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Major diff between 12AX7 and 12AU7

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They're very very different tubes, with different purposes in mind. Only the pinout is identical, heater is the same and the power rating is similar. But mu, rp, gm, noise figure, distortion, microphonics... all different.

(the difference is that ECC83 sounds good where is needed, ECC82 cannot sound good because it's not an audio tube :D )
 
Yup, A 12AX7 is a high mu/gain (100), high rp-low gm tube used as an AF low-level/hi-gain twin triode.
A 12AU7 on the other hand is a medium mu (17), medium everthing, designed for vertical oscillator/vertical amplifier in B&W Television sets! Not considered good for quality audio :yuck: by many because of not-so good linearity when used at the operating points used for AF voltage amplifiers. Seen many times used as a concerta phase splitters in power amps tho and cathode follower circuits because of the resulting low(er) output impedance when used in this manner. And because of the indentical filament requirements as the 12AX7 and permissable maximum voltage specs. A 12BH7 would've been a little better!
With different bases and filament requirements the 6FQ7/6CG7 or the octal 6SN7 are better alternatives w/approx. the same gain.
Maybe too much info but you asked for it! :D

Wayne
 
There appears to be a cult following around the 12au7. I have noticed some asking good money for some select 12au7's. They seem to be known for being a little coloured but it is pronounced 'rich mids' :D

I have tried them, they seem to work a little better at more than 8mA or so. They are capable of giving a fine performance if you happen to like the flavour (and if you get a good one).
 
In his excellent book “Valve Amplifiers”, Third Edition, Morgan Jones presents a useful analysis of the distortion spectra of several tube types using a common test bed. The big winner was the good old 6SN7, not surprisingly. One the worst of the common tube types was the 12AU7, which he labeled “particularly ghastly”. It produced significantly more odd and even distortion that many of the other tube types. Of course he didn’t test every brand and style of 12AU7 / ECC82, nor every possible bias / operating point, but the trend is clear among the 28 samples that he did test. IMO, there appears to be crowding of the plate curves in the high voltage region which would explain this problem.
 
Originally posted by Giaime (the difference is that ECC83 sounds good where is needed, ECC82 cannot sound good because it's not an audio tube :D ) [/B]

Please Giaime, stop to spread around this urban legend. Take a look at one datasheet, for example this one:
http://www.mif.pg.gda.pl/homepages/frank/sheets/127/1/12AU7A.pdf

it is clearly stated that this tube is designed for audio amplifier.
The people that wrote these documents weren't used to qualify themselves as 'tube guru', they simply knew their job very well.
So 12AU7 is definitively an audio suitable tube.


On the 12AX7 versus 12AU7 issue, I remind You all that there is a tube that comprises the best of both: 7077, a small ceramic tube of 10mA/V gm, Rp of 9000ohm, for a mu of 90 and Ip of 11mA.
Difficut to use because of the ring metal contacs but of absolute quality.
 
The 1973 Mullard databook lists:

ECC81 - R.F. double triode
ECC82 - double triode
ECC83 - double triode
ECC85 - R.F. double triode

ECC83 is often listed as low noise and intended for audio, but NOT in the 1973 book.

Commonly you find the ECC83 in the front end, and the ECC82 as the driver or phase-splitter in a push/pull amp. From a servicing point of view I would replace them with the designed valves - but from a fault finding point of view, or a "oh my god the amps broke and we've got to play NOW", I would happily exchange them. It really makes VERY little difference in practice!.
 
plovati said:


Please Giaime, stop to spread around this urban legend. Take a look at one datasheet, for example this one:
http://www.mif.pg.gda.pl/homepages/frank/sheets/127/1/12AU7A.pdf

it is clearly stated that this tube is designed for audio amplifier.
The people that wrote these documents weren't used to qualify themselves as 'tube guru', they simply knew their job very well.
So 12AU7 is definitively an audio suitable tube.

Ahahaha!!! A legend!!!! :D It's years that here we say that ECC82 isn't so linear. And I'm spreading a legend? Morgan Jones too, Stuart Yaniger too? We're all deaf and stupid? :xeye:

Have you read Morgan Jones book? :xeye: :xeye: :xeye:

I only trust measurements, Piergiorgio. Here are some:
http://www.geocities.com/la1zka/hifi/distortion.html

Now, will you say that those measurements are bad and the one who made them doesn't understand nothing about tubes, or will you admitt that ECC82 is a very nonlinear tube?
 
Ahhh ...another Internet "Tempest inna Teapot" :)
12au7's do work 'jes fine' thank you.. In a adequately designed circuit.
Nothing wonderfull about ax7's either , just even easier to implement but there is always a 'better' solution in any effort.

Old obsolete tubes are Yesterdays News... get over it.
 
Giaime said:


Ahahaha!!! A legend!!!! :D It's years that here we say that ECC82 isn't so linear. And I'm spreading a legend? Morgan Jones too, Stuart Yaniger too? We're all deaf and stupid? :xeye:

Have you read Morgan Jones book? :xeye: :xeye: :xeye:

I only trust measurements, Piergiorgio. Here are some:
http://www.geocities.com/la1zka/hifi/distortion.html

Now, will you say that those measurements are bad and the one who made them doesn't understand nothing about tubes, or will you admitt that ECC82 is a very nonlinear tube?

12AU7 is dsigned for audio. 12AU7 linearity is comparable to tubes like 12BH7 and 5687, very well regarded in audio community and used by the audiocraft masters.
Measurements taken by the same condition for all different tubes are meaningless. Each audio glass wants be fueled by the proper wine.
I saw the Morgan Jones books and exactly for this reasons I do not care too much of his sentences.

I strongly reccomend You and all the Italian speaking people to read the Luca Chiomenti articles, published under 'una valvola al mese' series in Costruire HiFi (the good old one, till numbers 55 or so).
 
Whether or not a particular tube was originally designed for audio service is only a minimally useful discriminator for us today. Many of the best tubes for audio were originally intended for other applications, such as RF service, computer switching, or series pass service in power supplies. Some tubes designed for audio, such as the 7199, don’t have great performance by purist standards and were merely optimized to lower costs in commercial stereo gear or had heaters optimized to cancel AC heater supply hum, allowing cheaper supplies to be used. This is hardly an advantage where DC heaters supplies are used today. A few tubes intended for audio DID live up to the hype of course, such as the 300B.
 
Giaime said:
Those are the reasons why I don't trust 50 years ago tube datasheets about tube applications.

So, I suppose You measure plate characteristics of every single tube You wants to use?
Why trust people that designed and made millions of vacuum tubes, extrapolating data from a really statistical significative lot?
Why trust data that were provided by manufactorer when tube businnes were strong, competition too, and customers very competent and carefull?
Why engineers that send man on the moon (with tubes and bubbles memory) should be more skilled in the art than the Space Shuttle Columbia engineers?
 
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