Hi all,
I’m trying to learn the basics of tube amp building and I’m reading Robert Megantz “design and construction of tube guitar amps”. Most of it makes sense to Me but one thing I don’t understand. It’s probably the simplest thing but the book doesn’t explain!
When you use a guitar pickup, it can create a voltage signal up to 1v. He constantly talks about the control grid voltage as being negative. -.05 to -1.5 for example. He also references the negative grid voltage relationship to plate current.
How is it possible to achieve a negative voltage at the grid, when the guitar signal is a positive voltage signal? I’m confused, and feel really dumb lol
I’m trying to learn the basics of tube amp building and I’m reading Robert Megantz “design and construction of tube guitar amps”. Most of it makes sense to Me but one thing I don’t understand. It’s probably the simplest thing but the book doesn’t explain!
When you use a guitar pickup, it can create a voltage signal up to 1v. He constantly talks about the control grid voltage as being negative. -.05 to -1.5 for example. He also references the negative grid voltage relationship to plate current.
How is it possible to achieve a negative voltage at the grid, when the guitar signal is a positive voltage signal? I’m confused, and feel really dumb lol
the negative voltage is with respect to the cathode, not necessarily ground. Usually a resistor is used on the cathode, or a battery is used in the grid circuit.
Here's a good place to start. http://www.sportsbil.com/other/Basic%20Electronics,%20Volumes%201-5,%20(1955).pdf
Here's a good place to start. http://www.sportsbil.com/other/Basic%20Electronics,%20Volumes%201-5,%20(1955).pdf
It is negative in respect to cathode voltage. Cathode current causes positive voltage drop on cathode resistor, while the grid is referenced to 0 volt on the common wire AKA "Ground". That means, the voltage on the grid in respect to cathode is negative, because a voltage on cathode in respect to 0 is positive.
Ahhhh! Thank you! This explains it much better than my book. I found a section that had a diagram of a working grid and it makes perfect sense now.
Also keep in mind that when talking about bias voltage it is a steady state DC voltage. The output of the guitar pickup is an AC signal that will be superimposed on the DC voltage on the grid.
The signal goes both ways, its average is zero. This is the same as a guitar string that vibrates around its resting position.
What mashaffer said is key!
Audio signals go both positive and negative. Tubes only work with negative signals. How to marry the two, then? Add a steady negative voltage to the audio signal: now it goes from a wee bit negative to even more negative, rather than from positive to negative. Presto, problem solved.
-Gnobuddy
As said, the guitar swings both ways around zero volts.
We normally set the tube grid to zero volts (cheap and cheerful), bias-up the cathode to positive 1V-2V.
The "signal is negative" relative to *cathode* until peaks rise above cathode bias.
Through the 1950s, pickups and input stages converged to the levels and biases we still use today. (Before mid-1950s we sometimes had hard-grounded cathodes and trickery for small to medium negative grid bias; humbuckers' high output changed fashion away from those designs.)
We normally set the tube grid to zero volts (cheap and cheerful), bias-up the cathode to positive 1V-2V.
The "signal is negative" relative to *cathode* until peaks rise above cathode bias.
Through the 1950s, pickups and input stages converged to the levels and biases we still use today. (Before mid-1950s we sometimes had hard-grounded cathodes and trickery for small to medium negative grid bias; humbuckers' high output changed fashion away from those designs.)
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OH: and tube-men commonly refer all electrode voltages to *cathode*.
But in practical builds we often jack-up the cathode a volt or two with a resistor. Then a ground-referenced grid is negative *with respect to cathode*.
But in practical builds we often jack-up the cathode a volt or two with a resistor. Then a ground-referenced grid is negative *with respect to cathode*.
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