use to combine or separate the two. By the way, that is important – combine – or – separate. If you are guessing diode or rectifier, you just failed the quiz. Although a diode can separate 2 AC signals (called a detector), I’m not talking about a detector or rectifier.

    The first is a capacitor. Let’s visualize (because I do not have the time or ambition to do a drawing at the moment) a wire with a DC voltage on it only. Now, let’s connect a Capacitor and Resistor in series, and then to ground, and then wait “a long time” – well beyond the RC time constant of the “R” and the “C”. Who said “Filter”? – 20 yard penalty. – I said the DC voltage is pure DC – There is nothing to filter. Also, there is a resistor in there, so if there was “noise” on the DC, it wouldn’t be a very good filter. What have I built? Answer – nothing. The capacitor is charged to the DC voltage. There is no longer any current flow through the capacitor or resistor. Electrically, the capacitor and resistor are not there to a steady state DC voltage.

    But now, what if I take that exact same circuit – a wire at the top, and a capacitor and resistor connected in series, and then to ground -- but instead of DC, I apply an AC voltage to the wire. Assume that the AC voltage or signal is much higher in frequency than the RC time constant of the “R” and the “C". Now the capacitor is a short, and all of the current in the wire flows through the resistor. An oscilloscope would (in theory – nothing is perfect) show exactly the same signal in the wire and on the other side of the capacitor (between the capacitor and resistor.)

    OK, so what if there is both? Say 100 volts DC, and a 1 volt peak-to-peak AC signal (again, at a "high enough" frequency). The capacitor will still do the same thing for each case, but it will now be doing them simultaneously. It will block the DC voltage, and pass (without change) the AC signal (audio, perhaps?) onto the resistor. This is using the capacitor as a “DC block”.

    BTW, this works the other way, too. I can use a capacitor to add an audio signal to a DC offset (assuming the DC wire has sufficient impedance, which is beyond the scope of this article.)

    We see this on the input to a tube amp, and between stages (Say it with me: “Coupling Capacitor”). Yes, it is oversimplification, but think of coupling capacitors as doing nothing to (passing) the audio, while fully blocking the passage of DC.

    One more note – This is exactly what happens when you select “AC” or “DC” on your Digital Multi-meter. The AC setting places a capacitor of "sufficiently large value" in the circuit; the DC setting bypasses that capacitor. Checking plate voltage with the meter set to AC will show you the amplitude of any signal (audio) that is playing, but it could give you a dangerously misleading idea of the DC voltage. Similarly, setting it to DC will show only a DC offset, and not the magnitude of any AC that is present.

    So what is the other device? A Transformer. You may have learned that transformers “transform” one voltage to another. Forget that for just a moment. I didn’t say it isn’t true. In fact in the case of a tube amplifier’s power transformer, that is exact what it does, but for just a moment, and for exactly half of all of those moments when you think of output transformers or inter-stage transformers, I want you to forget that.

    A transformer is also a DC block.

    Again, think of that wire with pure (again in theory) DC on it. I connect that wire to one side of a transformer's primary winding, and the other side of the transformer primary winding to ground (and we wait longer than the time constant of the winding). Again, I have built absolutely nothing useful. Assuming that I am within the ratings of the transformer, it shouldn’t even get hot. The transformer is said to be "saturated", but there will not be any voltage on the secondary regardless of the turns ratio or the (steady state, DC) voltage across the primary (again, within transformer ratings.)

    OK, this may be getting a bit repetitive at this point, but what if I put AC on the primary instead of DC. Let’s assume for the moment that it is a 1:1 transformer, and that it is “ideal” – you know, just as things in the real world are not. Also, my AC signal is of sufficiently high frequency to be within the bandwidth of the transformer. Just as with the capacitor, whatever AC signal I see on the primary, I should see exactly the same thing on the secondary. Again, I have manage to build -- and use lots of words to describe -- exactly nothing.

    OK, what if there are several hundreds of volts DC on the wire that is connected to the primary, and a relatively small AC signal superimposed on it. The AC is going to cross the theoretically perfect transformer unchanged, but none of the DC is.

    Guess what – again, assuming a perfect transformer, and sufficient impedance on the DC source – this works the other way, too. I can use a transformer to couple an AC signal onto a DC potential.

    Yes, turns ratio, impedance etc. counts for tranformers, but for half the time you think about it, think of a signal transformer as a DC block.

    So, if both block DC, and pass AC, and both can be used to couple AC onto a DC bias, what is the difference? Why do we use both in tube amplifiers?

    I'll continue if there is any interest...


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  2. BillWojo

    BillWojo AK Subscriber Subscriber

    Burlington, NJ
    Interesting read, I like the way you explain all of this. Some of those concepts took a long time for me to understand when I was learning basic electricity but I've never really had any formal training. Just years of troubleshooting machines and when I got into audio, especially tubes, constant reading about what makes these things tick.
    Continue with your writing, I'll go along for the ride.

  3. dsndblm

    dsndblm AK Subscriber Subscriber

    Central Arizona
    Great information. I'm following along!
  4. manu et deo

    manu et deo I'm loving it! Subscriber

    SW Riverside County, CA
    Please do, I'm enjoying the style as well.

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