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popilin 25th February 2012 08:20 PM

ECC82/12AU7 Line Preamp
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Hi All
ECC82/12AU7 is like a Cinderella between tubes, as a simple exercice I sim an SRPP Line preamp and the result is, at least, unexpected.
It seems to me that we can drive an 845 whith this modest circuit.
Cheers :cheers:

jmillerdoc 15th March 2012 11:58 AM


Salas 15th March 2012 12:00 PM


popilin 15th March 2012 01:33 PM

Yes, +B=650V , filaments float at +180V.
Only a few valves can tolerate this level.
The problem with low +B is the well known electron cloud effect.
Try the same circuit at +350 and you will see the difference.

DF96 15th March 2012 01:36 PM

Electron cloud effect?

SY 15th March 2012 01:50 PM

If you look at how a 12AU7 mu varies with plate voltage, the seemingly nutty idea of putting 320V across it starts making sense. I suspect the actual results won't look much like the sim- even with operation on the flatter part of mu vs Vp, there will be a lot of higher order harmonics. But if you're going to extract the most out of a tube with poor linearity, you have to take extreme measures.

Of course, you could get better performance with a different tube and different circuit and not deal with 650V supplies, but that's not a polite thing to say. :D

DF96 15th March 2012 01:56 PM

Yes, the 12AU7 is a reasonable approximation to a square-law device over part of its characteristic so OK for LTP or cathode-coupled amp (so-called 'grounded grid') where the 2nd gets balanced out. Not so good for SRPP. The plot shows 2nd only. 2nd harmonic OK, 2nd IM not so nice.

During start up one triode could get 650V for a while, so scope for flashover?

popilin 15th March 2012 02:01 PM

Yes, electron cloud effect, that's one reason for tetrode and pentode invention.
The electrons between cathode and anode disturb the pass of new electrons.
Charges with same sign tend to repel each other.
I had seen that many times in the lab, didn't you made Thermionic Effect experiment, or measure the Planck constant?

popilin 15th March 2012 02:06 PM

This is the idea, make that Cinderella can sing better.

Hi DF96
Start up isn't a problem with a delay circuit that connect +B with filament warm

DF96 15th March 2012 03:56 PM

I'm not sure if by 'electron cloud' you mean space charge. If so, it was not the reason for inventing the tetrode/pentode. The tetrode gives more gain, especially at higher frequencies, by shielding the potential on the anode from the electron stream.

I hope you are not running the triodes with sufficiently high voltage and current that it overcomes the cathode space charge. This will create noise and shorten valve life.

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