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Re: Grid resistors would ruin the source impedance? (nt)

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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'

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Re: Grid resistors would ruin the source impedance? - KevinC 12:38:29 09/01/03 (0)
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)
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)
  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)
    nt

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