12AU7 Phono Stage - Page 2

DF96 · 11th October 2010, 12:15 PM

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.

Salas · 11th October 2010, 12:41 PM

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.

Merlinb · 02:01 PM

Quote:

Originally Posted by Salas

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.)

SY · 11th October 2010, 02:30 PM

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.

Merlinb · 04:01 PM

Quote:

Originally Posted by SY

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)

Eli Duttman · 11th October 2010, 04:38 PM

Quote:

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.

Salas · 11th October 2010, 07:20 PM

Quote:

Originally Posted by Merlinb

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.

The Space Egg Corp · 11th October 2010, 07:34 PM

Anyone ever checked out the 12AD7, got a matched pair here waiting for a job to do.

Simon.

The Space Egg Corp · 11th October 2010, 07:55 PM

Quote:

Originally Posted by Salas

U. Pictured with Russian NO

NNIICCEE !!

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

Simon...

Salas · 11th October 2010, 08:16 PM

Quote:

Originally Posted by The Space Egg Corp

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.

11th October 2010, 02:30 PM	#14
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	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. __________________ If there's a sucker born every minute, where do the rest of them come from?

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11th October 2010, 12:15 PM	#11
DF96 diyAudio Member Join Date: May 2007	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.

11th October 2010, 12:41 PM	#12
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	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.

11th October 2010, 07:34 PM	#18
The Space Egg Corp diyAudio Member Join Date: Sep 2010 Location: London & Miami Blog Entries: 13	Anyone ever checked out the 12AD7, got a matched pair here waiting for a job to do. Simon.