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Jeffrey D. Medwin

Anyone experienced with a Coronet or Coronet 2 Phono Stage Preamp ?

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What are your thoughts of it?  How does it compare to other tube phono stages ? 

 

I have been thinking of using the 12AX7 stages' circuit, my power supply,  and adding an anode-driven 12B4 output stage, instead of Jim Hagerman's Cathode Follower.

 

Jeff 

 

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On 11/5/2019 at 6:19 AM, canyonman said:

Why? What's wrong or not to like about the Cathode Follower?

 

Thats easy. 

 

When both choices ( anode-drive, cathode-drive ) are properly executed, and generally speaking, a cathode follower does not sound quite as good - as the music's signal being anode-driven.

 

In more familiar-to-audiophiles' terms   this is perhaps why some prefer the sonics of a simple, modest two tube DYNA PAS-3 RIAA EQ, when it is well modified, over the three tube ARC SP3A-1 RIAA EQ, having a cathode follower.  

 

In my home system, I purposely use no line stage preamp, to avoid the extra circuitry.  I just have a DIY 10K System Attenuator,  Ladder type, with source-switching ( selection ) , to feed my SE amp(s). 

 

A solid state Blu-Ray player (Pioneer Elite BDP-09FD ) in use, can easily drive 10K.  But a tube RIAA stage, normally can not.  Jim Dowdy ( " The Dowdy Lama " ) came up with the idea this year of using a high current triode, to DRIVE 10K from the anode,  to avoid a cathode follower.

 

This takes over 33 mA. of idle current, a whopping amount -  to do this !!  So, I eliminated the Coronet's 12AU7 Cathode Follower third stage.   I figured out how to directly couple the 12B4A to the preceding two Coronet RIAA EQ stages.  The direct couple, avoids a cap in the circuit path,  between the two stages.  Good !!

 

Its hilarious in a way, the stock Coronet's four 12AX7 RIAA EQ sections draw 3/4s of a mA. each,  ( right " on the money " ) or 3 mA. total, but the two 12B4A tubes will use a total current draw of  66.66 mA, ....to drive 10K from the plate.  The 12B4A  triode is a linear tube, decent sounding. 

 

Jeff Medwin

 

868230618_12B4AGraphLARGE.thumb.JPG.bd5090b66f87d6e36a93b2051c55fe72.JPG

 

 

 

 

12B4A.pdf    ------------------------------------------Click on this to see the interesting G.E. datasheet on a 12B4A.

 

Next is a comment "The Dowdy Lama" just made to me in a email this morning, on 11-7, re Cathode Follower avoidance :

 

786917561_JimonPlateFollowers11-7-19.thumb.JPG.9ab875c59b10cbf678530662e3d6270c.JPG

 

 

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Something new :  'Am thinking of doing a phono stage using Russian 6C45Pi tubes, and a 12B4A Finals stage,  in lieu of 12AX7s for the EQ.  

 

Any of you tube listeners have experience with the 6C45pI?   How does it strike you ?   Comments welcomed.  ( Bruce Rosenblit's Phono stage uses 

a 6C45Pi in the first stage, and a 12AX7 in the second ).  I've never heard it.  Have you ??

 

I think, use of a 6C45Pi in both EQ stages, is commercially unavailable, and limited to a DIY build effort,  I am talking to Jeremy presently.

 

http://home.earthlink.net/~jeremyepstein/6c45riaa.html

 

IF I do this, I designed a brute-force power supply, tube rectified, ' got it all PSUD simulated - for it.

 

Jeff Medwin

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Interesting learning experience, last few weeks and days.   

 

A vacuum tube Passive RIAA EQ circuit, that is feed forward, ( and not using negative feedback ), typically has many thousands of Ohms of series resistances added to its plate-driven circuit.  This is done by almost everyone, as " conventional EE wisdom " because the added resistance assists as tubes age, and, in primarily - getting the circuit to very closely follow the RIAA EQ.

 

Here are some passive RIAA circuits that come to my mind, with their added series resistances :

 

Thomas Meyer ( 6SF5-6J5)  200,000 Ohms added  

Original RCA phono EQ  ( 12AX7-12AX7)  470,000 Ohms added

Eli Duttman's modified RCA ( 12AX7-12AX7) 255,000 to 265,000 Ohms added

Jeremy Epstein's ( 6C45Pi-6C45Pi) phono stage, 28,670 Ohms added.

Bruce Rozenblit's ( 6C45Pi, 12AX7 ) phono stage 91,000 Ohms added.

Andrae Ciuffoli ( D3A-D3A) 220,000 Ohms added. 

Salectric  ( D3A-5687)  21,500 Ohms added

 

Whenever you take the audio signal in a passive phono EQ, and pass it through a any such series resistor, ( as above ) you have a sonic degradation, VS a resistor not being there at all.  People who design and listen, will all tell you " the less resistance in series there is, for the signal to drive through, the better the preamp sounds. "

 

I discovered, over the last three weeks, that perhaps three high end phono Manufacturers, ( or more - perhaps some that I am unaware of ), might be using NO added series resistances at all, for the signal to pass through.  They all believe an L.P. music-source,  played back through such a phono stage,  SOUNDS BETTER to them !! 

 

How VERY interesting.  The decades-long conventional strict adherence to the RIAA curve, is possibly totally trumped by play back, sounding better, with zero series resistances used !!!! 

 

Well, as part of this thread, and as part of my tube phono preamp's design, I have decided to have NO added series resistances in my upcoming phono build. I also decided to not use the lovely 6C45Pi ( highly variable, and it can oscillate ).  Rather, I'll design with the 12AX7, or one of its variants, as a more conservative possibility, including perhaps even the use of a 12BZ7.  I have several really good thoughts on how to precisely implement a 12AX7,  it takes extra care.   

 

B+ supplies in the new design will ( uniquely / heavily ) be double-series-shunt fed,  for each B+ use-point.  If the entire audio circuit draws 70 milliamperes, this planned double-series-shunted supply may draw an additional 110 milliampreres, so it becomes a 180 milliampere Phono stage ..... like a small power amp, will be the current requirement.  Two tube rectifiers, not one, and three series B+ chokes, all three chokes, each under 7 Ohms in DCR.  Henry Pasternak's "Flywheel" input filter.  15 Ohms total - for all three, is the series choke DCR.   Phono preamp will use a 10 AWG m22759/11 Mil Spec ( stranded copper-silver plated )  ground buss .  Am PSUD-optimizing this supply, doing many iterations of simulations these days.  Truly much work, but fun stuff. 

 

ADDED  11-26-2019  :

 

 

1844710286_SNIPMASTEREDITED2.jpg.32011ac2986f63f1cc5c2b7a495cca5c.jpg

 

 

 

 

No wheres in audio, has anyone ever built a supply with the topology as seen above. 

 

Most people seriously underestimate the need for a truly great preamp ( and amp )  power supply.

 

Ever see a phono stage that starts with a 525 mA. Power Transformer, dual directly-heated tube rectifiers,  and uses a below 1 Ohm low DCR Input choke ??  And has a separated brute force double-series-shunted B+....... for each and every triode section?   What is this design going to sound like???   I think I know already, from my 2019 KT88 DC amp build, which I had designed to be double-shunted, ( for the amp's Input/Driver stage B+. )  This new phono preamp, funds permitting, is to be built and its performance determined - hopefully in 2020. 

 

Jeffrey Medwin 

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