• ...... LATE POST..... Request for comments on schematic - drlowmu 21:49:11 02/11/07 (1)
  In Reply to: Request for comments on schematic posted by vanofmonks on January 20, 2007 at 17:58:17:

  Hi Chris,

  Just came across your schematic and post today. My two cents are as follows:

  In the main audio circuit, I would simply reduce the circuit's grid-stopper resistance values some as follows:

  R2, input tube, from 240 ohms to 220 to the 208 ohms region. R5, the 2A3 tube, from 240, go to 120 ohms, and NOT lower than 115.0 ohms. Rhoderstein MK-3s ( NOT ever PRPs!! ) can sound good there. I really LIKE the BPC-16 where you have it, I ordered the medium NI ones from Mikey last year, a very very nice part. I LIKE a two-stage 2A3 amp too !!

  In the supply, much redo is needed:

  The 5U4GB is OK, but I would really want to viciously pare-away at all the ugly series resistances, which are harmful to the musical presentation, AND use a double pi choke input filter, preferably one that does not meet critical inductance at the standard convention of 60 HZ.

  So, Chris, totally eliminate R8, R9 and R10. Eliminate the 15 uF input cap and also sadly, the very nice MQ 06-008 inductor.

  After the 5U4GB, connect to a double pi filter as follows : L1/C1/L2/C2. L1 and L2 can both be the Triad C-40X at under $11 from Allied Electronics on line, C1 and C2 can be from 47 to 50 uF, either round canned ASC oils or the 47 plus 47 @ 500 VDC Black Gate WKZs if your budget allows.

  A power transformer with a different secondary will NOT be needed!! Here's why: Your cap input usually produces a VDC B+ of 1.41 times the VAC of the P.T.'s secondary, minus the rectifier drop . A Triad C-40X in the L1 position ( DCR 10 ohms, 320 mHY ) will have about 1.35 times ratio of the VAC of the P.T.'s secondary. But you are also eliminating 238 ohms of UNWANTED DCR, which will help to restore the .06 difference in multipliers !!

  Consider these humble power supply suggestions, as you are eliminating 50 ohms ( R8 ) plus 88 ohms ( L1 ) plus 100 ohms ( R10 ) in your schematic's supply ( 238 ohms total ) , and obtaining to a supply of only 10 plus 10 ohms, ( 20 ohms total ) ie: 1/13 th the DCR !! That redesign WILL positively and absolutely creme the original supply from purely a listening-to-music point of view.

  Minor final points, you may want to ( or have to ) increase the value of R11 slightly upward, maybe another 100 ohms or so, for a little more filtering to the input stage. Keep C6 at 50 uFs or less, film !! Use heavy duty and highest-quality ground wires. Keep your input wiring ( RCA jack to input tube grid ) short.

  Please do let me know if you execute this, and exactly how good it sounds. Bet 'ya a dollar if 'ya built and heard it, there would positively be no returning to the original schematic's power supply ever !! Oh well. Cheers.

  Jeff Medwin

  
  
  

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  • Follow Up - drlowmu 22:06:58 02/21/07 (0)
    In Reply to: ...... LATE POST..... Request for comments on schematic posted by drlowmu on February 11, 2007 at 21:49:11:

    See below URL :
    

    • http://www.audioasylum.com/forums/tubediy/messages/117315.html   (Open in New Window)

    
    

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• Re: Request for comments on schematic - vanofmonks 04:36:32 01/23/07 (0)
  In Reply to: Request for comments on schematic posted by vanofmonks on January 20, 2007 at 17:58:17:

  Thanks for all the help. Looks like I had a typo and should have 155v above the 9002. I was worried about the resonance with the grid choke, so it looks like DrP is the way to go. Is there any thing else I might want to change?

  Thanks,
  Chris
  
  

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• I'd Try DrP across C1+ Excel Spread Sheet - VoltSecond 14:50:58 01/21/07 (4)
  In Reply to: Request for comments on schematic posted by vanofmonks on January 20, 2007 at 17:58:17:

  

  Add a .66 uF cap with a series resistor across C1 to damp the tank made by C1 and the BCP-16.

  I've been playing with a Parafeed spread sheet and it is on the web site below. I'm not done tweaking the web site or the spread sheet. However, I think the site and spread sheet is done enough for a few people to look at it.

  I just updated the site and Excel Spread Sheet a few minute ago.

  Note: This web site is not linked on my main web page
  
  Play safe and play longer! Don't be an "OUCH!" casualty.
  Unplug it, discharge it and measure it (twice) before you touch it.

  . . .Oh!. . .Remember: Modifying things voids their warranty.
  

  • Parafeed Spread Sheet Web Page   (Open in New Window)

  
  

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  • Re: I'd Try DrP across C1+ Excel Spread Sheet - vanofmonks 16:02:42 01/21/07 (3)
    In Reply to: I'd Try DrP across C1+ Excel Spread Sheet posted by VoltSecond on January 21, 2007 at 14:50:58:

    Thanks VS,

    That was very interresting to see the difference when the tank is damped. I thought that might be a problem, but I wasn't sure how everything interracts in that type of circuit. One question though, Mike says that the Hi Ni bcp-16 has over 2500H of inductance compared to the 750H you used, which when put into the spread sheet, if I am using it right, would give me a series resistor of 79.5k along with the .66u cap. Is this one of those times where in practice there is a certain point where it doesn't matter if the value is higher?

    As an aside, I was origionally going to use your delay circuit, and then had boards made up for it using 1200v schottkeys. Then I read your post on the bottlehead forum about the necessary voltage for safety and decided it would be easier to just use a 5u4 than get new boards with stacked uf4007s and one schottkey. My question is, do you know if that circuit would work in the seduction? Or should I just save them for another project?

    Thanks for the help,
    Chris
    
    

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    • 79.5k is what you'll want with the .66u cap - VoltSecond 18:48:43 01/21/07 (2)
      In Reply to: Re: I'd Try DrP across C1+ Excel Spread Sheet posted by vanofmonks on January 21, 2007 at 16:02:42:

      The DrP resistor isn't very sensitive to its value. 75K to 82K should work. If you want lower peaking, the DrP capacitor has to be about 1 u instead of .66U. The damping with .66u should be good enough.

      I don't have the current seduction schematic so I can't answer. I built one of the proto-runs which was different than the production runs.
      
      Play safe and play longer! Don't be an "OUCH!" casualty.
      Unplug it, discharge it and measure it (twice) before you touch it.

      . . .Oh!. . .Remember: Modifying things voids their warranty.
      
      

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      • Re: 79.5k is what you'll want with the .66u cap - mqracing 20:20:40 01/21/07 (1)
        In Reply to: 79.5k is what you'll want with the .66u cap posted by VoltSecond on January 21, 2007 at 18:48:43:

        Hello VS and van of monks:

        I just simulated some parameters with VS version2.G

        here were my "inputs"

        r plate tube 7000 ohms

        r load 990Kohms

        L primary xfmr 2500 henries

        R primary DCR 4000 ohms

        C parafeed .5uf (half a mic)

        L choke 250 henries

        R choke DCR 1000 ohms

        
        C DrP 1uf (one mic)
        R DrP 75000 ohms

        and got output peaking of .22db (point 22 db) at about 5 or so hertz.

        
        My Strategy:

        on purpose I used a plate choke (as opposed to CCS) and kept it's nominal L at about 1/10th the L used in the sim of the grid choke.

        For DrP cap size I used Voltsec's guidance of 2 times blocking cap size.

        For DrP resistance I used 75K figuring it was more than ten times the value of r sub p.

        
        Another thought: cool as VS calculator is.... and it is a GREAT tool... it still may not capture reality... it may overstate the resonance problem since we are not incorporating the impedance (i.e., losses) of the iron core of the grid choke. So bear this in mind... at least preliminarily.

        The issue of resonances is one of them hot-potato issues... and ya want to drop it or get it out of your hand as quick as you can. Yet... another issue (that at least I think relevant) is the question of... taking a horrid resonance say at five hertz (which is two octaves below 20 hertz) we still need to (or should?) ask... how will this resonance be excited? Do we have program material that will excite a 5 hertz resonance? If your playing cd's... rarely will there be much material recorded below what... 40 hertz or so? I, too love to crunch the numbers and VS's calculator is godsend (and thanks also goes to
        gianluca and the nice gentleman from NY who got the ball rolling initially)... but... we still need to use these programs with a bit of reserve or caution. Use them as tools and use them to learn the relationships that exist... changing numbers and variables around can give good insight into the workings or tendencies of the circuit taken as a whole... but... for those blessed with good test equip... scoping the circuit and looking at the actual behaviour may yet be the best way to do the final tuning.

        
        and one last general question: would an alternative to DrP be to use a high pass filter on the input of the amp? Or would (could?) the slope of an filter be sufficient to keep the amp from "producing" the resonance???

        msl
        
        

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        • DrP (Damped Resonance Parafeed) - VoltSecond 18:59:28 01/22/07 (0)
          In Reply to: Re: 79.5k is what you'll want with the .66u cap posted by mqracing on January 21, 2007 at 20:20:40:

          The following is not criticism of Parafeed, it is a discussion of how to avoid POTENTIAL problems using Parafeed. Any, and I do mean any, circuit has trade offs and options. I like having options.

          The DrP (Damped Resonance Parafeed) capacitor needs to be equal to or larger than the Parafeed capacitor. 1X is quite useful, 5X is usually the point of diminishing returns.

          DrP is an option I hope more people come to appreciate.

          The DrP resistor is a function of the square root of ( Lprimary/ Cparafeed ) and the ratio of the DrP cap / Parafeed cap. The DrP resistor has little to do with the plate resistance of the tube. The DrP calculator in the excel spreadsheet assumes no help from the DCR of the transformer or the plate resistance of the tube.

          Peaking can occur in any lightly loaded LC tank.
          …The plate resistance of the tube,
          …the DCR of the primary of the transformer,
          …the reflected load resistance and
          …the core loss of the Parafeed transformer will reduce the Q peaking at low frequencies in a Parafeed.

          I would not add series resistance to the primary of the transformer or add resistance to plate of the tube to reduce with ringing because it will
          …hurt the damping factor,
          …could reduced the gain of the stage and
          …cause high frequency roll off.

          Parafeed gain peaking only really occurs in very lightly loaded outputs like preamps and driver stages. This peaking can be a good thing, a bad thing or neither. We can add resistance across the output to help damp Parafeed ringing. This resistance can
          …Reduce gain,
          …make the tube work harder and/or
          …change the sound of the tube.
          
          The core loss of the output transformer (and plate choke) help damp the Parafeed LC tank. Unfortunately, I don't have a rule of thumb for the core loss at this point. The core loss at 5Hz can be greatly different from the core loss at 60 and 1000 Hz regardless of the manufacture of the transformer. I’d rather not use core loss for this damping because I want to minimize core loss for sonic reasons.

          DrP is a way to add damping with the only draw back I see so far being added cost.
          - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
          Thoughts on 1-20 Hz gain peaking issues:

          Because the “5 Hz” gain peaking can be excited by B+ and grid conduction, rolling off the gain of previous stages does not completely remove the risk of having problems.

          1-20 Hz is a problem area with LPs

          1-20 Hz could be a problem with bias recovery from repetitive clipping (either at the drive tube or forward biasing the grid of a tube on the output of the Parafeed stage.) Asymmetrical clipping is usually worse than symmetrical clipping.

          1-20 Hz can be a problem with 115V line modulation. I've seen a 3 Hz B+ ripple I can't explain. Line dips and peaks could cause large excursions in the bias point. Plug a 500W lamp in to the same outlet as the amp to check for problems when the lamp is turned on and off.

          1-20 Hz could be a problem with motor boating between tube stages.

          IF there are more than one gain stage in a system, make sure if there is peaking that the peaking does not occur at the same frequency. Three 15 dB peaks at 5 Hz gives a total system peak of 45 dB. With multiple stages and amps, either split the frequencies where the peaks occur an octave apart or damp the peaking some how.

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          Other notes

          Because of the DrP resistor in series with the DrP capacitor, the Parafeed capacitor shorts out the sound of the DrP cap at almost all frequencies. This means the DrP cap can be a slightly lower grade than the Parafeed capacitor. I happen to like metal foil capacitors. I could use a metal foil (film and foil) capacitor for the Parafeed capacitor and then use a metalized cap for the DrP cap.

          Note: Mike has asked for permission to post the spreadsheet in his magnetics library. After we're done tweaking the spreadsheet, I'm going to let Mike put a copy in his library.
          
          
          Play safe and play longer! Don't be an "OUCH!" casualty.
          Unplug it, discharge it and measure it (twice) before you touch it.

          . . .Oh!. . .Remember: Modifying things voids their warranty.
          
          

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