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Magnatone Schematic Question

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  • Magnatone Schematic Question

    Hi everyone,

    I feel like this question might have been asked before some time, but I wasn't able to find anything searching old posts. Forgive me if I'm repeating old info. I've been reading a lot of old Magnatone schematics recently, and I keep getting confused by their use of "A" and "B". This is most likely a dumb question to ask, but is it safe to assume that every line with an arrow pointing to "A" should be connected together, and the same for "B"?

    Sorry again if this is really too simple. It just seems strange that all these leads would just connect together in the same place, unless it was ground, which they clearly mark in other places.

    Thanks everyone!

  • #2
    Yes, it is exactly that simple. A and B are B+ voltages from the power supply. Imagine how much more cluttered the drawing would be if they drew lines all over to connect all those points. At each point in the circuit that connects to a B+ supply, they just used the A or B symbol. This is extremely common practice. A,B,C are one way, while some other drawers use B, B+, B++, etc. X,Y,Z would be just as good. As long as the drawing is consistent. Every tube section needs a connection to the power supply, so yes, they all connect to the same place. Instead of a letter, they could have used just a + in a circle.

    unless it was ground, which they clearly mark in other places.
    Well, if you look, there is the three-line ground symbol in at least 13 places that I see. It is not surprising they all connect to the same place. Yet, there they are, 13 separate ground points. Think of the ground symbol and imagine a G in a circle instead. Look at B in each preamp channel - at each B I see four resistors going down into the circuit. But if I look at the input stages, I see the ground symbo, and 4 or 5 parts coming up from it. It really is the same thing.

    Other things will also use such a convention. For example in a complex amp with channel switching, there might be several JFETs or optos or something, and they all run off the same control signal. Instead of drawing wires all over, they just show each device with its control pin arrowed to some abbreviation - CTRL for example. Maybe in a large mixer, there are mute transistors on all the output jacks. The schematics would just show the transistors with their gates again controlled by a signal called MUTE. Somewhere on the drawing would be the circuit that generates the MUTE signal.

    Many drawings are split into several pages, and signals and other things must get from one page to another. Most drawers have a convention for that as well. A signal might exit page 1 as EQ-OUT - some point in the circuit would have an arrow pointing to that label. Page 2 would also have a point labelled EQ-OUT, and the signal would continue from there on about its business.
    Education is what you're left with after you have forgotten what you have learned.