The bad reputation is imho for the people who perpetuate others' perceptions.
The Williamson started before 12AU7 Noval was in general manufacture (early 1950), and so it is interesting to see various clone/variant manufacturers take on the 12AU7 instead of the 6SN7 during the 1950's. As such the 12AU7 was deployed in to a large number of quality hifi amps and in huge quantity, at the time that hi-fi valve amps were in full swing.
Although the 12AU7 does impart a higher level of distortion than a 6SN7 in that particular amplifier (I've run my own distortion testing of 15x 6SN7 and 8x 12AU7), by far the dominant amp distortion contribution in the amp is from the output stage valves as signal level approaches rated power.
The Williamson started before 12AU7 Noval was in general manufacture (early 1950), and so it is interesting to see various clone/variant manufacturers take on the 12AU7 instead of the 6SN7 during the 1950's. As such the 12AU7 was deployed in to a large number of quality hifi amps and in huge quantity, at the time that hi-fi valve amps were in full swing.
Although the 12AU7 does impart a higher level of distortion than a 6SN7 in that particular amplifier (I've run my own distortion testing of 15x 6SN7 and 8x 12AU7), by far the dominant amp distortion contribution in the amp is from the output stage valves as signal level approaches rated power.
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I agree, and I ran similar tests with a somewhat smaller overall sample size more than 25 years ago.
Again compactness, lower cost (including sockets) was a motivator, just imagine a Leak Stereo 20 or Acrosound 20-20 amplifier and how much larger either would have been if octal tubes had been used.
A large component even then had to be economics and size reduction. (I design consumer electronics for a living)
Again compactness, lower cost (including sockets) was a motivator, just imagine a Leak Stereo 20 or Acrosound 20-20 amplifier and how much larger either would have been if octal tubes had been used.
A large component even then had to be economics and size reduction. (I design consumer electronics for a living)
i agree, and the Bottlehead's Foreplay line amp is nothing to scoff at...
An externally hosted image should be here but it was not working when we last tested it.
Similar, but the curves tell a somewhat different story. The 6CG7 and 6SN7 are closer to each other than the 12AU7A is to either of them. All can give satisfactory results in well designed circuits.
My early budget line stage design used the 12AU7A in the then trendy SRPP circuit where it acquitted itself well. (I was trying to make things people would buy, and the whole world of good HV mosfets was more than a decade in the future)
That's not the tube's fault, it's yours (the designer). If your circuit needs a more linear tube and you choose one that's less linear, it's on you.
There are many reasons and many circumstances where you need or want to make certain choices and compromises. For example the 12AU7 takes half the heater power of a 6SN7. And many others described in posts above. If the choice is a bad one, it's the circuit designer/manufacturer that deserves a bad reputation, not the tube type.
pre pandemic, our 12au7 line amp buildout in 2019 was a success, those who build their kits, were even surprised that our line amp sounds better than a McIntosh tube kit....of course that was a china kit.
Not sure we have an argument over that.. LOL Cost, size, power consumption, and expected performance are all valid choices. Ultimately if you apply global feedback around everything the linearity of individual amplifier stages is less critical, if you don't then a good understanding of trade offs vs goals is required.
My 12AU7A line stage sounded pretty good actually, a similar design using 6SN7 measured somewhat better and that was intentional, both nice in the intended context.
The tube operating points in that demonstration were each optimized for that particular tube. All of the tubes were running all the time, and the switch merely selected which one we were listening to. The result that struck me most forcefully was that, while almost every tube was number one on somebody's list, a large majority selected the 12AX7 for second place ...
Incidentally, a good 12AX7 is remarkably linear, more than even a good 6SN7. Its limitations are high input miller capacitance and low output current capability, not distortion.
If you want linearity and no distortion use a transistor. What's the point of designing a valve amp as a straight wire with gain. When the OPT is thrown into the mix linearity and ultra low THD is harder to achieve,for me building valve amps isn't about 0.001% THD and a frequency response of less than 1dB from DC to sunlight. Personally I think the ECC82/12AU7 is absolutely fine, not shite at all.
Diabolical Artificer,
Transistors are so linear, that amplifiers employing them can use 50 or 60 dB of negative feedback.
Ask those who know about Portland's own Hi Fi company, Audionics, that had a high negative feedback transistor power amplifier that tested so good on a power resistor load.
When the amplifiers were sold, many of them blew up, and had to be returned, and either repaired or the money had to be returned. Those wonderful amplifiers could not deal with real loudspeakers.
The next amplifier from Audionics had more distortion, less negative feedback, but they actually survived when connected to real loudspeakers.
And, I believe they sounded good.
Transistors are so linear, that amplifiers employing them can use 50 or 60 dB of negative feedback.
Ask those who know about Portland's own Hi Fi company, Audionics, that had a high negative feedback transistor power amplifier that tested so good on a power resistor load.
When the amplifiers were sold, many of them blew up, and had to be returned, and either repaired or the money had to be returned. Those wonderful amplifiers could not deal with real loudspeakers.
The next amplifier from Audionics had more distortion, less negative feedback, but they actually survived when connected to real loudspeakers.
And, I believe they sounded good.
THD v frequency won't show anything interesting, just a flat line. THD V level is what you want. My book is full of such plots.
A 12AX7 will not put out large signal swings into a medium impedance load.
But instead, used in a circuit that is appropriate for it, it will have lots of gain.
A 12AU7 will be able to drive a medium impedance load, but it used in a circuit that is appropriate for it, it will only have medium gain.
Both tubes are good, when used appropriately for what they were designed for.
Do not expect a high rev low torque Indy Gas Turbine engine (it only raced one year) to directly drive the rear tires differential gear (or more likely a posi-drive shaft).
But instead, used in a circuit that is appropriate for it, it will have lots of gain.
A 12AU7 will be able to drive a medium impedance load, but it used in a circuit that is appropriate for it, it will only have medium gain.
Both tubes are good, when used appropriately for what they were designed for.
Do not expect a high rev low torque Indy Gas Turbine engine (it only raced one year) to directly drive the rear tires differential gear (or more likely a posi-drive shaft).
Also something to consider is that although none of these tubes, even the 12AU7, need negative feedback you can use a touch with any one of them if you want to get some more linearity and consistency. With transistors you pretty much have to use feedback but no law says we can't employ some local feedback with our tubes.
