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Old 12th December 2011, 12:04 PM   #11
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Quote:
Originally Posted by SY View Post
OK, so no measurement or calculation, just assertion from the seller.

Using a more conventional engineering approach, one should expect 120-150pF, maybe somewhat more, depending on strays from wiring (which can be considerable with four paralleled sections) and how optimistic the datasheets are.
Yes!
This design is not the same as datasheets, as you've seen before I like to use datasheets, each tube 6.30 40mA using bias, Pre almost like a power amplifier.
Sounds very, very hard and expensive electricity too!

you can use 120-150pF , if you like it! or not use the value or me!
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Old 12th December 2011, 12:19 PM   #12
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OK, measurement
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Old 12th December 2011, 12:22 PM   #13
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Quote:
Originally Posted by quanghao View Post
By practical experience!
Curious: what is the 'practical experience' you have had that provided you with the 10pF number?

I can understand if you say, "well in my experience the input capacitance does not negatively affect the sound", it's another thing entirely to say your experience provided you with a number.
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Old 12th December 2011, 12:59 PM   #14
DF96 is offline DF96  England
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Does the OP mean that his anode-grid capacitor is 10pF? If so, that means Miller capacitance of around 500pF at the input, when the valve capacitance is included too. To get a -3dB point at 20kHz requires 16k source impedance, so volume pot must be less than 64k. A 50k pot would give -3dB at 25.5kHz at mid setting, and better at higher or lower volume.

Feasible, but I am not a great fan of circuits which significantly vary their frequency response with volume setting. This could, however, in the hands of a journalist or saleman be called a positive feature as the sound could come into 'focus' at certain settings, this demonstrating the 'fine discrimination' of the poorly designed equipment.
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Old 12th December 2011, 01:26 PM   #15
Charm is offline Charm  Russian Federation
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You had to write my best preamp.
Miller is more of course. And of course the parallel path is not good for fidelity.
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Old 12th December 2011, 03:02 PM   #16
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For 6N30P: Cgk=6.3pF, Cga = 6.0pF
Given circuit gain = 21 dB, A = 11.2
Miller capacitance: 11.2 x 6.0 pF = 67.2 pF (for one tube)
Input capacitance: Miller + Cgk + strays (say, 5 pF) = 78.5 pF (for one tube)
Total Cin for two tubes = 157 pF
Source inpedance = 47 k, ---> f(-3dB) = 21.5 kHz

Does anyone agree ?
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Old 12th December 2011, 03:10 PM   #17
SY is offline SY  United States
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Art, it appears that there are four sections in parallel, so minimum of 300pF by your calculations. My guess would be a bit higher because all of that wiring is likely to increase Cgp.

I'm still scratching my head about the 10pF.
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Old 12th December 2011, 03:13 PM   #18
DF96 is offline DF96  England
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Don't forget the 10pF cap he added. He has four triodes in parallel. I used voltage gain of 30 (valve mu), as it didn't occur to me that he might be deliberately running them into a low anode load to get extra distortion. Anyway, we are still talking about 100's of pF so the line stage input C could exceed likely cable C.

The source impedance is likely to be lower than 47k. Even a bad source will have an impedance not much above a few k, and then a 100k volume pot will give a maximum source impedance just above 25k.
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Old 12th December 2011, 04:14 PM   #19
Yvesm is offline Yvesm  France
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Quote:
Originally Posted by artosalo View Post
For 6N30P: Cgk=6.3pF, Cga = 6.0pF
Given circuit gain = 21 dB, A = 11.2
Miller capacitance: 11.2 x 6.0 pF = 67.2 pF (for one tube)
Input capacitance: Miller + Cgk + strays (say, 5 pF) = 78.5 pF (for one tube)
Total Cin for two tubes = 157 pF
Source inpedance = 47 k, ---> f(-3dB) = 21.5 kHz

Does anyone agree ?
Almost !

You've just forgoten the "magical" deliberatly added 10pF that makes this preamp "the best"

Here is "The Better Preamplifier Of The Whole Know And Unknow Universe" :

Click the image to open in full size.



Yves.
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Old 12th December 2011, 04:29 PM   #20
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I looked carelessly at the schematic and did not notice the added capacitor from the grid to anode.
But I could not find that there would be 4 tube halves ( 2 x 6N30P) in parallel.
Is it really so ?
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