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K&K Audio / Lundahl Transformers

which LL1660 should I buy?

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I'm thinking of parallel 6sn7s for driving the IT and the IT driving 6b4g's in push pull.
I understand the 6sn7 is not the best IT driver, so which 1660 do I buy that will let me change my mind and try different driver tubes?
Thanks........Tre'

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Re: which LL1660 should I buy? - KevinC 17:03:31 08/30/03 (12)
Tre,
I think that you might want to buy the LL1660S/10mA. You can use it as you state in your post employing connection alternative T. This will give you a bit more gain and still drive the 6B4Gs adequately. The LL1660S is a new version of the LL1660 that has shielded secondaries, providing better phase-splitting performance. Using the same connection there are a lot of other tubes that will work with up to 20mA of plate current, such as the 5842, and have lower Rp and more gain.

Kevin Carter
K&K Audio
www.kandkaudio.com
- thanks, but now I'm a little confused... - Tre' 17:30:17 08/30/03 (11)
  I'm not understanding the "Primary DC current for .9 Tesla"
  Spec.
  In Alt T, it's 20ma. for .9 tesla, in Alt S it's 10ma.
  Is this differents because of the way the primarys are hooked up?
  Also wouldn't I want to use Alt V for SE to PP?
  Getting back to the .9 Tesla....Would I need to draw 18ma. for things to be happy? Or is that just a Max rating?
  Thanks again Kevin, If I can get to the point of understanding, I'm going to try this and I think, from all I've read, that I'll like the way transformer loaded and driven tubes sound.
  - Re: thanks, but now I'm a little confused... - KevinC 03:54:33 08/31/03 (10)
    Tre,
    Thinking one thing then writing another; if this is a qualification for declaration of onset of old age, then..
    You are perfectly correct, of course, that connection alternative V is almost what I meant to type. However, if you go up to the Lundahl website and download the datasheet for the LL1660S, you will find a variation on V named B that includes the shields I mentioned in last night's post. That is what I should have typed.
    The allowable primary current depends, as you guessed, on the primary hook-up. With the primaries in parallel, as they would be in alternative B, that nearly doubles the maximum current to 18mA with the transformer that is rated for 10mA service with the primaries in series. The usual silicon iron transformer core material saturates at something like 1.6 Tesla, so 0.9T is roughly the half saturation point, which is the highest point on the magnetic curve that you would want to "bias" the transformer and still get all of the magnetic capacity from it in audio use.
    Sorry for the confusion,
    
    Kevin Carter
    K&K Audio
    www.kandkaudio.com
    - Re: thanks, but now I'm a little confused... - Tre' 09:24:07 08/31/03 (9)
      Kevin, I'm getting this. Alt B for the 1660S is more or less the same as Alt V for the 1660. And with the 1660S sheilds it's better for PP.
      I would be able to draw UP TO 18ma. of DC with this setup.
      One more question please, what about the 1635/20ma.?
      Couldn't I run the primarys in parallel and use it for SE to PP?
      It has such a good freq. response.
      Thanks Kevin
      P.S. could I get prices including shipping to 93023.
      - LL1635 not suitable for phase splitting - KevinC 14:44:19 08/31/03 (8)
        
        In Reply to: Re: thanks, but now I'm a little confused... posted by Tre' on August 31, 2003 at 09:24:07:
        
        Tre,
        Due to the winding structure of the LL1635 it can't properly be used for phase splitting. The phase splitting accuracy is not good.
        LL1660S/10mA are currently priced at US$90 each and an extra $14 will get them to your coast in 2-3 days. However, unfortunately I will have none in stock until about Sept. 23rd. :=((
        
        Kevin Carter
        K&K Audio
        www.kandkaudio.com
        
        Re: LL1635 not suitable for phase splitting - Tre' 16:01:17 08/31/03 (7)
        
        In Reply to: LL1635 not suitable for phase splitting posted by KevinC on August 31, 2003 at 14:44:19:
        
        Help me out here,
        Why would one need to use the center tap of the secondary in a SE to PP setup?
        Couldn't I just use grid resistors (putting the reference to ground between the grid resistors) and wouldn't the phase splitting
        accuracy be as good as the resistors were matched?
        Back to the LL1660S, What would be the impedance of the load on the drive tube if the secondary was left open (no grid resistors)?
        With the LL1660s out of stock, it will give me more time to study.
        Tre'
        
        Grid resistors would ruin the source impedance? (nt) - Tre' 21:15:34 08/31/03 (6)
        
        In Reply to: Re: LL1635 not suitable for phase splitting posted by Tre' on August 31, 2003 at 16:01:17:
        
        nt
        
        Re: Grid resistors would ruin the source impedance? (nt) - KevinC 06:49:11 09/01/03 (5)
        
        In Reply to: Grid resistors would ruin the source impedance? (nt) posted by Tre' on August 31, 2003 at 21:15:34:
        
        Tre,
        I am sorry, but I don't understand your question about leaving the secondary open and grid resistors. Could you reword it?
        
        Kevin Carter
        K&K Audio
        www.kandkaudio.com
        
        Re: Grid resistors would ruin the source impedance? (nt) - Tre' 10:03:27 09/01/03 (4)
        
        In Reply to: Re: Grid resistors would ruin the source impedance? (nt) posted by KevinC on September 01, 2003 at 06:49:11:
        
        Sorry Kevin, What I meant to say is: if the transformer secondary's center tap is grounded and the ends are hooked to the grids of each output tube, the source impedance (impedance as seen by the output tube grid) will be low. If the center tap is left ungrounded and large grid resistors are used, the source impedance will be high, or at least as high as the resistor value. If I were to try to "fix" this with small grid resistor values, I would lose gain and load down the driver tube.
        Also, what would be the load on the driver tube if there were not grid resistors (the secoudary left open)? These are all questions. I just trying to see if I'm getting this. Thanks again........Tre'
        
        Re: Grid resistors would ruin the source impedance? - KevinC 12:38:29 09/01/03 (0)
        
        In Reply to: Re: Grid resistors would ruin the source impedance? (nt) posted by Tre' on September 01, 2003 at 10:03:27:
        
        Tre,
        The source impedance "seen" by the grid of the output tube is essentially that of the driver tube plate impedance reflected through the transformer ratio. The grid leak resistance has nothing to do with this. If the driving tube has a plate impedance of 10K and the transformer ratio is 1:1, then the grid "sees" a source impedance of 10K. If the transformer ratio were 1:2, then the source impedance is 40K (the impedance ratio is the square of the voltage (turns) ratio.
        However, you are right about the grid leak resistance being the load for the driver stage (at frequencies below which the Miller effect is significant). One of the good things about coupling driver to output with a transformer is that the output grid sees a very low DC resistance (good for tube runaway prevention), but the driver doesn't have to drive that nasty grid leak resistor, because there isn't one! With cathode biasing, the other end of the IT secondary goes to ground and with "fixed" bias it goes to the negative bias supply; no resistors are necessary.
        Hope that this helped.
        
        Kevin Carter
        K&K Audio
        www.kandkaudio.com
        
        Re: Grid resistors would ruin the source impedance? (nt) - Tre' 12:06:13 09/01/03 (2)
        
        In Reply to: Re: Grid resistors would ruin the source impedance? (nt) posted by Tre' on September 01, 2003 at 10:03:27:
        
        Kevin, I'm not trying to confuse you. Thanks for your patience.
        Using the example of the K&K Audio P-P Amp Input Circuit
        http://hometown.aol.com/kevinc927/images/PPAmpInput.pdf
        The first stage has 25k grid resistors.
        Two things about this first stage.
        Is there 50k reflected back to the rca input through the LL1676 input transformer?
        Does there need to be grid resistors?
        If there were no grid resistors what would be the input impedance at the rca jack?
        Since there are grid resistors, does the center tap need to be grounded and if so why?
        I also have questions about the LL1660s/PP in this circuit.
        If there are no grid resistors on the output tubes that this transformer is driving, what is the load for the 6bx7's?
        And if there are grid resistors on the output tubes, does the LL1660s/PP center tap need to be grounded?
        As for the question about leaving the center tap ungrounded and having grid resistors, I was trying to answer my own question and thinking that the source impedance would be higher if the center tap was left ungrounded.
        I hope this helps and doesn't make it more confusing. I'm trying to look up these questions on my own as well.
        Thanks........Tre'
        
        Re: Grid resistors would ruin the source impedance? - KevinC 12:49:33 09/01/03 (1)
        
        In Reply to: Re: Grid resistors would ruin the source impedance? (nt) posted by Tre' on September 01, 2003 at 12:06:13:
        
        Tre,
        You aren't confusing me, just challenging me to keep up with your questions.
        In the web page example you cited, the two 25K resistors are unnecessary for input tube biasing. They are there to improve phase splitting at high frequencies. With them there, the input impedance is 50K with the transformer wired 2:1+1, as it is. If they were omitted, then the input inpedance would be dominated by the space charge resistance of the tube at low frequencies (megohms) and the Miller effect impedance at high frequencies. The CT needs to be connected to ground to give each input tube a grid ground reference. Otherwise, the grid/cathode voltage relationship would not be established appropriately and the tube would conduct wildly.
        The same considerations apply for the output stage driven through an interstage transformer.
        
        Kevin Carter
        K&K Audio
        www.kandkaudio.com
        
        Thanks Kevin, I think I've got it now (nt) - Tre' 13:36:42 09/01/03 (0)
        
        In Reply to: Re: Grid resistors would ruin the source impedance? posted by KevinC on September 01, 2003 at 12:49:33:
        
        nt

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