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Old 10th December 2012, 04:21 PM   #1
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Default Driving 211 with VT25 or EL34?

I'm building a 211 tube amp and I don't know to driver the 211 with EL34 or VT-25. personal preference is VT25 but I want your idea
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Old 14th December 2012, 07:15 AM   #2
tyo94 is offline tyo94  Indonesia
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With EL34 to drive you got more 'power' and tight bass, as I experience in my friend's 300B amp. But I don't know about VT25, I haven't experience that tube for driver function yet. But i like the sound of VT25 (10Y) as power tube....very nice detail and stagging
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Old 14th December 2012, 07:28 AM   #3
cotdt is offline cotdt  United States
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Of course VT25 if you want a linear driver tube.
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Old 14th December 2012, 07:55 AM   #4
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Originally Posted by cotdt View Post
Of course VT25 if you want a linear driver tube.
+1
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Old 14th December 2012, 08:22 AM   #5
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Can somebody tell me the reason to use power tubes to drive 211 and 845 triodes ?
In case of A2 it is obvious that a low impedance driver is required.
I have understood that mostly these tubes are driven in A1 and then I can not see any technical reason to use EL34 sort of tubes.
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Old 14th December 2012, 08:58 AM   #6
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Originally Posted by artosalo View Post
Can somebody tell me the reason to use power tubes to drive 211 and 845 triodes ?
In case of A2 it is obvious that a low impedance driver is required.
I have understood that mostly these tubes are driven in A1 and then I can not see any technical reason to use EL34 sort of tubes.
Because even if you are in A1 class you have to override the miller capacitance of the next tube. Try for instance to drive an 211 using a 12ax7 and you will hear the difference. have a look over here, Miller Capacitance
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Old 14th December 2012, 09:18 AM   #7
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Originally Posted by satelitis View Post
Because even if you are in A1 class you have to override the miller capacitance of the next tube. Try for instance to drive an 211 using a 12ax7 and you will hear the difference. have a look over here, Miller Capacitance
look over here too Miller Capacitance AGAIN!
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Old 14th December 2012, 10:58 AM   #8
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Originally Posted by satelitis View Post
Because even if you are in A1 class you have to override the miller capacitance of the next tube....
The Miller capacitance of 211 is ridiculously small, smaller than Cm of 12AX7.

Mu of the 211 = 12, but when the stage is loaded, the gain is some 7,5...8.
According to the RCA 211 data sheet, Cgp = 14 pF and Cgf = 5,4 pF.
This gives the total input capacitance: Cin = 14 pF x 8 + 5,4 pF = 117,4 pF.

As a reference: Cin of 12AX7 is some 150...170 pF.

So it is obvious that the Miller capacitance of 211 or 845 does not require EL34 to be (over) driven.

So can somebody give other reasons ?
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Old 14th December 2012, 12:06 PM   #9
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Hi!

If you want to go for the DHT driver, I would rather suggest the 801A or VT62 because you can run it at higher plate voltage and thus it has more headroom.

I have built a 211 amp with the 801A. Some pics can be seen here:

VinylSavor: Making of a 211 Amplifier, Part 1: Planning

The article is about another 211 amp but it contains pics of the 801A/211 monos.

DHT tubes are very linear and they can provide very clean drive voltage swings which such tubes require. Since the 801A runs at about half the plate voltage of the 211 you can use a power transformer with a center tap. use a full wave bridge for the output stage B+. Put another filter chain at the center tap and it will give half that voltage.

The filament supply is even more critical at the driver stage than the output stage.

However keep in mind that DHT driver will only have a mu of 8. if you want to have reasonable input sensitivity an input transformer withs tep up is needed, say 1.4


Best regards

Thomas
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Old 14th December 2012, 12:11 PM   #10
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Hi!

Quote:
Originally Posted by artosalo View Post
So can somebody give other reasons ?
Grid current effects start to kick in way before the grid of the output tube gets positive. Even if the amp is designed to operate in Class A1, it will still have to fight with that at volume peaks. In case it is over driven it can recover more quickly with a power driver.

Another reason is headroom. Few small signal tubes can provide the voltage swings needed to drive a 211 or even a 845 cleanly. If you want headroom this becomes only viable with power tubes. I want 6dB headroom minimum.

Best regards

Thomas
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