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Old 11th October 2010, 12:15 PM   #11
DF96 is offline DF96  England
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I have used an ECC81 as the first stage in a modified 5-10 power amp, to replace the EF86. Signal levels at low volume are not much higher than a phono input. Noise is very low. Gain is high enough to overcome noise in a passive RIAA network, and the low anode impedance means that the network can be lower impedance than one driven from an ECC83.

However, we need to bear in mind that a valve which is designed for low noise at VHF is not necessarily low noise at audio frequencies, as it depends on where flicker noise starts to be a problem. Flicker noise depends on details of cathode construction, so can vary significantly for the same nominal valve type.

I would only use an ECC82 (or 6SN7) as a cathode follower if I happened to have a spare triode available, and it was a non-critical position. I am surprised by the number of people who use it. I suspect that some of them are confused: they may think that having a low anode impedance means that it is a 'low impedance' valve so OK for CF service. If I had to add an extra valve anyway then ECC81 or ECC88 would be better.
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Old 11th October 2010, 12:41 PM   #12
Salas is online now Salas  Greece
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If we go examine the equivalent noise resistance of triodes for use in phono head amp then ECC81 is 3 nVrtHz when ECC83 is 5.6 nVrtHz. Any Rk and grid stopper will push it up. We generally look for high gm low Rp. Some pentodes like E810F or D3A can go low enough en when trioded and holding enough gain still. I would highly recommend to read SY's ''His Master's Noise'' article, has an excellent and easy to follow intro on phono tube head amps and noise concerns. SY'S link.
Amplifying from MC level with tubes competently is out of the question without help of an input transformer. That is low Rbb BJTs and paralleled high gm JFETs territory.
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Old 11th October 2010, 01:48 PM   #13
Merlinb is offline Merlinb  United Kingdom
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Originally Posted by Salas View Post
If we go examine the equivalent noise resistance of triodes for use in phono head amp then ECC81 is 3 nVrtHz when ECC83 is 5.6 nVrtHz.
Where do you get these figures from?

By the size of them I suspect you used the old EIR = 2.5/gm formula. Does anyone know the origin of this formula, as I find it highly dubious, at room temperature anyway... (Yeah I know it's in RDH4, but it's not referenced.)

Last edited by Merlinb; 11th October 2010 at 02:01 PM.
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Old 11th October 2010, 02:30 PM   #14
SY is offline SY  United States
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There's actually an excellent derivation of the 2.5/gm approximation for triodes in Elmore and Sands. Their discussions of low noise tube design overall are first-rate.
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Old 11th October 2010, 03:42 PM   #15
Merlinb is offline Merlinb  United Kingdom
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There's actually an excellent derivation of the 2.5/gm approximation for triodes in Elmore and Sands.
You mean this book?
http://www.archive.org/details/Elect...ntalTechniques
(still looking for the derivation)

Last edited by Merlinb; 11th October 2010 at 04:01 PM.
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Old 11th October 2010, 04:38 PM   #16
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Amplifying from MC level with tubes competently is out of the question without help of an input transformer.
I AGREE. However, that does not stop people from trying, with things like the 12AX7. Maybe, just maybe, it can be done with the 6C45П (6s45p). IMO, the bother involved to obtain a marginal result is unwarranted.

JMO, use either a SUT or a hybrid cascode (ala Allen Wright), with LOMCs.
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Old 11th October 2010, 07:20 PM   #17
Salas is online now Salas  Greece
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Quote:
Originally Posted by Merlinb View Post
Where do you get these figures from?

By the size of them I suspect you used the old EIR = 2.5/gm formula. Does anyone know the origin of this formula, as I find it highly dubious, at room temperature anyway... (Yeah I know it's in RDH4, but it's not referenced.)
From Vogel's ''Sound Of Silence''. RNtriode=3.06/gm for cathode temp 1100K, room temp 300K.
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Old 11th October 2010, 07:34 PM   #18
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Anyone ever checked out the 12AD7, got a matched pair here waiting for a job to do.

Simon.
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Old 11th October 2010, 07:55 PM   #19
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U. Pictured with Russian NO
NNIICCEE !!

Looks like your packing more iron than Dirty Harry with that little baby dude !

Simon...
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Old 11th October 2010, 08:16 PM   #20
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Anyone ever checked out the 12AD7, got a matched pair here waiting for a job to do.

Simon.
High impedance low gm (1.6). Can do as good as an ECC83. Its low microphony and hum special though, and preferable when signal is low.
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