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Phono preamp with modern pentode as first stage

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karsten21

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Joined 2010
#1
Dear folks,

After building a MM Phono preamp using JJ´s ECC803S ( It sounds absolutly great! ) I would like to start to build an MC version. I read some threads and the marvelous Artikle from SY which lead me to try to use a modern pentode as first stage. I know, there are quite better tubes like D3a as NOS available but I love challanges: So I decide to use tube which are in current production only. I bought tow JJ´s EF806S which are my preferred pentodes for this project.

Here my question:
Any hints about best connection ( wired as triode or pentode ) and/or bias point of the EF806S?
Are there any other modern (new, current production ) small signal pentodes available?

Thanks a lot for your input!

best regards
Karsten
 
#3
Hi Merlinb

For sure: I would like to use input transformer 1:10 like Sowter 9570 or Lundahl LL9206 or Jensen JT-44K-DX. It depends on mechanical dimensions and price. I think the difference of quality is negligible.

Thanks for your question!
Best regards
Karsten
 
#5
Hey Sodacose,

Thanks for this hint. I know Pete´s Phone Pramp.His solutio is one of the reason why to try to use a pentode in the first stage. But Pete use an 5879 pentode which is a NOS part. I Stuart´s article the wonderful D3a is used. Wenn unfortunatey there is no similar new tube in market. Lot of this great famous old tubes will be lost for ever in the next years. It´s a sad trueness..

Best regards
Karsten
 
#6
Pete Millett touched on an important point, noise. There is no way around the fact that pentodes introduce partition noise. JMO, the EF86 family is just shy of being quiet enough for the 1st gain block of a phono preamp. Perhaps a CCS loaded 6922 section in the 1st gain block and a voltage follower buffered EF86 in the 2nd gain block would get the job done, while exhibiting adequate S/N performance.

High gm contributes to better noise performance. While not in production, the 6AC7 has the sort of high gm (9 mA./V.), which suggest it could be quiet enough, partition noise taken into account, for 1st gain block service.
 
#8
I'd strongly recommend you reconsider the pentode and go for a cascode front end instead.

http://www.diyaudio.com/forums/analogue-source/246756-muscovite-mini-6-9-6z9p-phono-stage.html

http://www.diyaudio.com/forums/analogue-source/273817-muscovite-mini-iii-6n23p-phono-stage.html

The two threads chronicle my experience designing a pentode based phono stage using the 6J9P, and what followed from that. (There are other threads if you search on "Muscovite")

Interesting project and I will be following.
 
#9
Eli Duttman said:
<snip>
High gm contributes to better noise performance. While not in production, the 6AC7 has the sort of high gm (9 mA./V.), which suggest it could be quiet enough, partition noise taken into account, for 1st gain block service.
Click to expand...

I used much higher gm pentodes and still had some noise problems relating mostly to operating point and its relationship to 1/f noise. I also had some problems with the audible signature with 6J9P and D3A pentodes.

As an aside most of the really good phono stage projects are located in analog source, this one certainly seems worthy, let me know if you'd like it moved.
 
#12
Bondini, Kevinkr

Sorry for my delayed answer. I had a hevay working woeekend.. without tubes 😉

There is hope now! Half a Year ago I read the patent from Mr. Blöhbaum using a current source for G2 to prevent partition noise. ( Well, seemed to me a "state of the art" technique from older ages, but now it is fixed to Mr. Blöhbaum... )

Unfortunately the prefered EF806 has a gm of 2.2mA/V only. This lead to a higher ra ( If I understand the dependencie µ=ra*gm correctly ) and therefore to a higher noise.

Your tips and suggestions show me, that I should start to "play" a little with the EF806.
Thanks a lot for your input!!

Best regards
Karsten
 
#13
Hello Karsten,

I´m using the E810F in pentode mode as the first stage in a phonopreamplifier. Even with 0A2 for the g2. I don´t have any problerms with noise.

You should read the following article from Frank:
https://linearaudio.net/article-detail/2071

There he explains how to use a pentode with less noise and distortions as in triode mode. It also contains noise measurements of different tubes.

Best regards,
Martin
 
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