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I ordered a couple of Siemens ECC82's described as "medium mu". I understand "mu" means amplification. Does it mean these "medium mu" ECC82's have lower gain that normal ECC82's, i.e., of lower quality?
Thanks!
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TubeAcolyte got it, low, medium and high are common descriptors for tubes based on their Mu. High gain tubes are generally useful for preamps and low signal applications. They aren't for delivering power. Low Mu tubes generall have more grunt and are used in power supplies. Most OTL amps use low Mu tubes because of their current passing ability. The medium Mu guys are tweeners, as they approach the borders of low and high, some designers have learned how to coax the desired performance out of them. Don't think that medium or low Mu is a deficiency due to (relatively) reduced gain.Their designs are part of the witches brew of capabilities exploited by designers with a purpose. As stated the three fundamental triode parameters, mu, gm and rp, are generally functions of the plate current you can tolerate. many tube data sheets show which are more sensitive to Ip. If you don't know or understand this you should find a basic book on tubes, the three parameters are coupled and constrained by the tube's power handling ability. if you think there are too many balls in the air at one time, you're right. That's why many of the classic designers found something that worked and stuck with it, and why we amateurs explore new combinations of tubes.
Sorry for being a redundant windbag,.
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
Calling the ecc82 a medium mu tube was written by the department of redundancy department.
It's not an indication of quality. "Quality" (electrical characteristics, tolerances, and life expectancy) are almost always identified by a suffix or industrial/military designation. It's kind like when you learn that calling a square a rectangle and vice versa are two different things; there's only ONE right answer. So, for example, a 7025 is a 12AX7, but not the reverse; a 7025 is a compatible but superior design. Just the same, a CV4004 is a military-quality ECC83 (12AX7), but a standard ECC83 is not a CV4004. And it's not all cherry-picking within tolerances, but sometimes a question of materials and overall design (spiral filament heater, better cathode materials, different plate construction, welding, getter design, quality of the vacuum, and more)
"High, mid, low mu" is more of a designation of intended purpose and sorta translates to the layman into high, medium and low gain. Of course, there's loads more to a tube than just its amp factor, but that's the gist of it.
If I'm correct, low-mu is around 10-30, medium is 30-60, high-mu is 60-100. I may be slightly off, but that's the idea. If you look up, for example, the GE data sheets on the 12AU7, AV7, AT7, AY7, and AX7, you'll get an idea as to how they throw around the terms.
Careful! My experience is that power tubes are not advertised in the same fashion. They have an amplification factor, yes, but it's not the same (a question of current rather than voltage gain). It's only small signal tubes, triodes specifically, that get high-mid-low treatment. I may be wrong, but I can't remember any examples of power tubes advertised in the same sense as tiny tubes.
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May your tubes be lively and long-lasting. Holy be thy heater.
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