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RE: ? How to Calc optimum grid Leak R?

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In Reply to: RE: ? How to Calc optimum grid Leak R? posted by kellymon on November 02, 2009 at 12:13:30

hey-Hey!!!,
So far I've found that lower is better. As low as the preceeding stage is able to drive. If your driver is up to it, I suspect you'll find 200k better than 270, and 100k better than 200.
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

Follow Ups:

RE: ? How to Calc optimum grid Leak R? - kellymon 14:13:43 11/02/09 (22)
I'm curious as to why you feel lower is better?
If I'm calculating my freq rolloff right, then 270K and .22uf results in 2.7hz. Lower would require additional uf.
Also, doesn't lower also result in a more attentuated signal to the output tube grid?
robert

Edits: 11/02/09
- RE: ? How to Calc optimum grid Leak R? - PakProtector 14:32:11 11/02/09 (21)
  -----I'm curious as to why you feel lower is better?
  
  Simply because every time I played with it, lower *ALWAYS* sounded better when the preceeding stage drive ability was taken into account.
  cheers,
  Douglas
  
  Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
  - RE: ? How to Calc optimum grid Leak R? - kellymon 14:49:31 11/02/09 (20)
    Ok, thanks.
    When I complete this build, I'll play with it lower and see what it sounds like to me;
    Actually, when it comes down to the listening evaluations, I also have a couple of MQ grid chokes to give a try...... but that's down the road.....
    In the meantime, I'd still like to develop a better understanding of how to optimize this resistor value.
    thanks,
    robert
    - RE: ? How to Calc optimum grid Leak R? - Tre' 16:06:33 11/02/09 (19)
      Go with the grid choke.
      
      A grid choke gives you the best of both worlds. High impedance so the driver stage is happy and low DCR so the tube's grid leakage won't matter.
      
      You won't be sorry.
      
      Tre'
      Have Fun and Enjoy the Music
      "Still Working the Problem"
      - RE: ? How to Calc optimum grid Leak R? - kellymon 17:59:04 11/02/09 (18)
        
        In Reply to: RE: ? How to Calc optimum grid Leak R? posted by Tre' on November 02, 2009 at 16:06:33
        
        Tre',
        I previosly did not care for the grid choke, but that was a different driver. My current driver (6GK5) has a much lower rp and may work better.
        Since these are monoblocks, and 1 is set up for easier experimentation, I will cobble in the grid choke tonight and give another listen.
        But I still want to learn how to calc the optimum grid resistor.....
        robert
        
        RE: ? How to Calc optimum grid Leak R? - Tre' 18:26:08 11/02/09 (17)
        
        In Reply to: RE: ? How to Calc optimum grid Leak R? posted by kellymon on November 02, 2009 at 17:59:04
        
        I don't know if there is a calculation for that.
        
        If you know how much grid leakage current the output tube has you can calculate how much the bias will wander. But tubes will have grid current when over driven as well.
        
        I think you want to use the biggest grid resistor you can (for the sake of the driver) while keeping the bias stable. The problem is, how much of the time does the output tube spend in over drive? And once in over drive, the lowest value grid resistor will be the best (for the output tube, not the driver stage).
        
        BTW Dave has a spice model to show the LF resonance of the tank circuit when cap coupling to a grid choke. The cap value can make or break the overall outcome when using grid chokes.
        
        Let me know and I'll send it to you. Or tell me the inductance and DCR of the choke, cap value and output impedance and I'll post a screen shot of the sim.
        
        Tre'
        Have Fun and Enjoy the Music
        "Still Working the Problem"
        
        RE: ? How to Calc optimum grid Leak R? - kellymon 20:12:35 11/02/09 (16)
        
        In Reply to: RE: ? How to Calc optimum grid Leak R? posted by Tre' on November 02, 2009 at 18:26:08
        
        Thanks Tre, I think I do understand the potentially very bad runaway issues that can occure with too high a grid resistor and tube is over driven.... grid current flows, plate res goes down, causing more plate current causing more pos grid current etc right?
        No bueno....
        Here is where I am now regarding the grid choke I just installed:
        MQ BCP-16 Ni, 2 parallel K40Y9 .22uf coupling caps. I'd like to just use one .22uf......
        I think Mike used to say this grid choke is about 4000hy but now just says real high or something like that....and my OPT is also MQ 5k.
        Right now it all sounds pretty dang fine.
        I just listened to Jorma Kaukonen "Blue Country Heart" and Jewel "Pieces of You" followed by Neil Young "Prarie Wind"......
        All accoustic, all stuff I listen to and can use to evaluate.
        It's getting late, but I will just say that it sounds very nice, clear string notes, good bass and vocals are not strident, but carry emotion.
        Over all, an initially perceived significant improvement.
        More listening will tell.
        So yes, by all means, I would love to see what you have in a sim.
        I have downloaded and played with LT Spice a little, but am nowhere near ready for something like this.
        robert
        
        Edits: 11/02/09
        
        RE: ? How to Calc optimum grid Leak R? - Tre' 08:05:47 11/03/09 (15)
        
        In Reply to: RE: ? How to Calc optimum grid Leak R? posted by kellymon on November 02, 2009 at 20:12:35
        
        I will need to know the dc resistance of the choke and the driver stage output impedance to do the sim.
        
        Tre'
        Have Fun and Enjoy the Music
        "Still Working the Problem"
        
        Grid choke scheme and specs - kellymon 20:35:29 11/03/09 (14)
        
        In Reply to: RE: ? How to Calc optimum grid Leak R? posted by Tre' on November 03, 2009 at 08:05:47
        
        Thanks for the help on this.....
        Here is a schematic of the driver stage with what I think is the needed info.
        Since the 6GK5 driver is listed with 5400 rp and the plate resistor is 15K I assume the output impedance is 3970 ohms.
        The MQ inductance spec is kind of vague, Mikey used to call this a 4800hy choke but since that varies with voltage etc I think he said "1000hy" and now just says it's a lot.
        
        I am not including the PS 150hy choke/cap in this calc because I am considering it part of the CLCLCLC driver B+ filter.
        We can discuss this more if anyone would like.
        
        These are the current measured specs, the info in brackets is where I plan to go based on more recent fine tuning of the calcs. (no listening tests on those yet)
        
        I look forward to seeing the sim results. I can see that I am seriously going to need to get up to speed with LT Spice soon.....
        Robert
        
        RE: Grid choke scheme and specs - Tre' 08:30:09 11/04/09 (13)
        
        In Reply to: RE: Grid choke scheme and specs posted by kellymon on November 03, 2009 at 20:35:29
        
        I used the numbers from the website for the choke.
        
        The parallel resistor will dampen this resonance but defeats one of the reasons for using a grid choke, high impedance.
        
        Tre'
        Have Fun and Enjoy the Music
        "Still Working the Problem"
        Specs on choke (Open in New Window)
        
        RE: Grid choke scheme and specs - Tre' 08:40:50 11/04/09 (12)
        
        In Reply to: RE: Grid choke scheme and specs posted by Tre' on November 04, 2009 at 08:30:09
        
        Here it is with a .22uf cap.
        
        The peak is sub-sonic either way and not that large.
        
        Tre'
        Have Fun and Enjoy the Music
        "Still Working the Problem"
        
        Interesting.... - kellymon 10:20:56 11/04/09 (1)
        
        In Reply to: RE: Grid choke scheme and specs posted by Tre' on November 04, 2009 at 08:40:50
        
        Your link to the MQ site is different from mine.... good to know that's there.
        
        I assume that your plot represents the resonance with the 20 meg bypass R in place?
        What does it look like with just the choke R?
        Thanks for the sim, seeing how you represented the circuit helps.
        robert
        http://www.magnequest.com/products.htm (Open in New Window)
        
        RE: Interesting.... - Tre' 10:56:07 11/04/09 (0)
        
        In Reply to: RE: Interesting.... posted by kellymon on November 04, 2009 at 10:20:56
        
        I made the resistor 20meg ohms to take it out of the picture. If I make it 100 megs there is no difference.
        
        If you make it small enough to make a difference, the load presented to the driver will be low. That defeats one of the reasons for using a grid choke.
        
        Tre'
        
        Have Fun and Enjoy the Music
        "Still Working the Problem"
        
        RE: Grid choke scheme and specs - dave slagle 10:19:17 11/04/09 (9)
        
        In Reply to: RE: Grid choke scheme and specs posted by Tre' on November 04, 2009 at 08:40:50
        
        I have been looking at the periodic A2 recovery from grid chokes lately and have noticed some interesting behaviors.
        I used basically the same values as you did in your sims but placed the grid choke on the grid of an 801 biased at -30V. I then fed it with a 20V 100hz sine and then stepped the voltage up 6dB for 2 complete cycles and then returned things back to the original voltage.
        The sample input signal
        
        The behavior at the grid of the 801 with a .22 u cap and a 100K grid resistor.
        
        The 100K resistor replaced with a Grid choke.
        
        An FFT of the above response to show harmonic content.
        
        The response in the frequency domain
        
        Now consider that the 5.5hz resonance is 16dB below the fundamental and is obviously being excited by a pure 100hz tone. Also consider that even though it is 1/7th the magnitude of the fundamental, is is also at 1/18th the frequency which means it produces 2.5X more flux in the core than the fundamental transient that created it. I do want to be clear that this was all the result of temporary A2 operation of an output tube (40V peak hitting a tube biased at -30V)
        dave
        
        Edits: 11/04/09
        
        Dave, what happens with an interstage transformer? - Ivan303 12:35:46 11/05/09 (3)
        
        In Reply to: RE: Grid choke scheme and specs posted by dave slagle on November 04, 2009 at 10:19:17
        
        Not that I understand what's happening with the grid choke, mine you!
        
        Boats and Fish!
        
        RE: Dave, what happens with an interstage transformer? - dave slagle 12:46:35 11/05/09 (2)
        
        In Reply to: RE: Dave, what happens with an interstage transformer? posted by Ivan303 on November 05, 2009 at 12:35:46
        
        If it is cap coupled (parafed) then the behavior is the same since the same CL resonance still exists. If it is series fed, then the A2 overload does not cause the same ringing since there is no C for the L to resonate with
        
        dave
        
        RE: Dave, what happens with an interstage transformer? - mqracing 05:48:43 11/06/09 (0)
        
        In Reply to: RE: Dave, what happens with an interstage transformer? posted by dave slagle on November 05, 2009 at 12:46:35
        
        Doesn't this series fed power supply have any caps in it? In most power supply designs the last element of the power supply is a relatively large capacitor shunted to ground.
        Therefore, most of the series type power supplies--- will have all the requisite C's, L's and R's to garner the same potential resonance issues. And may actually be moreso irksome since the whole power supply may have multiple resonances as well as a systemic resonance due to it's relative complexity and the number of reactive elements contained therein.
        Paul Joppa in a post several years ago--- pointed this out--- and provided an example to demonstrate that the series fed IT along with the last cap in the power supply--- can also create quite a bit of mischief (i.e., resonance) as I recall from his post.
        Ironically-- to my way of thinking--- the parafeed IT will have several less resonant elements in it than the typical series fed power supply/IT combination. In the series fed circuit (depending the on degree of decoupling provided by the last capacitor in the power supply) the signal could "see" several of the reactive elements that make up your series power supply. Gotta remember that (at a minimum) the last cap (which tends towards being large in value) in a series supply is IN the AC signal path.
        
        Builder of MagneQuest™ & Peerless™ transformers since 1989
        
        Edits: 11/06/09 11/06/09 11/07/09 11/07/09 11/07/09 11/07/09
        
        I knew that! - Ivan303 16:11:16 11/05/09 (0)
        
        In Reply to: RE: Dave, what happens with an interstage transformer? posted by dave slagle on November 05, 2009 at 12:46:35
        
        I just didn't know that I did!
        
        Boats and Fish!
        
        RE: Grid choke scheme and specs - Tre' 10:57:17 11/04/09 (4)
        
        In Reply to: RE: Grid choke scheme and specs posted by dave slagle on November 04, 2009 at 10:19:17
        
        Dave, can we keep our output tube out of A2?
        
        Tre'
        Have Fun and Enjoy the Music
        "Still Working the Problem"
        
        RE: Grid choke scheme and specs - dave slagle 11:13:56 11/04/09 (3)
        
        In Reply to: RE: Grid choke scheme and specs posted by Tre' on November 04, 2009 at 10:57:17
        
        Do you mean for simulations or in reality?
        
        For simulations yes, In reality...I guess. If we consider an amp that hits A2 @ 8W and assume that peaks can be 18dB above the average, we need to be listening to 0.125Wrms to avoid A2 on the peaks.
        
        dave
        
        RE: Grid choke scheme and specs - tube wrangler 23:20:32 11/11/09 (0)
        
        In Reply to: RE: Grid choke scheme and specs posted by dave slagle on November 04, 2009 at 11:13:56
        
        Hi, Dave!
        
        Boy, do I like your statement! It's just like my measured responses.
        
        I am so happy that you, and not myself, have calculated this RMS figure.
        
        I totally agree with you, and I hope that others will look into Hi-Eff. speaker designs and will work to make them more tonally ideal, since they already are ideal-- mostly, that is, electrically.
        
        Speakers are electric motors. The more power is put into them, the more they "kick back"-- inductively. Our amplifiers view this reverse motion as a chance to slip out-of-time-- musically, and amplifiers also see it as a great opportunity to waste energy.
        
        Kudos to you and your insight.
        
        ---Dennis---
        
        RE: Grid choke scheme and specs - Tre' 11:59:52 11/04/09 (1)
        
        In Reply to: RE: Grid choke scheme and specs posted by dave slagle on November 04, 2009 at 11:13:56
        
        Reality.
        
        I clearly need to get some more efficient speakers!
        
        Tre'
        Have Fun and Enjoy the Music
        "Still Working the Problem"
        
        RE: Grid choke scheme and specs - Naz 04:16:18 11/08/09 (0)
        
        In Reply to: RE: Grid choke scheme and specs posted by Tre' on November 04, 2009 at 11:59:52
        
        Or design for proper drive into A2 on peaks. It's worth it IMO.
        
        Naz

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