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ECC82 pre with cathode follower
ECC82 pre with cathode follower
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Old 13th November 2019, 02:56 PM   #1
disco is offline disco  Netherlands
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Default ECC82 pre with cathode follower

As a first finger exercise in LTSpice I composed this little volume control. For some reason the ECC82 (12AU7, double 6C4) is popular for making distortion, so let's use it. Low output impedance is a necessity, so the CF is king. There is a misunderstanding about the cathode follower giving it a bad reputation. For driving a huge load like a SS poweramp with 10K input impedance (sic) it takes a type that combines high current with high gain (a property called conductance). ECC82 is like 6SN7 a low mu type which makes a lousy CF. Keeping SS out, I opted for the available 6C45n (or 6S45pi but it's all russian to me..) with high gain and high transconductance. A (really) wise old tech once told me to formulate goals before ordering parts, so one doesn't have to lie about its performance when finished. Let's christen her Bourgeois Sandwich and let her do these tricks:
- be a nice girl and behave with little distortion, unless pleasing
- no hysteria, so behaviour should not suffer from oscillations
- naturally, what comes in gets out, so no amplification unless ordered
- turning the knobs should give the desired effect: half volume at 12 'o clock.

A first design.
Attached Images
File Type: jpg ECC82 pre V2b.JPG (92.0 KB, 632 views)
File Type: jpg 6C45pi.JPG (54.8 KB, 616 views)
File Type: jpg whatsthis.JPG (174.3 KB, 613 views)
File Type: jpg transconductance.JPG (8.3 KB, 639 views)
File Type: jpg home-entertainment2.JPG (134.5 KB, 612 views)
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File Type: zip take2.zip (13.7 KB, 12 views)

Last edited by disco; 13th November 2019 at 03:01 PM.
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Old 13th November 2019, 04:11 PM   #2
nigelwright7557 is offline nigelwright7557  United Kingdom
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I would have just used one ECC82 instead of two different valves.
I am a big fan of the ECC82.
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Old 13th November 2019, 05:21 PM   #3
disco is offline disco  Netherlands
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Are you possibly related to Dirk Wright? It didn't end well with him either.. Personally I think ECC82 is a stinker but what happens when we arrange a marriage to a sputnik? Bear with me, fireworks to be expected..
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Old 13th November 2019, 05:56 PM   #4
artosalo is offline artosalo  Finland
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Quote:
Personally I think ECC82 is a stinker but what happens when we arrange a marriage to a sputnik?
Just a disappointment if Sputnik is grounded with 33k cathode resistor.
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Old 13th November 2019, 06:01 PM   #5
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by disco View Post
Are you possibly related to Dirk Wright? It didn't end well with him either.. Personally I think ECC82 is a stinker but what happens when we arrange a marriage to a sputnik? Bear with me, fireworks to be expected..
No relation !
The ECC882 has got me out of bad situations with ECC83's a few times.
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Old 14th November 2019, 04:31 PM   #6
kodabmx is offline kodabmx  Canada
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ECC82 pre with cathode follower
ECC83 is a stinker, too

6N*P >> 12A*7 IMHO.
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Old 15th November 2019, 09:42 AM   #7
disco is offline disco  Netherlands
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Both smell funny but they have a different field of application. I've just finished a test rig and the model does not hold: 6C45P promised 20mA standing current where practice shows just over 4mA. Back to the drawing board
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Old 15th November 2019, 10:35 AM   #8
artosalo is offline artosalo  Finland
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What is that operating condition?
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Old 15th November 2019, 11:37 AM   #9
disco is offline disco  Netherlands
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See att. R7 has to come down but it's in four parallel resistors. I'm aiming at 2,5V/20mA=125R
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Old 15th November 2019, 11:58 AM   #10
artosalo is offline artosalo  Finland
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Your operating point is quite extreme, i.e. -Ug is high (-4.3 V) and therefore Gm is low.
At the attached photo it is marked with red dot.
I would use something like the operating point marked with green dot.

Increase R8 to some 5k6 and fine tune (=search) R7 to get some 20 mA anode current.
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File Type: jpg 6S45P-E.jpg (114.8 KB, 195 views)

Last edited by artosalo; 15th November 2019 at 12:02 PM.
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