• Re: Progress of my 6550 amp// NEEDS some GRID-TUNING. - drkowmu 21:10:47 11/28/03 (1)
  In Reply to: Progress of my 6550 amp posted by akltam on November 21, 2003 at 06:09:14:

  Hi,

  To improve the sonics of the 6C45 / P-P 6550 simple two stage amp, you should experiment with values of ALL the grid-associated resistors.

  I would substitute 100 K for the existing 470K input grid to ground, and see if the music becomes more immediate. If better, go lower 'till it gets worse, and then increase the value, all by ear. (Its like focussing the lens on a camera).

  On the 100 ohm grid stopper to the 6C45, I suspect it is too low, and I would start with 220 ohms and reduce it by paralleling HIGH ohmic value resistors across it, ( 100's of K ohms) and determine final ohmic value by ear. My guess is your final selected value will be higher than the 100 ohms arbitrarily selected by you (and others). No one I know has set this ohmic value by ear yet on this tube !!

  Finally, the 1K ohm 6550 grid stopper needs to start at 2.2 or 2.4 K ohm and then be reduced by ear to a optimal point.

  In general, parallel similar and much higher ohmic value resistors across the start-value grid resistor, till it gets "edgy" sounding, and raise the final ohmic value a bit.

  Also, with this tweeking, use of a digital music source is good because it is an easily repeatable source, without the degradation an LP has when played more than two or three times in a row.

  Have fun grid resistor tuning, and please let me know your final decisions on ear-selected values, and TRUST your ears and brain and senses.

  Jeff Medwin
  
  

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  • Re: Progress of my 6550 amp// NEEDS some GRID-TUNING. - drlowmu 21:30:19 11/28/03 (0)
    In Reply to: Re: Progress of my 6550 amp// NEEDS some GRID-TUNING. posted by drkowmu on November 28, 2003 at 21:10:47:

    Gee, sorry, but my correct moniker is drlowmu and not drkowmu.
    
    

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• Re: Progress of my 6550 amp - Danny T 23:55:59 11/23/03 (3)
  In Reply to: Progress of my 6550 amp posted by akltam on November 21, 2003 at 06:09:14:

  When you're working with fixed bias it's possible that there's high frequency rolloff.
  Because the induction of the interstage and the resistors of the negative bias circuit create a low pass filter through which the signal passes.
  A solution is to add a capacitor to ground between the interstage and the bias resistors.
  
  

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  • Re: adding capacitors - Jim Doyle 16:36:07 02/11/04 (0)
    In Reply to: Re: Progress of my 6550 amp posted by Danny T on November 23, 2003 at 23:55:59:

    I should SPICE model this rather than just ceremoniously add standard
    circuit elements and be surprised by the results.

    One thing that worries me immediately about adding C.... I'm using an 0B2
    gas regulator to stabilize my bias reference to -105V. The -105V gets fed
    through 10K worth of potentiometer. The gas regulator deities forbid me from
    adding C for fear of exciting resonances with the gas tube...

    Oh god. I hope I dont have to put a cathode follower into the bias ckt.

    -- Jim
    
    

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  • Further progress on the 6550 amp - akltam 17:29:20 11/24/03 (1)
    In Reply to: Re: Progress of my 6550 amp posted by Danny T on November 23, 2003 at 23:55:59:

    Thank you for your comment. When I go for the SRPP version, I found that the treble section does not perform that well for the first time (where there is no cap as mention in your recommendation). Then I add it (as a measure after referencing some schematics), The SRPP version is performing close or similar to the single driver stage version IN THE TREBLE SECTION.

    The SRPP version has a gain of about 46 which is more than what I need also I found that the bass of the amp roll off starting JUST under 100 hz.

    As I am using a 1660/25mA for the interstage. Also I am using a configuration of 2.25: 2+2. I think this is probably due to the inductance of this 25mA version is on the low side, so the bass does not get too low. The 2.25 side is measuring at around 18H from LCR meter.

    I am running the 12B4 at 32mA. So maybe the solution is to use a 1660/18mA instead to get the extra H I need. Any comments ?

    Overall the SRPP version (using 6dj8) is performing close to the single stage (6c45) version with more than enough gain. (The 6C45 single has 17 whereas the 6dj8 version has 46). The 6C45 outperform the SRPP (6dj8) version I think in all areas except the required gain.

    I think I will be trying some other similar SRPP design such as using te 6H30 with less gain. Any comments.

    Alan
    

    
    
    

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    • Re: Further progress on the 6550 amp - Danny T 13:29:20 11/26/03 (0)
      In Reply to: Further progress on the 6550 amp posted by akltam on November 24, 2003 at 17:29:20:

      I have the LL1660/18ma in step down mode 4 : 2.25 + 2.25
      and it sounds very good.
      Primary inductance approx 70H
      source impedance 7.5k
      Primary DC current for 0.9 Tesla 16 mA

      Aoother option is the LL1660S/10ma with both primaries in parallel (18ma) 2.25 : 2 + 2
      Primary inductance 42H
      source impedance 3.5k
      Primary DC current for 0.9 Tesla 18 mA

      The LL1660S is a version of the LL1660 with internal Faraday shields to improve balance in phase splitting interstage
      applications.
      
      

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• Re: Progress of my 6550 amp - KevinC 19:15:50 11/21/03 (2)
  In Reply to: Progress of my 6550 amp posted by akltam on November 21, 2003 at 06:09:14:

  Alan,

  The sight of all of those Lundahl transformers warms my heart! ;=))

  You are doing the right thing by trying a number of configurations and evaluating the results by ear. BTW, a gain of 16 (about 24dB) is not a problem with CD as a source and medium sensitivity (90dB) speakers. I shoot for 26-30dB as optimum. Too much gain is a bad thing.
  
  Kevin Carter
  K&K Audio
  www.kandkaudio.com
  
  

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  • What does a gain of 16 =24db ? Thanks. - akltam 06:30:59 12/03/03 (1)
    In Reply to: Re: Progress of my 6550 amp posted by KevinC on November 21, 2003 at 19:15:50:

    Please show me the maths. Thanks.

    Alan
    
    

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    • Re: What does a gain of 16 =24db ? Thanks. - KevinC 06:35:32 12/03/03 (0)
      In Reply to: What does a gain of 16 =24db ? Thanks. posted by akltam on December 03, 2003 at 06:30:59:

      voltage gain (dB) = 20 log (Vout/Vin)

      so,

      voltage gain (dB) = 20 log 16 = 24.08dB
      
      
      

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