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Old 27th August 2011, 06:35 PM   #31
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Ask yourself why VT1 ,VT2, and VT3 all have working points dependant upon
Gm curve, rather than linear Mu? This design wastes 3 SITs by disabling their
best secret weapon, dumbing them down to ordinary FETs.
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Old 27th August 2011, 06:37 PM   #32
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Quote:
Originally Posted by VladimirK View Post
VT1 is an ordinary jFET, not SIT.
OK then, 1 transistor not wasted. I am still not seeing the other two
utilized in a way that makes use of Mu.
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Old 27th August 2011, 06:49 PM   #33
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Quote:
Originally Posted by kenpeter View Post
OK then, 1 transistor not wasted. I am still not seeing the other two
utilized in a way that makes use of Mu.
You can be right in some aspects, and of course can design very different thing. I have tried some other modifications, SRPP-like, and did not choose them.
I would be extremely interested in your listening reports, about more perfect schematics. There are many of them, at the tubecad pages.
In all cases, when I loved some schematics theoretically, no cap in the signal chain etc., I did not manage to get a sound. Main tricks relate to noise level, using of shunt PS concept, parts selection, design induced EMI, slew-rate, etc., i.e. to aspects not directly linked with linearity. In spite that this schematics looks as very trivial, it can not be approached sound-wise by varios other preamps laying around.

Last edited by VladimirK; 27th August 2011 at 07:04 PM.
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Old 27th August 2011, 07:01 PM   #34
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Do you have triode model for the 2sk79?
I have no SITs, only a box of 6DJ8 if I had to prove something...
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Old 27th August 2011, 07:38 PM   #35
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Default 2SK79 spice model

Kenpeter, see if this will help you.
Don't ask me about it.
Attached Files
File Type: txt 2SK79spice.txt (783 Bytes, 86 views)
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Old 27th August 2011, 08:05 PM   #36
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Faking Mu aint all that hard either.

Granted, some cropping was involved.
Looks less convincing outside the box.
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File Type: gif SillySIT.gif (34.1 KB, 365 views)

Last edited by kenpeter; 27th August 2011 at 08:11 PM.
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Old 27th August 2011, 08:36 PM   #37
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This type of output curves (artificial triode) is the general property of
all transconductance amplifiers (high output imp.) with the voltage FB.
Vbe multiplier is one example. Opamps with the voltage FB will also
have this type of output curves.
Sure, one transistor is the second shortest FB path (loop), (after "emitter"
follower).
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