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Old 17th April 2008, 10:25 AM   #11
SY is offline SY  United States
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Quote:
Originally posted by oldeurope
Good moring,
I recommend the ECC83 12AX7.

Kind regards,
Darius
It is generally not a good choice for a CF. Besides the low gm, the current is low which means available drive current is very low- try to swing more than 0.1mA and distortion will rise precipitously.
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Old 17th April 2008, 10:27 AM   #12
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I think the input impedance of the next stage is 50KΩ.
Thus the ECC83 has the lowest loss in gain. (0,98 @2mA)
This is why I like the ECC83 here.

Kind regards,
Darius
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Old 17th April 2008, 10:31 AM   #13
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Quote:
Originally posted by oldeurope
I think the input impedance of the next stage is 50KŮ.
Thus the ECC83 has the lowest loss in gain. (0,98 @2mA)
This is why I like the ECC83 here.

Kind regards,
Darius
At 2mA, you'll have some grid current issues with actual 12AX7s.

The gain for an ECC88 in this position is 0.97. The difference between that and a tube with a mu of 100 is less than 0.1dB.
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Old 17th April 2008, 10:46 AM   #14
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Quote:
Originally posted by SY


At 2mA, you'll have some grid current issues with actual 12AX7s.

The gain for an ECC88 in this position is 0.97. The difference between that and a tube with a mu of 100 is less than 0.1dB.
Hello,
for 2mA at 240VDC you have -1,5V. Where is the problem?

Kind regards,
Darius
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Old 17th April 2008, 11:01 AM   #15
SY is offline SY  United States
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Put one on the bench and do some measurement with non-zero source impedance.
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Old 17th April 2008, 11:14 AM   #16
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Quote:
Originally #15 posted by SY
Put one on the bench and do some measurement with non-zero source impedance.
Hello SY
The source has a high impedance this is why I use a cathode follower.
Scroll down here:
http://radio-darius.blogspot.com/
V10 is the cathode follower V9 is its constant current draw.

Kind regards,
Darius
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Old 17th April 2008, 11:24 AM   #17
SY is offline SY  United States
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Quote:
The source has a high impedance this is why I use a cathode follower.
Exactly. That's why you want bench tests of the CF to be taken with the CF being driven by a high source Z. The distortion spectrum will be illuminating, and the source of the distortion will be evident as you look at the spectrum while reducing the driving source Z.
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Old 17th April 2008, 11:57 AM   #18
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Default D3a, E280F

Not exactly cheap and not even double triode, but D3a and E280F are definitely excellent (maybe the best?) for cathode followers when triode strapped and even better in pentode mode CF.
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Old 17th April 2008, 01:35 PM   #19
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Quote:
Originally #18 posted by panos29
Not exactly cheap and not even double triode, but D3a and E280F are definitely excellent (maybe the best?) for cathode followers when triode strapped and even better in pentode mode CF.
Hello panos29,
whow yes, the D3a looks good. µ(g2)=80 and high transconductance.

Kind regards,
Darius
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Old 17th April 2008, 01:44 PM   #20
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Question ???

#17
Hi SY,
I am still confused. What do you want to tell us?
Something like no cathode follower is better?

Kind regards,
Darius
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