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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Medium mu posted by Adagio on March 7, 2014 at 15:04:37:
Please refer to any vacuum-tube textbook about tube parameters.
For triodes (or triode-connected pentodes) the "mu" is a "good" parameter, i.e. it is reasonably constant over a range of operating voltage and current. For an ideal tube, it is the ratio of the effects on the electrostatic field at the cathode due to the plate votage and the grid voltage. In a high-mu device, the grid is a more effective shield (compared with a low-mu device) against the voltage on the plate affecting the field at the cathode. Therefore, all other things being equal, the high-mu device will have a lower negative cutoff voltage than a low-mu device, and the low-mu device will have higher plate current at zero bias than the high-mu device.
If the tube is operated with a constant-current (very high resistance) load, it is then the voltage gain from grid to plate.
The other two parameters are the transconductance "Gm" and the plate resistance "Rp". As Gm is a strong function of plate current, so is Rp, since the three parameters are related by mu = Gm x Rp.
This is true for "normal" low-power and power triodes.
An important exception to this are "variable-mu" or "remote cutoff" tubes, where the mu is a function of grid voltage (think of a high-mu and a medium-mu triode connected in parallel, where the high-mu triode cuts off at a reasonable grid negative voltage, leaving only the low-mu device in the circuit). These tubes use a non-constant winding pitch on the grid.
For pentodes, "mu" is not a useful parameter, since it is very high and not constant. There is a "screen to grid 1" mu value, which is basically the mu of an equivalent triode formed by the cathode, grid, and screen (in lieu of plate), which is useful for calculating operating current when changing the screen voltage.
Edits: 03/10/14
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Follow Ups
- RE: Medium mu - TimFox 09:34:29 03/10/14 (0)
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