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Jeffrey D. Medwin

Thinking of doing a Dynaco / DYNA PAS Preamp

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Hello, 

 

Last Wednesday, I blew up my 1979 Fulton /Nexus Bravura stereo phono and line stage  preamplifier, as I was adding mods to it.  I am NOT good at all, doing solid state work, so this was no surprise.  'A risk I took.

 

I can't personally fix it, ( its solid state ) and there are only two people living who know the circuit.  I'd have to take it to Kansas City, where a GOOD technician with 40 years high-end audio-repair experience may ( or may not ) be willing to work on it.

 

As I see it, I have three choices :

 

1)  See if the Kansas City audio repair technician will repair the Fulton dual-mono Bravura preamp

 

2) During  Q4, 2019,  on-paper,  I fully designed an elaborate / deluxe all- tube  phono preamp, ( B+ nodes double-series-shunt-regulated ).  I could scratch-build that,  'have many of the parts on hand

 

3) Safest path of all, simply modify / rebuild a Dynaco / DYNA PAS 2 or PAS 3.

 

Since 1979, I have learned a lot from two bright mentors, on tube design and building, that should be applied - to any PAS in 2020 !!  

 

As of today, Option number three appeals to me the most.   These contemplated  Dynaco  PAS mods would be UNLIKE all the typical ones everyone does.   I have well established design prejudices and priorities !!  These are firmly established from all the years of tube amplifier building. I've never built a phono preamp !! 

 

As an experiment, Saturday night I augmented the runs on the stock Dyna PC  board, to determine if it could be done.  Everyone else uses the stock PC board runs, with out much thought.  The traces on the Dyna boards are THIN, so they exhibit poor transfer efficiency.   Also, the PC board traces are of lower sonic quality, compared to using some of the better-quality, real-wire available !! 

 

My first experiment I did last night : to see if myself, or others, could augment those PUny thin traces by adding ( soldering to them )  copper multi-stranded, silver-plated Mil Spec wire, ( and NOT ruin the known-to-be-flimsy stock PC boards. )  Well, 16 AWG was too large, but 20 AWG wire augmentation seems to be carefully doable .  Hooray.  See :

 

     P1010057.thumb.JPG.ffbfa0d99bc9e3652a580be201061110.JPG

 

                               (   Above :   2-29-20,  Experimental  -  BOTTOM wire augmentation is for B+, TOP wire augmentation is for GROUND.  Real wire !!    )

 

 

 

Some other design thoughts I have - are as follows :

 

1) Use no one else's after-market mod-boards or circuits, stay close to the stock Dyna approach

 

2) Retain the tube rectifier,  a 12X4.

 

3) Eliminate stock ( and all ) electrolytic capacitors from the supply, such as DYNA's electrolytic  " quad can ".  Uniquely,  give the component an ALL FILM cap power supply, hopefully, WIMA DC LINK film caps.  ( Am PSUDing this more, next week )

 

4) Unlike any other mods I have seen, input the 12X4 rectifier tube into a choke, and NOT into a capacitor.  Choke input filter - only.  There is room to do this, if I neatly bolt the stock Power Transformer to the rear panel, away from the RCA jacks.  Install the  added supply choke where the stock Power Transformer was usually positioned. 

 

5) Apply extensive use of QUALITY internal wire, such as, Mil Spec ( m22759/11 ) and also, solid silver - content wire, for audio signal path wiring.  No silly PC board traces for the audio' signal runs.

 

6) Mount the two PC boards, new choke, and external power transformer off the vibrating thin chassis, using the brass " Pyramid" washer stacks as employed in the KT88 thread of 2019 .

 

7) Mount the two PAS PC boards above the inner chassis metalwork, raise the boards 9/16th of an inch off that internal chassis floor, using brass isolation / anti-vibration pyramids.  This will create room to beneficially ADD parts below the boards.  Below the boards,  I can individually add a small high-quality B+ L/C localized final filter, intimately to each plate resistor, on all eight triode sections.  In effect, each of the eight triode sections  would each have their own local and independent power supply, with zero distance lead lengths, nicely-buffered from any and all of the other tubes' operations.  Wonderful.

 

I have figured out how to mount the ALL-WIMA film B+ main power supply caps into a Dyna unit, safely, space-wise, this Sunday afternoon.  Really cool / efficient  techniques, never ever done before.

 

I have a request to make.  I THINK I have located a ripped -apart PAS chassis / metalwork locally.  Not sure yet. It will be totally missing the top cover, missing the Power Transformer, and any of the stuffed P.C. boards. 

 

                                                                                                                                REQUEST,

 

Can anyone assist me ??  Do ANY of you gentlemen have some trashed - out, non - working, tube - less  Dynaco PAS units, in storage, collecting dust,  that you really won't be using, that you could donate to this build project ?? 

 

I would be happy to pay for packing and shipping of donated incomplete units !!  ETC. I am thinking of, and hoping to be able to post a Klipsch Forum phono preamp build article, with photos and descriptions, much like my March 2019 KT 88 DC SE amp Thread.  ( 138,225 Views so far )  In 2020, there should likely be no interference from the on-line stalker -  who purposely disrupted that 2019 thread.

 

Please P.M. me with odd-ball Dyna chassis or PAS parts you have, and I will proceed to ask my audio Mentor ( a high-end Manufacturer) if he is agreeable to me discussing some details that are HIS ideas, not fully mine !!

 

Thanks for taking your valuable time, to read this.  Thank you very much.

 

Imagine how such  a modest but well-wired PAS, with all-film B+ caps, and eight independent / buffered B+ supplies, one for each triode section,...... will play-back an L.P.

 

Jeffrey Medwin

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DONATION # ONE on March 5, 2020  :

 

A  PAS2  partial Chassis, received as of today.  Anyone got same or other PAS parts I can acquire ?   

 

Need a top cover, so I can take ALL the metal work , all at ONE time, down to a local Automotive Paint Shop, for sand blasting and professional painting.   

 

There is cadmium on the PAS  metal work,  that I'd really like to bury under a pro-paint job !! 

 

1454854097_P1010001edited2.thumb.jpg.94501406a9f2ed1596c4b60c507550d2.jpg

 

 

See JAC;s write up of a 6E5P-I, kissing cousin to a 6E6P-DR, here:

 

Triode-Curves-6E5P-I(2) JAC.pdf

 

 

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I love my phono stage and preamp (as separates) from tubes4HiFi.

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13 hours ago, Marvel said:

You could have started with  a replica board from Triode Electronics and had a good base, even if you wanted to add to it. They don't cost much,

 

http://www.triodeelectronics.com/paspc5blank.html

 

 

Hi, 

 

Thanks, that board looks nice, the traces are a bit wider than the stock early vintage Dyna boards which is good..  That would allow me to augment the traces a bit easier with Mil Spec m22759/11 and with solid silver wire.  I MAY buy that board, except, in the PC6 designation, which is the Phono RIAA EQ baord..  Appreciate you showing it to me.  Thanks.

 

The Triode Electronics PAS power transformer is advertised as having " half the DC Resistance" of the sock original 15 mA. power transformer.   However, their high voltage winding measures 1,155 Ohms,  which can be bettered by a Hammond Power Transformer, and it can fit on the rear vertical wall, at about 150 Ohms DCR.  The Hammond is over SEVEN times lower and far better IMHO, and 14 times lower in DCR then all the stock PASes in operation.   To read and understand the importance of having low DCR in a audio power supply, review Dean of E.E, Dr. Charles A. Halijak's 1987 letter,  "Clue Number Two " where he outlines " The Figure of Merit of a Power Supply ".

 

              https://community.klipsch.com/index.php?/topic/182623-kt88-direct-coupled-design/&do=findComment&comment=2381499

 

As per Dr. Halijak's 1989 letter, I will add and use a 10 Ohm input choke - right after the rectifier, and change the ( typically nice sounding ) tube rectifier, from the stock PAS's 12X4 ( 525 Ohm effective plate resistance ) to a quiet 5AR5, ( 160 Ohm effective plate resistance ) which is over three times less in effective DCR.

 

Lets give the Tin Man a new and strong HEART !!!

 

Out with the Brasso today, to see if I can " save"  that PAS 2's brass faceplate , and get it looking better.  Still need a PAS top chassis cover. Anyone have one for this build-project??

 

Jeff Medwin

 

 

PS : Notice how linear are the curves of the triode-connected tetrode 6E6Pi, the Line Stage's single tube stage tentative candidate :

 

                      1574869990_Bartola2013.JPG.a866765cb6e9d0f9d2e0ce97a37c9b01.JPG

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3-06-2020 PROGRESS

 

Removed stock PAS 2 brass face plate and prepped the surface.  Used 1,000 grit wet and 2,000 grit wet sandpaper for deep scratches, some Brasso to remove oxidation, and polished out the brass  with Mother's Mag and Aluminum Polish.  'Lotsa hand rubbing.  Had a bend in one corner, but I have a highly talented local friend who builds hot rods, full time.  He straightened the PAS 2's brass faceplate, bent maybe 5 /16ths of an inch, so one could not tell it was ever damaged.  Seven minutes work, and now I owe him lunch next week.  It looks very good to me now.

 

Anyone have a old ratty PAS, for parts , I can acquire by any means possible.??  Need a top - of- chassis cover, ASAP, so I can take ALL  the chassis metalwork parts in,  all at one time, for sand blasting, and professional automotive painting. 

 

 

477410761_P1010003edit.thumb.jpg.f8648a0f57cd88a4e7438c1d7067b96d.jpg

 

 

 

 

                                                                                            3-06-2020. below...... " What a Difference a Day Makes "

 

 

P1010010.thumb.JPG.24eeee343fb22598fc3b60afd6a7a014.JPG

 

1982162953_P1010007edit.thumb.jpg.e5a74082b059e810856dbb3f79673799.jpg

 

                                            Lots and lots of hand rubbing !!  Has a highly-reflective smooth brass finish.   Like Jadis equipment.

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3-07-2020 PROGRESS

 

Positioned the chassis mount power resistors today and drilled mounting holes, inside the rear vertical PAS's back panel..  These two Rs form the unique series-double-shunted B+ supply to the phono stage. (*1)  I really like these SIMPLE SHUNT arrangements.  They sound good to me, is the first and foremost thing about them, VS traditional; ( many parts ) active regulators. Secondly, in particular, I just LOVE the way two SHUNTS in series sounds, as it results in exceptionally high signal resolution.  This will be perfect for a phono EQ stage.

 

Need to add several other parts to the PAS's rear vertical panel next, a octal rectifier tube socket, a 10 Ohm choke, and a ON-OFF AC switch.  Am positioning parts and drilling holes, prior to sending this out for metalwork refinishing. 

 

Have been PSUD simulating power supplies this morning, and it looks like I have settled on a Hammond Power Transformer model, that will do justice to the circuit.  The stock PAS uses a 15 mA. power transformer, and I am installing one rated for 123 mA, continuous.  This will have a large difference in the power supply's DCR .

 

 

 

(*1)  No where in all of tube audio design's history, have "I" ever SEEN a schematic or gear that had a double series shunt B+ supply to the Phono EQ stage !!   It is unique, AFAICT,  to this preamp.

 

                                        272184836_P1010019edited.thumb.jpg.1a4f46dfad2da1b15cf24f56d44c85bc.jpg

 

 

 

                                     

 

Anyone have a PAS top cover I can buy, so I can get the metalwork over to the professional metal  refinishers ??

 

 

 

 

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3-08-2020 PROGRESS

 

Positioned and test-mounted the 10 Ohm choke, rectifier tube socket, and the 3 Ampere ON-OFF switch, onto the Dynaco PAS 2's rear vertical chassis panel, Sunday.

 

Will order a new Hammond ( over-speced ) power Transformer next, and keep looking for a chassis top-cover, so I can ship all metal work off to the professional refinishers, and have the work done just one time.  Slow progress being made.  Cool to do !!

 

The new tube rectifier, a 5AR4,  has 1/10th the effective resistance of a stock PAS 2's rectifier.  The 5AR4 inputs to the 10 Ohm DCR choke.

 

 

  1908490719_P1010022edited.thumb.jpg.0303ac7c018b51ebf3ba0c9fa04faf19.jpg

 

  66316007_P1010035edited.thumb.jpg.62ae6d0eadc4eca2d7dd5dd0e8664163.jpg

 

          

  

 

 

         

 

 

Jeff Medwin

 

     

 

     

 

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Jeff,

 

I'm curious about your polishing of the faceplate. I presume the lettering is silk screened or painted. Did you avoid sanding the lettering in order to preserve it?

 

Michael

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Here is a new idea !!!   I am contemplating this, over the next 24 hours : 

 

Instead of building a DYNA PAS - like phono stage,  with a negative feedback RIAA EQ on a PC (  on a real-wire augmented printed circuit board ) , why not build a Hagerman Coronet type RIAA EQ, which is not a NFB EQ, but rather, a two 12AX7 passive feed-forward circuit, and it would be ALL point -to-point wired.  Unlike Jim Hagerman's Coronet, however, I will couple the Coronet's two 12AX7 stages to each other DIRECTLY,  (and eliminate the Coronet's use of one series signal coupling cap, ). Use a short length of low-music-loss silver wire between those two 12AX7 stages.!!  

 

Very nicely, in Q2 2019, i TOTALLY ENGINEERED / designed such a preamp circuit,  inspired by the Coronet passive EQ.   The entire preamp, phono AND line stage, will have only three tubes in series, and two coupling caps, total.  No crummy sounding cathode followers.   All caps in it, film only !!  ZERO NFB . The 1.2K Rp Russian 6E6Pi , a Russian tetrode, triode connected single-tube line stage, will drive any of my SE tube amps fine !!

 

Put it ALL in the  uber-modest looking Dyna PAS 2 chassis , or in this case: " in and outside " the chassis..  Anyone got an old Dyna  TOP COVER I can buy, in any  condition ???

 

Jeff 

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9 hours ago, michaelpi said:

Jeff,

 

I'm curious about your polishing of the faceplate. I presume the lettering is silk screened or painted. Did you avoid sanding the lettering in order to preserve it?

 

Michael

 

 

Hi Michael,

 

The PAS 2  lettering  ( not PAS 3 ) is engraved, INDENTED in the brass panel !!!   I did two heavy scratches with 1,000 grit, wet, then all of the front surface with 2,000 grit CAREFULLY, over that, wet. a lot OF SANDING WET. 

 

Then,  it needs to be polished - out to shine brilliantly, which it will if you did the 2,000 grit wet /wellMother' Mag and Aluminum Polish gave it a BRILLIANT polished  finish, highly reflective, that I could not capture with my camera and photography.  My metal-working HOT ROD builder-friend advised me on this, and it was wonderful to do.

 

Do carefully tape OFF the brown painted bottom, as shown, before starting !!

 

Jeff 

 

 

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2100418206_Checkwith.JPG.33bf538c0a7da1283a55ebae403c249c.JPG

 

Check for specifically WHAT ????  A DYNA PAS top cover???

 

If so,  please clarify your communication, and thank you for your help.

 

Jeff

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As far as augmenting the traces on the PC boards with wire, no need to be too concerned if one of the traces lifts from the board while you're soldering on it. The acceptable MIL-2000 repair for a lifted trace was (is?) to reattach it to the board with epoxy, then apply pressure until the epoxy cures. We did that by using a C-clamp with a piece of plastic coffee can lid under it. If a trace is damaged beyond repair, it too can be replaced with wire that's been epoxied to the board.

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On 3/8/2020 at 2:07 PM, michaelpi said:

Jeff,

 

I'm curious about your polishing of the faceplate. I presume the lettering is silk screened or painted. Did you avoid sanding the lettering in order to preserve it?

 

Michael

 

The lettering is neither silk screened or painted on, its engraved below the brass surface.

 

(1) Remove face plate.

 

( 2) Carefully, precisely tape-off the brown painted on lettering across the bottom.

 

(3) A deep scratch, if any, WET sand with 1.000 grit

 

(4)  Entire brass surface, above your added tape-off, WET-sand with 2,000 grit, same direction, repeatedly. 

 

(5) Wash and wipe residue off, and view surface. ANY imperfection, WET-sand with 2,000 grit some more.  Should look uniform and nice.

 

(6)  Polish brass out, same direction, with MOTHER's Mag and Aluminum Polish, which puts on a lovely, highly reflective luxurious finish.   Reassemble and enjoy looks.

 

Jeff Medwin

 

 

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